Campbell Biology

Superset 53

Population Ecology

Figure 53.1  What causes the survival of turtle hatchlings to vary from year to year?

Key Concepts Turtle Tracks

53.1 Biotic and abiotic factors affect Each year along the Florida coast, thousands of loggerhead turtle (Caretta curette)
hatchlings break out of their eggshells, dig up through the sand, and crawl down
population density, dispersion, the beach for their first journey to the ocean (Figure 53.1). How many turtles will
and demographics successfully hatch, navigate the perils of the beach, and reach the water? This num-
ber can vary widely from year to year. The number of females returning to lay eggs
53.2 The exponential model describes fluctuates, as does the percentage of eggs eaten by raccoons and other predators.
Of the hatchlings that do manage to dig to the surface, some become disoriented
population growth in an by lights and wander away from the ocean or are eaten by birds or crabs before they
idealized, unlimited environment reach the water.

53.3 The logistic model describes how Investigating how factors such as predators and light affect the size of a loggerhead
turtle population is an example of population ecology, the study of populations
a population grows more slowly in relation to their environment. Population ecology explores how biotic and abiotic
as it nears its carrying capacity factors influence the abundance, dispersion, and age structure of populations.

53.4 Life history traits are products Recall that populations evolve as natural selection acts on heritable varia-
tions among individuals, changing the frequencies of alleles and traits over time
of natural selection (see Concept 23.3). Evolution remains a central theme as we now view popula-
tions in the context of ecology.
53.5 Density-dependent factors

regulate population growth

53.6 The human population is no

longer growing exponentially
but is still increasing rapidly

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1188

In this chapter, we will first examine some of the basic the habitat is fairly homogeneous. In other cases, instead of
characteristics of populations. We will then explore the counting single organisms, ecologists estimate density from
tools and models ecologists use to analyze populations and an indicator of population size, such as the number of nests,
the factors that can determine the abundance of organisms. burrows, tracks, or fecal droppings. Ecologists also use the
Finally, we will apply these concepts as we examine recent mark-recapture method to estimate the size of wildlife
trends in the size and makeup of the human population. populations (Figure 53.2).

Concept  53.1 Figure 53.2

Biotic and abiotic factors affect Research Method  Determining Population Size
population density, dispersion, Using the Mark-Recapture Method
and demographics
Application  Ecologists can-
A population is a group of individuals of a single species not count all the individuals
living in the same general area. Members of a population in a population if the
rely on the same resources, are influenced by similar envi- organisms move too
ronmental factors, and are likely to interact and breed with quickly or are hidden
one another. from view. In such
cases, researchers
Populations are often described by their boundaries and often use the mark-
size (the number of individuals living within those bound- recapture method to
aries). Ecologists usually begin investigating a population estimate population
by defining boundaries appropriate to the organism under size. Andrew Gormley
study and to the questions being asked. A population’s and his colleagues at ▲ Hector‘s
boundaries may be natural ones, as in the case of an island or the University of Otago dolphins
a lake, or they may be arbitrarily defined by an investigator— applied this method to a
for example, a specific county in Minnesota for a study of population of endangered Hector’s dolphins (Cephalorhynchus
oak trees. hectori ) near Banks Peninsula, in New Zealand.

Density and Dispersion Technique  Scientists typically begin by capturing a random sample
of individuals in a population. They tag, or “mark,” each individual
The density of a population is the number of individuals per and then release it. With some species, researchers can identify indi-
unit area or volume: the number of oak trees per square kilo- viduals without physically capturing them. For example, Gormley and
meter in the Minnesota county or the number of Escherichia colleagues identified 180 Hector’s dolphins by photographing their
coli bacteria per milliliter in a test tube. Dispersion is the distinctive dorsal fins from boats.
pattern of spacing among individuals within the boundaries
of the population. After waiting for the marked or otherwise identified individuals to
mix back into the population, usually a few days or weeks, scientists
Density: A Dynamic Perspective capture or sample a second set of individuals. At Banks Peninsula,
Gormley’s team encountered 44 dolphins in their second sampling,
In some cases, population size and density can be deter- 7 of which they had photographed before. The number of marked
mined by counting all individuals within the boundaries animals captured in the second sampling (x) divided by the total num-
of the population. We could count all the sea stars in a tide ber of animals captured in the second sampling (n) should equal the
pool, for instance. Large mammals that live in herds, such number of individuals marked and released in the first sampling (s)
as elephants, can sometimes be counted accurately from divided by the estimated population size (N):
airplanes.
x = s or, solving for population size, N= sn
In most cases, however, it is impractical or impossible to n N x
count all individuals in a population. Instead, ecologists use
various sampling techniques to estimate densities and total The method assumes that marked and unmarked individuals have
population sizes. They might count the number of oak trees the same probability of being captured or sampled, that the marked
in several randomly located 100 * 100 m plots, calculate the organisms have mixed completely back into the population, and
average density in the plots, and then extend the estimate that no individuals are born, die, immigrate, or emigrate during the
to the population size in the entire area. Such estimates are resampling interval.
most accurate when there are many sample plots and when
Results  Based on these initial data, the estimated population size
of Hector’s dolphins at Banks Peninsula would be 180 * 44/7 = 1,131
individuals. Repeated sampling by Gormley and colleagues suggested
a true population size closer to 1,100.

Data from  A. M. Gormley et al., Capture-recapture estimates of Hector’s dolphin abun-
dance at Banks Peninsula, New Zealand, Marine Mammal Science 21:204–216 (2005).

INTERPRET THE DATA Suppose that none of the 44 dolphins encountered
in the second sampling had been photographed before. Would you be able to
solve the equation for N? What might you conclude about population size in
this case?

chapter 53  Population Ecology 1189

Figure 53.3  Population dynamics. Deaths move farther from shore. Both immigration and emigration
Births represent important biological exchanges among popula-
tions through time.
Births and Deaths and
immigration emigration Animation: Techniques for Estimating Population
add individuals to remove individuals Density and Size
a population. from a population.
Patterns of Dispersion
Immigration Emigration
Within a population’s geographic range, local densities may
Density is not a static property but changes as individuals differ substantially, creating contrasting patterns of dispersion.
are added to or removed from a population (Figure 53.3). Differences in local density are among the most important
Additions occur through birth (which we define here to characteristics for a population ecologist to study, since they
include all forms of reproduction) and immigration, the provide insight into the biotic and abiotic factors that affect
influx of new individuals from other areas. The factors that individuals in the population.
remove individuals from a population are death (mortality)
and emigration, the movement of individuals out of a The most common pattern of dispersion is clumped, in
population and into other locations. which individuals are aggregated in patches. Plants and
fungi are often clumped where soil conditions and other
While birth and death rates influence the density of all environmental factors favor germination and growth.
populations, immigration and emigration also alter the Mushrooms, for instance, may be clumped within and
density of many populations. Studies of a population of on top of a rotting log. Insects and salamanders may be
Hector’s dolphins (see Figure 53.2) in New Zealand showed clumped under the same log because of the higher humid-
that immigration was approximately 15% of the total popu- ity there. Clumping of animals may also be associated with
lation size each year. Emigration of dolphins in the area mating behavior. Sea stars group together in tide pools,
tends to occur during the winter season when the animals where food is readily available and where they can breed
successfully (Figure 53.4a). The aggregation of individuals
into groups may also increase the effectiveness of preda-
tion or defense; for example, a wolf pack is more likely than
a single wolf to subdue a moose, and a flock of birds is more
likely than a single bird to warn of a potential attack.

A uniform, or evenly spaced, pattern of dispersion may
result from direct interactions between individuals in the
population. Some plants secrete chemicals that inhibit the
germination and growth of nearby individuals that could
compete for resources. Animals often exhibit uniform dis-
persion as a result of antagonistic social interactions, such

Figure 53.4  Patterns of dispersion within a population’s geographic range.

(a) Clumped (b) Uniform (c) Random

Sea stars group together where Nesting king penguins exhibit nearly uni- Dandelions grow from windblown
food is abundant. form spacing, maintained by aggressive seeds that land at random and later
interactions between neighbors. germinate.

? Patterns of dispersion can depend on scale. How might the penguin dispersion look from an airplane
flying over the ocean?

1190 Unit eight  Ecology

as territoriality—the defense of a bounded physical birth until all of the individuals are dead. Building the life
space against encroachment by other individuals table requires determining the proportion of the cohort
(Figure 53.4b). that survives from one age-group to the next. It is also nec-
essary to keep track of the number of offspring produced
In random dispersion (unpredictable spacing), the posi- by females in each age-group.
tion of each individual in a population is independent of
other individuals. This pattern occurs in the absence of strong Demographers who study sexually reproducing species
attractions or repulsions among individuals or where key often ignore the males and concentrate on the females in a
physical or chemical factors are relatively constant across the population because only females produce offspring. Using
study area. Plants established by windblown seeds, such as this approach, a population is viewed in terms of females
dandelions, may be randomly distributed in a fairly uniform giving rise to new females. Table 53.1 is a life table built in
habitat (Figure 53.4c). this way for female Belding’s ground squirrels (Urocitellus
beldingi) from a population located in the Sierra Nevada
Demographics mountains of California. Next, we’ll take a closer look at
some of the data presented in a life table.
The biotic and abiotic factors that influence population den-
sity and dispersion patterns also influence other characteristics Survivorship Curves
of populations, including birth, death, and migration rates.
Demography is the study of these vital statistics of popula- The survival rate data in a life table can be represented graph-
tions and how they change over time. A useful way to sum- ically as a survivorship curve, a plot of the proportion or
marize demographic information for a population is to make numbers in a cohort still alive at each age. As an example,
a life table. let’s use the data for female Belding’s ground squirrels in
Table 53.1 to draw a survivorship curve. Often, a survivor-
Life Tables ship curve begins with a cohort of a convenient size—say,

A life table summarizes the survival and reproduc- 1,000 individuals. To obtain the other points in the
tive rates of individuals in specific age-groups curve for the ground squirrel population, we
within a population. To construct a life table, multiply the proportion alive at the start of
researchers often follow the fate of a cohort, each year (the third column of Table 53.1) by
a group of individuals of the same age, from 1,000 (the hypothetical beginning cohort).

Table 53.1  Life Table for Female Belding’s Ground Squirrels (Tioga Pass, in the Sierra Nevada of California) 

Age (years) Number Alive Proportion Death Average Number
0–1 at Start of Alive at Start Rate† of Female
1–2 Year 0.614
2–3 653 of Year* 0.496 Offspring per
3–4 252 1.000 0.472 Female
4–5 127 0.386 0.478 0.00
5–6  67 0.197 0.457 1.07
6–7  35 0.106 0.526 1.87
7–8  19 0.054 0.444 2.21
8–9   9 0.029 0.200 2.59
  5 0.014 0.750 2.08
 9–10   4 0.008 1.00 1.70
  1 0.006 1.93
0.002 1.93
1.58

Data from  P. W. Sherman and M. L. Morton, Demography of Belding’s ground squirrel, Ecology 65: Researchers working with a Belding’s ground squirrel
1617–1628 (1984).

*Indicates the proportion of the original cohort of 653 individuals that are still alive at the start of
a time interval.
†The death rate is the proportion of individuals alive at the start of a time interval that die during
that time interval.

Table Walkthrough

chapter 53  Population Ecology 1191

Figure 53.5  Survivorship curve for female Belding’s steeply as death rates increase among older age-groups. Many
ground squirrels. The logarithmic scale on the y-axis allows the large mammals, including humans and elephants, that pro-
number of survivors to be visible across the entire range (2–1,000 duce few offspring but provide them with good care exhibit
individuals) on the graph. this kind of curve.

1,000 In contrast, a Type III curve drops sharply at the start,
reflecting very high death rates for the young, but flattens
Number of survivors (log scale) 100 out as death rates decline for those few individuals that sur-
vive the early period of die-off. This type of curve is usually
10 associated with organisms that produce very large numbers
of offspring but provide little or no care, such as long-lived
1 10 plants, many fishes, and most marine invertebrates. An
0246 8 oyster, for example, may release millions of eggs, but most
Age (years) larvae hatched from fertilized eggs die from predation or
other causes. Those few offspring that survive long enough
? What percentage of females survive to be three years old? to attach to a suitable substrate and begin growing a hard
shell tend to survive for a relatively long time. Type II curves
The result is the number alive at the start of each year. are intermediate, with a constant death rate over the organ-
Plotting these numbers versus age for female Belding’s ism’s life span. This kind of survivorship occurs in Belding’s
ground squirrels yields Figure 53.5. The approximately ground squirrels and some other rodents, many inverte-
straight line of the plot indicates a relatively constant rate brates, lizards, and annual plants.
of death.
Many species fall somewhere between these basic types
Figure 53.5 represents just one of many patterns of survi- of survivorship or show more complex patterns. In birds,
vorship exhibited by natural populations. Though diverse, mortality is often high among the youngest individuals (as
survivorship curves can be classified into three general types in a Type III curve) but fairly constant among adults (as in
(Figure 53.6). A Type I curve is flat at the start, reflecting a Type II curve). Some invertebrates, such as crabs, may show
low death rates during early and middle life, and then drops a “stair-stepped” curve, with brief periods of increased mor-
tality during molts, followed by periods of lower mortality
Figure 53.6  Idealized survivorship curves: Types I, II, when their protective exoskeleton is hard.
and III. The y-axis is logarithmic and the x-axis is on a relative scale
so that species with widely varying life spans can be presented In populations not experiencing large amounts of immi-
together on the same graph. gration or emigration, survivorship is one of the two key fac-
tors determining changes in population size. The other key
Number of survivors (log scale) 1,000 I factor that affects how population size changes over time is
reproductive rate.
100
II Reproductive Rates

10 As mentioned above, demographers often ignore the males
and concentrate on the females in a population because only
III females produce offspring. Therefore, demographers view
1 populations in terms of females giving rise to new females.
The simplest way to describe the reproductive pattern of a
0 50 100 population is to identify how reproductive output varies with
Percentage of maximum life span the number of breeding females and their ages.

Animation: Investigating the Survivorship Curve of Oysters How do ecologists estimate the number of breeding females
in a population? Possible approaches include direct counts
and the mark-recapture method (see Figure 53.2). Increasingly,
ecologists also use molecular tools. For example, scientists
working in the state of Georgia collected skin samples from
198 female loggerhead turtles between 2005 and 2009. From
these samples, they amplified nuclear short tandem repeats
at 14 loci using the polymerase chain reaction (PCR) and pro-
duced a genetic profile for each female (Figure 53.7). They
then extracted DNA from an eggshell from each turtle nest on
the beaches they studied and, using their database of genetic

1192 Unit eight  Ecology

Figure 53.7  Using genetic profiles from loggerhead turtle eggshells to identify
which female laid the eggs.

Part 1: Developing the Database:

TAC TAC TAC TAC TAC A genetic profile is
GTG GTG GTG GTG GTG GTG determined for each
TTG TTG TTG TTG turtle and stored in
a database.

In a lab, DNA is extracted
from each skin sample, and
short tandem repeats at 14
loci are amplified by PCR.

Skin samples are collected The genetic profile of the eggshell A match identifies the
from female loggerhead turtles. is compared with an established female that laid the
Part 2: Comparing Samples to the Database: database containing genetic profiles eggs in the nest.
of adult female loggerhead turtles.

TAC TAC TAC TAC TAC TAC Eggshell sample #74
GTG GTG GTG GTG GTG GTG
TTG TTG TTG TTG TTG TTG TTG A genetic profile is
determined for each
In a lab, DNA is extracted eggshell sample.
from the eggshell, and
short tandem repeats at 14
loci are amplified by PCR.

An eggshell is collected from
a loggerhead turtle nest.

VISUAL SKILLS Use the profiles displayed in the figure to determine which breeding female laid
the eggs in the nest from which eggshell sample #74 was taken.

profiles, matched the nest to a specific female. This approach and sperm in a spawning cycle. However, a high reproduc-
allowed them to determine how many of the 198 females were tive rate will not lead to rapid population growth unless
breeding—and how many offspring each female produced— conditions are near ideal for the growth and survival of
without having to disturb the females during egg laying. offspring, as you’ll learn in the next section.

Reproductive output for sexual organisms such as birds Concept Check 53.1
and mammals is typically measured as the average number of
female offspring produced by the females in a given age-group. 1. DRAW IT Each female of a particular fish species pro-
For some organisms, the number of offspring for each female duces millions of eggs per year. Draw and label the most
can be counted directly; alternatively, molecular methods can likely survivorship curve for this species, and explain
be used (see Figure 53.7). Researchers directly counted the off- your choice.
spring of the Belding’s ground squirrels, which begin to repro-
duce at age 1 year. The squirrels’ reproductive output rises to 2. WHAT IF? Imagine that you are constructing a life table
a peak at 4–5 years of age and then gradually falls off in older like Table 53.1 for a different population of Belding’s
females (see Table 53.1). ground squirrels. If 485 individuals are alive at the start
of year 0–1 and 218 are still alive at the start of year 1–2,
Age-specific reproductive rates vary considerably by spe- what is the proportion alive at the start of each of these
cies. Squirrels, for example, have a litter of two to six young years (see column 3 in Table 53.1)?
once a year for less than a decade, whereas oak trees drop
thousands of acorns each year for tens or hundreds of years. 3. MAKE CONNECTIONS A male stickleback fish attacks
Mussels and other invertebrates may release millions of eggs other males that invade its nesting territory (see Figure
51.2a). Predict the likely pattern of dispersion for male
sticklebacks, and explain your reasoning.
For suggested answers, see Appendix A.

chapter 53  Population Ecology 1193

Concept  53.2 that occur in the time interval. Thus, R = B - D, and we can
simplify our equation by writing:
The exponential model describes
population growth in an idealized, ∆N =R
unlimited environment ∆t

Populations of all species have the potential to expand greatly Next, we can convert our model to one in which changes
when resources are abundant. To appreciate the potential for in population size are expressed on a per individual (per
population increase, consider a bacterium that can reproduce capita) basis. The per capita change in population size (r∆t )
by fission every 20 minutes under ideal laboratory conditions. represents the contribution that an average member of the
There would be two bacteria after 20 minutes, four after population makes to the number of individuals added to or
40 minutes, and eight after 60 minutes. If reproduction con- subtracted from the population during the time interval ∆t.
tinued at this rate for a day and a half without mortality, there If, for example, a population of 1,000 individuals increases by
would be enough bacteria to form a layer 30 cm deep over the 16 individuals per year, then on a per capita basis, the annual
entire globe! But unlimited growth does not occur for long change in population size is 16/1,000, or 0.016. If we know
in nature, where individuals typically have access to fewer the annual per capita change in population size, we can use
resources as a population grows. Nonetheless, ecologists study the formula R = r  ∆t N to calculate how many individuals will
population growth in ideal, unlimited environments to reveal be added to (or subtracted from) a population each year. For
how fast populations are capable of growing and the condi- example, if r∆t = 0.016 and the population size is 500,
tions under which rapid growth might actually occur.
R = r∆t N = 0.016 * 500 = 8 per year
Changes in Population Size
Since the number of individuals added to or subtracted
Imagine a population consisting of a few individuals living in from the population (R) can be expressed on a per capita basis
an ideal, unlimited environment. Under these conditions, there as R = r∆t N, we can revise our population growth equation to
are no external limits on the abilities of individuals to harvest take this into account:
energy, grow, and reproduce. The population will increase in
size with every birth and with the immigration of individuals ∆N = r∆t N
from other populations, and it will decrease in size with every ∆t
death and with the emigration of individuals out of the popula-
tion. We can thus define a change in population size during a Remember that our equation is for a specific time interval
fixed time interval with the following verbal equation: (often one year). However, many ecologists prefer to use
differential calculus to express population growth as a rate
of change at each instant in time:

Change in        Immigrants      Emigrants dN = rN
population = Births +   entering    - Deaths - leaving dt
  size        population      population
In this case, r represents the per capita change in population
For now, we will simplify our discussion by ignoring the size that occurs at each instant in time (whereas r∆t repre-
effects of immigration and emigration. sented the per capita change that occurred during the time
interval ∆t). If you have not yet studied calculus, don’t be
We can use mathematical notation to express this simpli- intimidated by the last equation; it is similar to the previous
fied relationship more concisely. If N represents population one, except that the time intervals ∆t are very short and are
size and t represents time, then ∆N is the change in population expressed in the equation as dt. In fact, as ∆t becomes shorter,
size and ∆t is the time interval (appropriate to the life span r∆t and r become increasingly close to one another in value.
or generation time of the species) over which we are evaluat-
ing population growth. (The Greek letter delta, ∆, indicates Exponential Growth
change, such as change in time.) Using B for the number of
births in the population during the time interval and D for the Earlier we described a population whose members all have
number of deaths, we can rewrite the verbal equation: access to abundant food and are free to reproduce at their
physiological capacity. In some cases, a population that expe-
∆N =B-D riences such ideal conditions increases in size by a constant
∆t proportion at each instant in time. When this occurs, the pat-
tern of growth that results is called exponential population
Typically, population ecologists are most interested in growth. The equation for exponential growth is the one
changes in population size—the number of individuals that presented at the end of the previous section, namely:
are added to or subtracted from a population during a given
time interval, symbolized by R. Here, R represents the difference dN = rN
between the number of births (B) and the number of deaths (D) dt

1194 Unit eight  Ecology

In this equation, dN/dt represents the rate at which the Figure 53.9  Exponential growth in the African elephantElephant population size
population is increasing in size at each moment in time, akin population of Kruger National Park, South Africa.
to how a glance at the speedometer of a car reveals the speed
at that instant in time. As seen in the equation, dN/dt equals 8,000
the current population size, N, multiplied by a constant, r.
Ecologists refer to r as the intrinsic rate of increase, the 6,000
per capita rate at which an exponentially growing population
increases in size at each instant in time. 4,000

The size of a population that is growing exponentially 2,000
increases at a constant rate per individual, resulting even-
tually in a J-shaped growth curve when population size is 0
plotted over time (Figure 53.8). Although the per capita rate 1900 1910 1920 1930 1940 1950 1960 1970
of population growth is constant (and equals r), more new
individuals are added per unit of time when the population Year
is large than when it is small; thus, the curves in Figure 53.8
get progressively steeper over time. This occurs because pop- large number of elephants eventually caused enough damage
ulation growth depends on N as well as r, and hence more to vegetation in the park that a collapse in their food supply
individuals are added to larger populations than to small was likely. To protect other species and the park ecosystem
ones growing at the same per capita rate. It is also clear from before that happened, park managers began limiting the ele-
Figure 53.8 that a population with a higher intrinsic rate phant population by giving females birth control medication
of increase (dN/dt = 1.0N) will grow faster than one with a and by exporting elephants to other countries.
lower intrinsic rate of increase (dN/dt = 0.5N).
Concept Check 53.2
The J-shaped curve of exponential growth is characteristic
of some populations that are introduced into a new environ- 1. Explain why a constant per capita rate of growth (r) for a
ment or whose numbers have been drastically reduced by a population produces a curve that is J-shaped.
catastrophic event and are rebounding. For example, the pop-
ulation of elephants in Kruger National Park, South Africa, 2. Where is exponential growth by a plant population more
grew exponentially for approximately 60 years after they were likely—in an area where a forest was destroyed by fire or
first protected from hunting (Figure 53.9). The increasingly in a mature, undisturbed forest? Why?

Figure 53.8  Population growth predicted by the 3. WHAT IF? In 2014, the United States had a population
exponential model. This graph compares growth in a population of 320 million people. If the (annual) per capita change
with r = 1.0 (blue curve) to growth in a population with r = 0.5 in population size (r∆t ) was 0.005, how many people were
(red curve). added to the population that year (ignoring immigration
and emigration)? What would you need to know to deter-
2,000 dN = 1.0N mine whether the United States is currently experiencing
1,500 dt exponential growth?
1,000 For suggested answers, see Appendix A.
Population size (N) dN = 0.5N
dt Concept  53.3

500 The logistic model describes how
a population grows more slowly
0 as it nears its carrying capacity
0 5 10 15
The exponential growth model assumes that resources
Number of generations remain abundant, which is rarely the case in the real world.
Instead, as the size of a population increases, each individual
? How many generations does it take these populations to reach a size has access to fewer resources. Ultimately, there is a limit
of 1,500 individuals? to the number of individuals that can occupy a habitat.
Ecologists define the carrying capacity, symbolized by K,
BioFlix® Animation: Exponential Growth as the maximum population size that a particular environ-
ment can sustain. Carrying capacity varies over space and
time with the abundance of limiting resources. Energy,
shelter, refuge from predators, nutrient availability, water,

chapter 53  Population Ecology 1195

and suitable nesting sites can all be limiting factors. For growth, r(K - N)/K, will be close to (but slightly less than) r,
example, the carrying capacity for bats may be high in a habi- the intrinsic rate of increase seen in exponential population
tat with abundant flying insects and roosting sites but lower growth. But when N is large and resources are limiting, then
where there is abundant food but fewer suitable shelters. (K - N)/K is close to 0, and the per capita population growth
rate is small. When N equals K, the population stops growing.
Crowding and resource limitation can have a profound Table 53.2 shows calculations of population growth rate for
effect on population growth rate. If individuals cannot obtain a hypothetical population growing according to the logistic
sufficient resources to reproduce, then the per capita birth model, with r = 1.0 per individual per year. Notice that the
rate will decline. Similarly, if starvation or disease increases overall population growth rate is highest, +375 individuals
with density, the per capita death rate may increase. Falling per year, when the population size is 750, or half the carrying
per capita birth rates or rising per capita death rates will cause capacity. At a population size of 750, the per capita popula-
the per capita rate of population growth to drop, a very differ- tion growth rate remains relatively high (one-half the value
ent situation from the constant per capita growth rate (r) seen of r), but there are more reproducing individuals (N) in the
in a population that is growing exponentially. population than at lower population sizes.

The Logistic Growth Model As shown in Figure 53.10, the logistic model of popula-
tion growth produces a sigmoid (S-shaped) growth curve
We can modify our mathematical model so that the per when N is plotted over time (the red line). New individuals
are added to the population most rapidly at intermediate
capita population growth rate decreases as N increases. In population sizes, when there is not only a breeding popula-
the logistic population growth model, the per capita tion of substantial size, but also lots of available space and
rate of population growth approaches zero as the population other resources in the environment. The number of indi-
viduals added to the population decreases dramatically
size nears the carrying capacity (K). as N approaches K. As a result, the population growth rate
(dN/dt) also decreases as N approaches K.
To construct the logistic model, we start with the expo-
Note that we haven’t said anything yet about why the
nential population growth model and add an expression population growth rate decreases as N approaches K. For

that reduces the per capita rate of population growth as N Figure 53.10  Population growth predicted by the logistic
model. The rate of population growth decreases as population size
increases. If the carrying capacity is K, then K - N is the num- (N) approaches the carrying capacity (K) of the environment. The red
ber of additional individuals the environment can support, line shows logistic growth in a population where r = 1.0 and K = 1,500
individuals. For comparison, the blue line illustrates a population
and (K - N)/K is the fraction of K that is still available for continuing to grow exponentially with the same r.
population growth. By multiplying the exponential rate

of population growth rN by (K - N)/K, we modify the change
in population size as N increases:

dN = rN (K - N)
dt K

When N is small compared to K, the term (K - N)/K is close Exponential
to 1. In this scenario, the per capita rate of population growth

2,000 dN = 1.0N
dt
Table 53.2  Logistic Growth of a Hypothetical Population
(K =  1,500)  Population size (N ) 1,500
K = 1,500
Per Capita Population Logistic growth
1,000
Intrinsic Population Growth dN = 1.0N 1,500 – N
Rate of Growth Rate Rate* dt 1,500
Population Increase K-N (K - N) (K - N)
Size K r K rN K
(N) (r )
    25 1.0 0.983 0.983   +25 Population growth rate
begins slowing here.
   100 1.0 0.933 0.933   +93 500

   250 1.0 0.833 0.833 +208

   500 1.0 0.667 0.667 +333 0
0 5 10
   750 1.0 0.500 0.500 +375 Number of generations 15

1,000 1.0 0.333 0.333 +333 BioFlix® Animation: Logistic Growth

1,500 1.0 0.000 0.000    0

*Rounded to the nearest whole number.

1196 Unit eight  Ecology

Figure 53.11  How well 1,000Number of Paramecium/mL 180
Number of Daphnia/50 mL 150
do these populations 800 120

fit the logistic growth 600 90
model? In each graph, the 400 60
black dots plot the measured 200 30
growth of the population,
and the red curve is the 0 0
growth predicted by the 0 0
logistic model.

5 10 15 20 40 60 80 100 120 140 160
Time (days) Time (days)

(a) A Paramecium population in the lab. (b) A Daphnia population in the lab. The growth (black dots)
The growth (black dots) of Paramecium of a population of water fleas (Daphnia) in a small laboratory
aurelia in a small culture closely culture does not correspond well to the logistic model (red
approximates logistic growth (red curve) curve). This population overshoots the carrying capacity of its
if the researcher maintains a artificial environment before it settles down to an
constant environment. approximately stable population size.

a population’s growth rate to decrease, the birth rate must The logistic model provides a useful starting point for
decrease, the death rate must increase, or both. Later in the thinking about how populations grow and for construct-
chapter, we’ll consider some of the factors affecting these ing more complex models. As such, its role is similar to that
rates, including the presence of disease, predation, and played by the Hardy-Weinberg equation for thinking about
limited amounts of food and other resources. the evolution of populations. The logistic model is also impor-
tant in conservation biology for predicting how rapidly a par-
The Logistic Model and Real Populations ticular population might increase in numbers after it has been
reduced to a small size and for estimating sustainable harvest
The growth of laboratory populations of some small ani- rates for wildlife populations. Conservation biologists can also
mals, such as beetles and crustaceans, and of some micro- use the model to estimate the critical size below which popu-
organisms, such as bacteria, Paramecium, and yeasts, fits lations of certain organisms, such as the northern subspecies
an S-shaped curve fairly well under conditions of limited of the white rhinoceros (Ceratotherium simum), may become
resources (Figure 53.11a). These populations are grown in extinct (Figure 53.12).
a constant environment lacking predators and competing
species that may reduce growth of the populations, condi- Figure 53.12  White rhinoceros mother and calf. The two
tions that rarely occur in nature. animals pictured here are members of the southern subspecies, which
has a population of more than 20,000 individuals. The northern
Some of the assumptions built into the logistic model subspecies is critically endangered, with just a few known individuals.
clearly do not apply to all populations. The logistic model
assumes that populations adjust instantaneously to growth
and approach carrying capacity smoothly. In reality, there
is often a delay before the negative effects of an increas-
ing population are realized. If food becomes limiting for a
population, for instance, reproduction will decline eventu-
ally, but females may use their energy reserves to continue
reproducing for a short time. This may cause the population
to overshoot its carrying capacity temporarily, as shown
for the water fleas in Figure 53.11b. In the Scientific Skills
Exercise, you can model what can happen to such a popu-
lation when N becomes greater than K. Other populations
fluctuate greatly, making it difficult even to define carrying
capacity. We will examine some possible reasons for such
fluctuations later in the chapter.

chapter 53  Population Ecology 1197

Scientific Skills Exercise ▶ Daphnia

Using the Logistic Equation Now calculate the population growth rate for the same four
to Model Population Growth cases, this time assuming that r = 2.0 (and with K still = 1,500).
3. Now let’s see how the growth of a real-world population of
What Happens to the Size of a Population When It Daphnia corresponds to this model. At what times in Figure 53.11b
Overshoots Its Carrying Capacity?  In the logistic population is the Daphnia population changing in ways that correspond to
growth model, the per capita rate of population increase approaches the values you calculated? Hypothesize why the population drops
zero as the population size (N) approaches the carrying capacity (K). below the carrying capacity briefly late in the experiment.
Under some conditions, however, a population in the laboratory
or the field can overshoot K, at least temporarily. If food becomes Instructors: A version of this Scientific Skills Exercise can be
limiting to a population, for instance, there may be a delay before assigned in MasteringBiology.
reproduction declines, and N may briefly exceed K. In this exercise,
you will use the logistic equation to model the growth of the hypo-
thetical population in Table 53.2 when N 7 K.

Interpret the Data

1. Assuming that r = 1.0 and K = 1,500, calculate the population
growth rate for four cases where population size (N) is greater
than carrying capacity (K): N = 1,510, 1,600, 1,750, and 2,000 indi-
viduals. To do this, first write the equation for population growth
rate given in Table 53.2. Plug in the values for each of the four
cases, starting with N = 1,510, and solve the equation for each
one. Which population size has the highest growth rate?

2. If r is doubled, predict how the population growth rates will
change for the four population sizes given in question 1.

Concept Check 53.3 organism are evolutionary outcomes reflected in its develop-
ment, physiology, and behavior.
1. Explain why a population that fits the logistic growth
model increases more rapidly at intermediate size than Diversity of Life Histories
at relatively small and large sizes.
We’ll focus on three key components of an organism’s life
2. WHAT IF? Given the latitudinal differences in sunlight history: when reproduction begins (the age at first reproduc-
intensity (see Figure 52.3), how might you expect the tion or age at maturity), how often the organism reproduces,
carrying capacity of plant species found at the equa- and how many offspring are produced per reproductive
tor to compare with that of plant species found at high episode. The fundamental idea that evolution accounts for
latitudes? the diversity of life is manifest in the broad range of these
life history characteristics in nature. For example, the age at
3. MAKE CONNECTIONS Suppose that a sudden change which reproduction begins varies considerably across species.
in environmental conditions caused a substantial drop A typical loggerhead turtle is about 30 years old when it first
in a population’s carrying capacity. Predict how natural crawls onto a beach to lay eggs (see Figure 53.1). In contrast,
selection and genetic drift might affect this population. the coho salmon (Oncorhynchus kisutch) is often only three or
(See Concept 22.2.) four years old when it spawns.
For suggested answers, see Appendix A.
Organisms also vary in how often they reproduce. The coho
Concept  53.4 salmon is an example of organisms that undergo a “one-shot”
pattern of big-bang reproduction, or semelparity (from the
Life history traits are products Latin semel, once, and parere, to beget). It hatches in the
of natural selection headwaters of a freshwater stream and then migrates to
the Pacific Ocean, where it typically requires a few years
Evolution Natural selection favors traits that improve to mature. The salmon eventually returns to the same
an organism’s chances of survival and reproductive success. stream to spawn, producing thousands of eggs in a single
In every species, there are trade-offs between survival and re- reproductive opportunity before it dies. Semelparity also
productive traits such as frequency of reproduction, number occurs in some plants, such as the agave, or “century plant”
of offspring (number of seeds produced by plants; litter or (Figure 53.13a). Agaves generally grow in arid climates with
clutch size for animals), or investment in parental care. The
traits that affect an organism’s schedule of reproduction and
survival make up its life history. Life history traits of an

1198 Unit eight  Ecology

Figure 53.13  Semelparity and the amount of resources a parent can devote to
and iteroparity. each offspring. Such trade-offs occur because organ-
isms do not have access to unlimited amounts of
(a) Semelparity, one-time (b) Iteroparity, repeat reproducer. resources. As a result, the use of resources for one
reproducer. An agave Organisms that reproduce repeat- function (such as reproduction) can reduce the
(Agave americana) is an edly, such as the bur oak resources available for supporting another func-
example of semelparity. (Quercus macrocarpa), undergo tion (such as survival). In Eurasian kestrels, for
The leaves of the plant iteroparity. One oak tree can example, caring for a larger number of young low-
are visible at the base of produce thousands of ered the survival rates of the parents (Figure 53.14).
the giant flowering stalk, acorns (inset) per year over the In another study, in Scotland, researchers found that
which is produced only at course of many decades. female red deer that reproduced in a given summer
the end of the agave’s life. were more likely to die the next winter than were
females that did not reproduce.
Selective pressures also influence trade-offs
between the number and size of offspring. Plants
and animals whose young have a low change of
survival often produce many small offspring. Plants

Figure 53.14

Inquiry  How does caring for offspring affect
parental survival in kestrels?

unpredictable rainfall and poor soils. An agave grows for Experiment  Cor Dijkstra and colleagues in the Netherlands studied
years, accumulating nutrients in its tissues, until there is an the effects of parental caregiving in Eurasian kestrels over five years.
unusually wet year. It then sends up a large flowering stalk, The researchers transferred chicks among nests to produce reduced
produces seeds, and dies. This life history appears to be an broods (three or four chicks), normal broods (five or six), and enlarged
adaptation to the agave’s harsh desert environment. broods (seven or eight). They then measured the percentage of male
and female parent birds that survived the following winter. (Both males
In contrast to semelparity is iteroparity (from the Latin and females provide care for chicks.)
iterare, to repeat), or repeated reproduction. As an example, Results
a female loggerhead turtle produces four clutches totaling
approximately 300 eggs in a year. It then typically waits Parents surviving the following winter (%) 100 Male Female
two to three years before laying more eggs; presumably, the 80
turtles lack sufficient resources to produce that many eggs
every year. A mature turtle may lay eggs for 30 years after the 60
first clutch. Horses and other large mammals also reproduce
repeatedly, as do many fish, sea urchins, and long-lived trees, 40
such as maples and oaks (Figure 53.13b).
20
Finally, organisms also vary in how many offspring they
produce. Some species, such as the white rhinoceros (see 0 Reduced Normal Enlarged
Figure 53.12), produce a single calf when they reproduce, brood size brood size brood size
while most insects and many plants produce large numbers
of offspring. Such variation in offspring number has other Conclusion  The lower survival rates of kestrels with larger broods
consequences as well; as you’ll read, a species that produces indicate that caring for more offspring negatively affects survival of
one or a few offspring may provision them better than does the parents.
a species that produces many offspring.
Data from  C. Dijkstra et al., Brood size manipulations in the kestrel (Falco tinnunculus):
“Trade-offs” and Life Histories effects on offspring and parent survival, Journal of Animal Ecology 59:269–285 (1990).

No organism could produce thousands of offspring yet provi- INTERPRET THE DATA The males of some bird species provide no paren-
sion each of them as well as does a white rhinoceros caring for tal care. If this were true for the Eurasian kestrel, predict how the experimental
its single calf. There is a trade-off between offspring number results would differ from those shown here.

chapter 53  Population Ecology 1199

Figure 53.15  Variation in the number and size of seeds maximize reproductive success in uncrowded environments
in plants. (low densities) is called r-selection. These names follow
from the variables of the logistic equation. K-selection is said
(a) Dandelions grow quickly and release a large number of tiny to operate in populations living at a density near the limit
fruits, each containing a single seed. Producing numerous seeds imposed by their resources (the carrying capacity, K), where
ensures that at least some will grow into plants that eventually competition among individuals is stronger. Mature trees
produce seeds themselves. growing in an old-growth forest are an example of K-selected
organisms. In contrast, r-selection is said to maximize r, the
(b) Some plants, such as the Brazil intrinsic rate of increase, and occurs in environments in
nut tree (right), produce a mod- which population densities are well below carrying capacity
erate number of large seeds in or individuals face little competition. Such conditions are
pods (above). Each seed’s large often found in disturbed habitats that are being recolonized.
endosperm provides nutrients Weeds growing in an abandoned agricultural field are an
for the embryo, an adaptation example of r-selected organisms.
that helps a relatively large
fraction of offspring survive. The concepts of K- and r-selection represent two extremes
in a range of actual life histories. The framework of K- and
that colonize disturbed environments, for example, usually r-selection, grounded in the idea of carrying capacity, also
produce many small seeds, only a few of which may reach a relates to the important question we alluded to earlier: Why
suitable habitat. Small size may also increase the chance of does population growth rate decrease as population size
seedling establishment by enabling the seeds to be carried lon- approaches carrying capacity? Answering this question is
ger distances to a broader range of habitats (Figure 53.15a). the focus of the next section.
Animals that suffer high predation rates, such as quail, sar-
dines, and mice, also tend to produce many offspring. Concept Check 53.4

In other organisms, extra investment on the part of the par- 1. Identify three key life history traits, and give examples
ent greatly increases the offspring’s chances of survival. Walnut of organisms that vary widely in each of these traits.
and Brazil nut trees produce large seeds packed with nutrients
that help the seedlings become established (Figure 53.15b). 2. In the fish called the peacock wrasse (Symphodus tinca),
Primates generally bear only one or two offspring at a time; females disperse some of their eggs widely and lay other
parental care and an extended period of learning in the first eggs in a nest. Only the latter receive parental care. Explain
several years of life are very important to offspring fitness. Such the trade-offs in reproduction that this behavior illustrates.
provisioning and extra care can be especially important in
habitats with high population densities. 3. WHAT IF? Mice that experience stress such as a food
shortage will sometimes abandon their young. Explain
One way to categorize variation in life history traits is how this behavior might have evolved in the context of
related to the logistic growth model discussed in Concept 53.3. reproductive trade-offs and life history.
Selection for traits that are advantageous at high densities is
referred to as K-selection. In contrast, selection for traits that For suggested answers, see Appendix A.

1200 Unit eight  Ecology Concept  53.5

Density-dependent factors regulate
population growth

What environmental factors keep populations from growing
indefinitely? Why are some populations fairly stable in size,
while others are not?

Answers to such questions can be important in practical
applications. Farmers may want to reduce the abundance
of insect pests or stop the growth of an invasive weed that is
spreading rapidly. Conservation ecologists need to know what
environmental factors create favorable feeding or breeding
habitats for endangered species, such as the white rhinoceros
and the whooping crane. Overall, whether seeking to reduce
the size of an unwanted population or increase the size of one
that is endangered, it is helpful to understand factors that
affect population abundance.

Population Change and Population Density population size. For example, a drought or heat wave can
cause a sharp increase in mortality rates, causing the abun-
To understand why a population stops growing when it dance of a population to plummet. Note, however, that a
reaches a certain size, ecologists study how the rates of birth, density-independent factor cannot consistently cause a
death, immigration, and emigration change as population population to decrease in size when it is large or increase
density rises. If immigration and emigration offset each other, in size when it is small—only a density-dependent factor
then a population grows when the birth rate exceeds the death can consistently cause such changes. With this in mind, a
rate and declines when the death rate exceeds the birth rate. population is said to be regulated when one or more density-
dependent factors cause its size to decrease when large
A birth rate or death rate that does not change with popu- (or increase when small).
lation density is said to be density independent. For
example, researchers found that the mortality of dune fescue BioFlix® Animation: Density Dependence
grass (Vulpia fasciculata) is mainly due to physical factors that
kill similar proportions of a local population, regardless of its Mechanisms of Density-Dependent
density. Drought stress that arises when the roots of the grass Population Regulation
are uncovered by shifting sands is a density-independent fac-
tor that can kill these plants. In contrast, a death rate that The principle of feedback regulation (see Concept 40.2)
increases with population density or a birth rate that falls with applies to population dynamics. Without some type of nega-
rising density is said to be density dependent. Researchers tive feedback between population density and the rates of
found that reproduction by dune fescue declines as population birth and death, a population would never stop growing. But
density increases, in part because water or nutrients become no population can increase in size indefinitely. Ultimately,
more scarce. Thus, the key factors affecting birth rate in this at large population sizes, negative feedback is provided by
population are density dependent, while death rate is largely density-dependent regulation, which halts population growth
determined by density-independent factors. Figure 53.16 through mechanisms that reduce birth rates or increase death
shows how the combination of density-dependent reproduc- rates. For example, a study of kelp perch (Brachyistius frenatus)
tion and density-independent mortality can stop population populations showed that the fish’s mortality rose propor-
growth, leading to an equilibrium population density in spe- tionally as its density increased (Figure 53.17). Without suf-
cies such as dune fescue. ficient kelp to hide in, the perch was vulnerable to predation
by another fish species, the kelp bass (Paralabrax clathratus).
Variation in density-independent factors such as tem- Predation and other mechanisms of density-dependent
perature and precipitation can cause dramatic changes in regulation are explored further in Figure 53.18.

Figure 53.16  Determining equilibrium for population These various examples of population regulation by
density. This simple model considers only birth and death rates. negative feedback show how increased densities cause popu-
(Immigration and emigration rates are assumed to be either zero or lation growth rates to decline by affecting reproduction,
equal.) In this example, the birth rate changes with population density,
while the death rate is constant. At the equilibrium density (Q), the Figure 53.17  Density-dependent regulation by predation.
birth and death rates are equal. As the density of kelp perch increased, predation by kelp bass increased,
resulting in a higher proportion of kelp perch deaths.
When the density is low, When the density is high,
there are more births than there are more deaths Kelp perch Kelp bass
deaths. Hence, the than births. Hence, the (prey) (predator)
population grows until the population shrinks until
density reaches Q. the density reaches Q. 1.0
0.8
Birth or death rate Equilibrium density (Q) 0.6
per capita 0.4
0.2
Proportional mortality
Density-independent 0
death rate 0

Density-dependent
birth rate

Population density 10 20 30 40 50 60
Kelp perch density (number/plot)
DRAW IT Redraw this figure for the case where the birth and death rates
are both density dependent, as occurs for many species.

chapter 53  Population Ecology 1201

Figure 53.18  Exploring Mechanisms of Density-Dependent Regulation

As population density increases, many density-dependent Predation
mechanisms slow or stop population growth by decreasing
birth rates or increasing death rates. Predation can be an important
cause of density-dependent
Competition for Resources mortality if a predator captures
more food as the population
Increasing population density intensifies competition for density of the prey increases.
nutrients and other resources, reducing reproductive rates. As a prey population builds
Farmers minimize the effect of resource competition on the up, predators may also feed
growth of wheat (Triticum aestivum) and other crops by applying preferentially on that species.
fertilizers to reduce nutrient limitations on crop yield. Population increases in the
collared lemming (Dicrostonyx
Disease groenlandicus) lead to density-
dependent predation by several
If the transmission rate of a predators, including the snowy owl
disease increases as a popula- (Bubo scandiacus).
tion becomes more crowded,
then the disease’s impact is
density dependent. In humans,
the respiratory diseases
influenza (flu) and tuberculosis
are spread through the air
when an infected person
sneezes or coughs. Both
diseases strike a greater
percentage of people in
densely populated cities than
in rural areas.

growth, and survival. Although negative feedback helps both populations appear to have been isolated from immigra-
explain why populations stop growing, it does not address tion and emigration since then. Despite this isolation, the
why some populations fluctuate dramatically while others moose population experienced two major increases and
remain relatively stable. collapses during the last 50 years (Figure 53.19).

Population Dynamics What factors cause the size of the moose population to
change so dramatically? Harsh weather, particularly cold
All populations show some fluctuation in size. Such popula-
tion fluctuations from year to year or place to place, called Figure 53.19  Fluctuations in moose and wolf populations
population dynamics, are influenced by many factors and on Isle Royale, 1959–2011.
in turn affect other species. For example, fluctuations in fish
populations affect populations of seabirds that eat fish. The 50 2,500
study of population dynamics focuses on the complex inter- 2,000
actions between biotic and abiotic factors that cause variation Wolves Moose 1,500
in population sizes. 1,000
40 500
Stability and Fluctuation Number of wolves 0
Number of moose30
Populations of large mammals were once thought to remain
relatively stable, but long-term studies have challenged that 20
idea. For instance, the moose population on Isle Royale in
Lake Superior has fluctuated substantially since around 1900. 10
At that time, moose from the Ontario mainland (25 km away)
colonized the island, perhaps by walking across the lake when 0 1965 1975 1985 1995 2005
it was frozen. Wolves, which rely on moose for most of their 1955 Year
food, reached the island around 1950 by walking across the
frozen lake. The lake has not frozen over in recent years, and BioFlix® Animation: Predator-Prey

1202 Unit eight  Ecology

Territoriality

Territoriality can limit population density when space
becomes the resource for which individuals compete.
Cheetahs (Acinonyx jubatus) use a chemical marker in
urine to warn other cheetahs of their territorial
boundaries. The presence of surplus, or nonbreeding,
individuals is a good indication that territoriality is
restricting population growth.

Toxic Wastes

Yeasts, such as the

Intrinsic Factors brewer’s yeast

Intrinsic physiological factors Saccharomyces
can regulate population size.
Reproductive rates of white-foot- cerevisiae, are used to
ed mice (Peromyscus leucopus) in a
field enclosure can drop even convert carbohydrates to
when food and shelter are
abundant. This drop in ethanol in winemaking.
reproduction at high population
density is associated with The ethanol that accumu-
aggressive interactions and
hormonal changes that delay lates in the wine is toxic to
sexual maturation and depress
the immune system. yeasts and contributes to

density-dependent regulation of yeast 5 μm

population size. The alcohol content of

wine is usually less than 13% because that

is the maximum concentration of ethanol that

most wine-producing yeast cells can tolerate.

winters with heavy snowfall, can weaken moose and reduce Figure 53.20  Population cycles in the snowshoe hare
food availability, decreasing the population size. When moose and lynx. Population counts are based on the number of pelts sold
numbers are low and the weather is mild, food is readily avail- by trappers to the Hudson Bay Company.
able and the population grows quickly. Conversely, when
moose numbers are high, factors such as predation and an 160 Snowshoe hare
increase in the density of ticks and other parasites cause the
population to shrink. The effects of some of these factors can Number of hares
be seen in Figure 53.19. The first major collapse coincided with (thousands)
a peak in the numbers of wolves from 1975 to 1980. The sec- Number of lynx
ond major collapse, around 1995, coincided with harsh winter (thousands)
weather, which increased the energy needs of the moose and 120 Lynx 9
made it harder for them to find food under the deep snow. 80 6

Population Cycles: Scientific Inquiry 40 3

While many populations fluctuate at unpredictable intervals, 0 1875 1900 0
others undergo regular boom-and-bust cycles. Some small 1850 1925
herbivorous mammals, such as voles and lemmings, tend
to have 3- to 4-year cycles, while some birds, such as ruffed Year
grouse and ptarmigans, have 9- to 11-year cycles.
INTERPRET THE DATA What do you observe about the relative timing of the
One striking example of population cycles is the roughly peaks in lynx numbers and hare numbers? What might explain this observation?
10-year cycling of snowshoe hares (Lepus americanus) and
lynx (Lynx canadensis) in the far northern forests of Canada
and Alaska. Lynx are predators that feed predominantly
on snowshoe hares, so lynx numbers might be expected
to rise and fall with the numbers of hares (Figure 53.20).

chapter 53  Population Ecology 1203

But why do hare numbers rise and fall in approximately Figure 53.21  The Glanville fritillary: a metapopulation.
10-year cycles? Two main hypotheses have been proposed.
First, the cycles may be caused by food shortage during On the Åland Islands, local populations of this butterfly (filled circles)
winter. Hares eat the terminal twigs of small shrubs such as
willow and birch in winter, although why this food supply are found in only a fraction of the suitable
might cycle in 10-year intervals is uncertain. Second, the
cycles may be due to predator-prey interactions. Many pred- habitat patches at any given time.
ators other than lynx eat hares, and they may overexploit
their prey. Individuals can move between local Åland
populations and colonize unoccupied Islands
Let’s consider the evidence for the two hypotheses. If patches (open circles).
hare cycles are due to winter food shortage, then the cycles
should stop if extra food is provided to a field population. EUROPE
Researchers conducted such experiments in the Yukon for
20 years—over two hare cycles. They found that hare popula- 5 km Occupied patch
tions in the areas with extra food increased about threefold in Unoccupied patch
density but continued to cycle in the same way as the unfed
control populations. Therefore, food supplies alone do not populations of the butterfly regularly appear and exist-
cause the hare cycles shown in Figure 53.20, so we can reject
the first hypothesis. ing populations become extinct, constantly shifting the

To study the effects of predation, ecologists used radio locations of the 500 colonized patches (Figure 53.21).
collars to track individual hares to determine why they
died. Predators, including lynx, coyotes, hawks, and owls, The species persists in a balance of local extinctions and
killed 95% of the hares in such studies. None of the hares
appeared to have died of starvation. These data support the recolonizations.
second hypothesis. When ecologists set up electric fences
to exclude predators from certain areas, the collapse in sur- An individual’s ability to move between populations
vival that normally occurs in the decline phase of the cycle
was nearly eliminated. Overexploitation by predators thus depends on a number of factors, including its genetic
seems to be an essential part of snowshoe hare cycles; with-
out predators, it is unlikely that hare populations would makeup. One gene that has a strong effect on the Glanville
cycle in northern Canada.
fritillary’s ability to move is Pgi, which codes for the enzyme
Immigration, Emigration, and Metapopulations
phosphoglucoisomerase. This enzyme catalyzes the second
So far, our discussion of population dynamics has focused
mainly on the contributions of births and deaths. However, step of glycolysis (see Figure 9.9), and its activity correlates
immigration and emigration also influence populations.
When a population becomes crowded and resource competi- with the rate of CO2 production from respiration by the but-
tion increases, emigration often increases. terflies. Ecologists studied butterflies known to be heterozy-

Immigration and emigration are particularly important gous or homozygous for a single nucleotide polymorphism
when a number of local populations are linked, forming
a metapopulation. Local populations in a metapopula- in Pgi. They tracked the
tion can be thought of as occupying discrete patches of
suitable habitat in a sea of otherwise unsuitable habitat. movements of individual A Glanville fritillary
Such patches vary in size, quality, and isolation from other butterflies using radar and (Melitaea cinxia) wearing
patches, factors that influence how many individuals move transponders attached to a tracking transponder.
among the populations. If one population becomes extinct,
the patch it occupied may be recolonized by immigrants the butterflies that emit an
from another population.
identifying signal. Butterfly
The Glanville fritillary (Melitaea cinxia) illustrates the
movement of individuals between populations. This but- movements ranged widely,
terfly is found in about 500 meadows across the Åland
Islands of Finland, but its potential habitat in the islands from 10 m to 4 km, in two-
is much larger, approximately 4,000 suitable patches. New
hour periods. Heterozygous
1204 Unit eight  Ecology
individuals flew more than

twice as far in the morning

and at lower ambient tem-

peratures than did homozy-

gous individuals. The results

indicated a fitness advantage

to the heterozygous genotype in low temperatures and a Figure 53.22  Human population growth (data as of 2015).
greater likelihood of heterozygotes colonizing new locations The global human population has grown almost continuously
in the metapopulation. throughout history, but it skyrocketed after the Industrial Revolution.
Though it is not apparent at this scale, the rate of population growth
The metapopulation concept underscores the significance has slowed in recent decades, mainly as a result of decreased birth
of immigration and emigration for the Glanville fritillary and rates throughout the world.
many other species. It also helps ecologists understand popu-
lation dynamics and gene flow in patchy habitats, providing 7
a framework for the conservation of species living in a net-
work of habitat fragments and reserves. 6 Human population (billions)

Concept Check 53.5 5

1. Describe three attributes of habitat patches that could 4
affect population density and rates of immigration and
emigration. 3

2. WHAT IF? Suppose you were studying a species that 2
has a population cycle of about ten years. How long
would you need to study the species to determine if its 1
population size were declining? Explain.
0
3. MAKE CONNECTIONS Negative feedback is a process
that regulates biological systems (see Concept 40.2). 8000 4000 3000 2000 1000 0 1000 2000
Explain how the density-dependent birth rate of dune BCE
fescue grass exemplifies negative feedback. BCE BCE BCE BCE CE CE
For suggested answers, see Appendix A.
adding a city the size of Amarillo, Texas. At this rate, it takes
Concept  53.6 only about four years to add the equivalent of another
United States to the world population. Ecologists predict there
The human population is no longer will be 8.1–10.6 billion people on Earth by the year 2050.
growing exponentially but is still
increasing rapidly Though the global population is still growing, the rate of
growth began to slow during the 1960s (Figure 53.23). The
In the last few centuries, the human population has grown annual rate of increase in the global population peaked at
at an unprecedented rate, more like the elephant population 2.2% in 1962 but was only 1.1% in 2014. Current models
in Kruger National Park (see Figure 53.9) than the fluctuating
populations we considered in Concept 53.5. No population Figure 53.23  Annual percent increase in the global human
can grow indefinitely, however. In this section of the chapter, population (data as of 2014). The sharp dip in the 1960s is due
we’ll apply the concepts of population dynamics to the spe- mainly to a famine in China in which about 60 million people died.
cific case of the human population.
Annual percent increase 2.2 2014
The Global Human Population 2.0 Projected
1.8 data
The human population has grown explosively over the last 1.6
four centuries (Figure 53.22). In 1650, about 500 million 1.4 1975 2000 2025 2050
people inhabited Earth. Our population doubled to 1 billion 1.2 Year
within the next two centuries, doubled again to 2 billion by 1.0
1930, and doubled still again to 4 billion by 1975. Notice that 0.8
the time it took our population to double in size decreased 0.6
from 200 years in 1650 to just 45 years in 1930. Thus, histori- 0.4
cally our population has grown even faster than exponential 0.2
growth, which has a constant rate of increase and hence a
constant doubling time. 0
1950
The global population is now more than 7.2 billion people
and is increasing by about 78 million each year. This translates
into more than 200,000 people each day, the equivalent of

chapter 53  Population Ecology 1205

project a growth rate of 0.5% by 2050, which would add A unique feature of human population growth is our abil-
45 million more people per year if the population climbs to ity to control family sizes through planning and voluntary
a projected 9 billion. The reduction in annual growth rate contraception. Social change and the rising educational
observed over the past four decades shows that the human and career aspirations of women in many cultures encour-
population is now growing more slowly than expected in age women to delay marriage and postpone reproduction.
exponential growth. This change resulted from fundamen- Delayed reproduction helps to decrease population growth
tal shifts in population dynamics due to diseases, including rates and to move a society toward zero population growth
AIDS, and to voluntary population control. under conditions of low birth rates and low death rates.
However, there is a great deal of disagreement as to how
Regional Patterns of Population Change much support should be provided for global family
planning efforts.
We have described changes in the global population, but pop-
ulation dynamics vary widely from region to region. In a stable Age Structure
regional population, birth rate equals death rate (disregarding
the effects of immigration and emigration). Two possible con- Another important factor that can affect population growth
figurations for a stable population are is a country’s age structure, the relative number of indi-
viduals of each age in the population. Age structure is com-
Zero population growth = High birth rate - High death rate monly graphed as “pyramids” like those in Figure 53.24.
or For Afghanistan, the pyramid is bottom heavy, skewed
toward young individuals who will grow up and perhaps
Zero population growth = Low birth rate - Low death rate sustain the explosive growth with their own reproduction.
The age structure for the United States is relatively even
The movement from high birth and death rates toward low until the older, postreproductive ages. Although the cur-
birth and death rates, which tends to accompany indus- rent total reproductive rate in the United States is about
trialization and improved living conditions, is called the 2.1 children per woman—near the replacement rate—the
demographic transition. In Sweden, this transition took population is projected to grow slowly through 2050 as a
about 150 years, from 1810 to 1975, when birth rates finally result of immigration. For Italy, the pyramid has a small
approached death rates. In Mexico, where the human popu- base, indicating that individuals younger than reproductive
lation is still growing rapidly, the transition is projected to age are relatively underrepresented in the population. This
take until at least 2050. Demographic transition is associ- situation contributes to the projection of a future popula-
ated with an increase in the quality of health care and sani- tion decrease in Italy.
tation as well as improved access to education, especially
for women. Age-structure diagrams not only predict a population’s
growth trends but also can illuminate social conditions.
After 1950, death rates declined rapidly in most develop- Based on the diagrams in Figure 53.24, we can predict that
ing countries, but birth rates have declined more variably. employment and education opportunities will continue
Birth rates have fallen most dramatically in China. In 1970, to be a problem for Afghanistan in the foreseeable future.
the Chinese birth rate predicted an average of 5.9 children In the United States and Italy, a decreasing proportion of
per woman per lifetime (total fertility rate); by 2011, largely younger working-age people will soon be supporting an
because of the government’s strict one-child policy, the total increasing population of retired “boomers.” This demo-
fertility rate was 1.6 children. In some countries of Africa, the graphic feature has made the future of Social Security
transition to lower birth rates has also been rapid, though and Medicare a major political issue in the United States.
birth rates remain high in most of sub-Saharan Africa. Understanding age structures is necessary to plan for
the future.
How do such variable birth rates affect the growth of
the world’s population? In industrialized nations, popula- Infant Mortality and Life Expectancy
tions are near equilibrium, with reproductive rates near the
replacement level of 2.1 children per female (over their life- Infant mortality, the number of infant deaths per 1,000 live
time). In many industrialized countries, including Canada, births, and life expectancy at birth, the predicted average
Germany, Japan, and the United Kingdom, total reproduc- length of life at birth, vary widely in different countries.
tive rates are in fact below the replacement level. These pop- In 2011, for example, the infant mortality rate was 149
ulations will eventually decline if there is no immigration (14.9%) in Afghanistan but only 2.8 (0.28%) in Japan. Life
and if the birth rate does not change. In fact, the population expectancy at birth was only 48 years in Afghanistan but
is already declining in many eastern and central European 82 years in Japan. These differences reflect the quality of
countries. Most of the current global population growth
occurs in less industrialized countries, where about 80%
of the world’s people now live.

1206 Unit eight  Ecology

Figure 53.24  Age-structure pyramids for the human population of three countries
(data as of 2010). The annual growth rate was approximately 2.6% in Afghanistan, 1.0% in the
United States, and 0.0% in Italy.

Rapid growth Slow growth No growth
Afghanistan United States Italy

Male Female Age Male Female Age Male Female

85+ 85+
80–84 80–84
75–79 75–79
70–74 70–74
65–69 65–69
60–64 60–64
55–59 55–59
50–54 50–54
45–49 45–49
40–44 40–44
35–39 35–39
30–34 30–34
25–29 25–29
20–24 20–24
15–19 15–19
10–14 10–14

5–9 5–9
0–4 0–4

10 8 6 4 2 0 2 4 6 8 10 54 3 21 01 23 45 54 3 21 01 23 45
Percent of population Percent of population Percent of population

Animation: Analyzing Age-Structure Pyramids

life faced by children at birth and influence the reproduc- Estimates of Carrying Capacity
tive choices parents make. If infant mortality is high, then
parents are likely to have more children to ensure that For over three centuries, scientists have attempted to esti-
some reach adulthood. mate the human carrying capacity of Earth. The first known
estimate, 13.4 billion people, was made in 1679 by Anton
Although global life expectancy has been increasing since van Leeuwenhoek (a Dutch scientist who also discovered
about 1950, it has recently dropped in a number of regions, protists; see the introduction to Chapter 28). Since then,
including countries of the former Soviet Union and in sub- estimates have varied from less than 1 billion to more than
Saharan Africa. In these regions, social upheaval, decaying 1,000 billion (1 trillion).
infrastructure, and infectious diseases such as AIDS and tuber-
culosis are reducing life expectancy. In the African country Carrying capacity is difficult to estimate, and scientists use
of Angola, life expectancy in 2011 was 43 years, about half of different methods to produce their estimates. Some research-
that in Japan, Sweden, Italy, and Spain. ers use curves like that produced by the logistic equation (see
Figure 53.10) to predict the future maximum of the human
Global Carrying Capacity population. Others generalize from existing “maximum”
population density and multiply this number by the area of
No ecological question is more important than the future habitable land. Still others base their estimates on a single
size of the human population. As noted earlier, population limiting factor, such as food, and consider variables such
ecologists project a global population of approximately as the amount of farmland, the average yield of crops, the
8.1–10.6 billion people in 2050. That means that an esti- prevalent diet—vegetarian or meat based—and the number
mated 1–4 billion people will be added to the population in of calories needed per person per day.
the next four decades because of the momentum of popula-
tion growth. But just how many humans can the biosphere Limits on Human Population Size
support? Will the world be overpopulated in 2050? Is it
already overpopulated? A more comprehensive approach to estimating the carrying
capacity of Earth is to recognize that humans have multiple

chapter 53  Population Ecology 1207

Figure 53.25  Per capita ecological footprint by country.

Per capita ecological
footprint (gha)

0–3
3–6
>6
Insufficient data

? Earth has a total of 11.9 billion gha of productive land. How many people could Earth support
sustainably if the average ecological footprint were 8 gha per person (as in the United States)?

constraints: We need food, water, fuel, building materials, yields an allotment of 1.7 gha per person—the benchmark
and other resources, such as clothing and transportation. The for comparing actual ecological footprints. Anyone who
ecological footprint concept summarizes the aggregate consumes resources that require more than 1.7 gha to pro-
land and water area required by each person, city, or nation duce is using an unsustainable share of Earth’s resources, as
to produce all the resources it consumes and to absorb all the is the case for the citizens of many countries (Figure 53.25).
waste it generates. For example, a typical ecological footprint for a person in
the United States is 8 gha.
What is a sustainable ecological footprint for the entire
human population? One way to estimate this footprint is Our impact on the planet can also be assessed using
to add up all the ecologically productive land on the planet currencies other than area, such as energy use. Average
and divide by the size of the human population. Typically, energy use differs greatly across different regions of the
this estimate is made using global hectares, where a global world (Figure 53.26). A typical person in the United States,
hectare (gha) represents a hectare of land or water with a Canada, or Norway consumes roughly 30 times the energy
productivity equal to the average of all biologically produc- that a person in central Africa does. Moreover, fossil fuels,
tive areas on Earth (1 hectare = 2.47 acres). This calculation such as oil, coal, and natural gas, are the source of 80%

Figure 53.26  The uneven electrification
of the planet. This composite image taken
from space of Earth’s surface at night illustrates
the varied density of electric lights worldwide,
one aspect of energy use by humans.

1208 Unit eight  Ecology

or more of the energy used in most developed nations. As Technology has substantially increased Earth’s carrying
Chapter 56 discusses in more detail, this unsustainable reli- capacity, but no population can grow indefinitely. After reading
ance on fossil fuels is changing Earth’s climate and increas- this chapter, you should realize that there is no single carrying
ing the amount of waste that humans produce. Ultimately, capacity. How many people our planet can sustain depends on
the combination of resource use per person and population the quality of life each of us has and the distribution of wealth
density determines our global ecological footprint. across people and nations, topics of great concern and political
debate. We can decide whether zero population growth will be
What factors will eventually limit the growth of the human attained through social changes based on human choices or,
population? Perhaps food will be the main limiting factor. instead, through increased mortality due to resource limitation,
Malnutrition and famine are common in some regions, but plagues, war, and environmental degradation.
they result mainly from the unequal distribution of food
rather than from inadequate production. So far, technologi- Concept Check 53.6
cal improvements in agriculture have allowed food supplies
to keep up with global population growth. In contrast, the 1. How does a human population’s age structure affect its
demands of many populations have already far exceeded the growth rate?
local and even regional supplies of one renewable resource—
fresh water. More than 1 billion people do not have access 2. How have the rate and number of people added to the
to sufficient water to meet their basic sanitation needs. The human population each year changed in recent decades?
human population may also be limited by the capacity of the
environment to absorb its wastes. If so, then Earth’s current 3. WHAT IF? Type “personal ecological footprint calculator”
human occupants could lower the planet’s long-term carrying into a search engine and use one of the resulting calcula-
capacity for future generations. tors to estimate your footprint. Is your current lifestyle sus-
tainable? If not, what choices can you make to influence
your own ecological footprint?

For suggested answers, see Appendix A.

53 Chapter Review Go to MasteringBiology™ for Videos, Animations, Vocab Self-Quiz,
Practice Tests, and more in the Study Area.
Summary of Key Concepts
Concept  53.2
Concept  53.1
The exponential model describes population
Biotic and abiotic factors affect growth in an idealized, unlimited environment
(pp. 1194–1195)
population density, dispersion,
If immigration and emigration are ignored, a population’s per
and demographics (pp. 1189–1193) VOCAB capita growth rate equals its birth rate minus its death rate.
SELF-QUIZ
Population density—the number of individuals per goo.gl/6u55ks The exponential growth equation dN/dt = rN represents a
population’s growth when resources are relatively abundant,
unit area or volume—reflects the interplay of births, where r is the intrinsic rate of increase and N is the number
deaths, immigration, and emigration. Environmental and social of individuals in the population.
factors influence the dispersion of individuals.

Patterns of dispersion

Population size (N) dN = rN
dt

Clumped Uniform Random

Populations increase from births and immigration and decrease Number of generations
from deaths and emigration. Life tables and survivorship
curves summarize specific trends in demography.

? Gray whales ( Eschrichtius robustus ) gather each winter near Baja ? Suppose one population has an r that is twice as large as the r of
California to give birth. How might such behavior make it easier another population. What is the maximum size that both populations

for ecologists to estimate birth and death rates for the species? will reach over time, based on the exponential model?

chapter 53  Population Ecology 1209

Concept  53.3 are influenced by complex interactions between biotic and
abiotic factors. A metapopulation is a group of populations
The logistic model describes how a population linked by immigration and emigration.
grows more slowly as it nears its carrying
capacity (pp. 1195–1198) ? Give an example of one biotic and one abiotic factor that contribute
to yearly fluctuations in the size of the human population.
Exponential growth cannot be sustained in any population.
A more realistic population model limits growth by incorporat- Concept  53.6
ing carrying capacity (K), the maximum population size the
environment can support. The human population is no longer growing
exponentially but is still increasing
According to the logistic growth equation dN/dt = rN rapidly (pp. 1205–1209)
(K - N)/K, growth levels off as population size approaches
the carrying capacity. Since about 1650, the global human population has grown
exponentially, but within the last 50 years, the rate of growth
Population size (N) K = carrying capacity has fallen by half. Differences in age structure show that while
some nations’ populations are growing rapidly, those of oth-
dN = rN (K – N) ers are stable or declining in size. Infant mortality rates and life
dt K expectancy at birth vary widely in different countries.

Ecological footprint is the aggregate land and water area
needed to produce all the resources a person or group of people
consume and to absorb all of their waste. It is one measure of how
close we are to the carrying capacity of Earth, which is uncertain.
With a world population of more than 7.2 billion people, we are
already using many resources in an unsustainable manner.

Number of generations ? How do humans differ from other species in the ability to “choose”
a carrying capacity for their environment?

The logistic model fits few real populations perfectly, but it is useful Test Your Understanding
for estimating possible growth.

? As an ecologist who manages a wildlife preserve, you want to increase Level 1: Knowledge/Comprehension
the preserve’s carrying capacity for a particular endangered species.

How might you go about accomplishing this?

1. Population ecologists follow the fate of same-age
Concept  53.4 cohorts to
(A) determine a population’s carrying capacity.
Life history traits are products of natural (B) determine the birth rate and death rate of PRACTICE
selection (pp. 1198–1200) TEST
goo.gl/CUYGKD
Life history traits are evolutionary outcomes reflected in the each group in a population.
development, physiology, and behavior of organisms. (C) determine if a population is regulated by
density-dependent processes.
Big-bang, or semelparous, organisms reproduce once and die. (D) determine the factors that affect the size of a population.
Iteroparous organisms produce offspring repeatedly.
2. A population’s carrying capacity
Life history traits such as brood size, age at maturity, and parental (A) may change as environmental conditions change.
caregiving represent trade-offs between conflicting demands (B) can be accurately calculated using the logistic growth model.
for time, energy, and nutrients. Two hypothetical life history (C) increases as the per capita population growth rate decreases.
patterns are K-selection and r-selection. (D) can never be exceeded.

? Explain why ecological trade-offs are common. 3. Scientific study of the population cycles of the snowshoe hare
and its predator, the lynx has revealed that
Concept  53.5 (A) predation is the dominant factor affecting prey population
cycling.
Density-dependent factors regulate population (B) hares and lynx are so mutually dependent that each species
growth (pp. 1200–1205) cannot survive without the other.
(C) both hare and lynx population sizes are affected mainly by
In density-dependent population regulation, death rates abiotic factors.
rise and birth rates fall with increasing density. A birth or (D) the hare population is r-selected and the lynx population
death rate that does not vary with density is said to be density is K-selected.
independent.
4. Analyzing ecological footprints reveals that
Density-dependent changes in birth and death rates curb popula- (A) Earth’s carrying capacity would increase if per capita meat
tion increase through negative feedback and can eventually sta- consumption increased.
bilize a population near its carrying capacity. Density-dependent (B) current demand by industrialized countries for resources
limiting factors include intraspecific competition for limited food is much smaller than the ecological footprint of those
or space, increased predation, disease, intrinsic physiological countries.
factors, and buildup of toxic substances. (C) it is not possible for technological improvements to
increase Earth’s carrying capacity for humans.
Because changing environmental conditions periodically (D) the ecological footprint of the United States is large because
disrupt them, all populations exhibit some size fluctuations. per capita resource use is high.
Many populations undergo regular boom-and-bust cycles that

1210 Unit eight  Ecology

5. Based on current growth rates, Earth’s human population in 11. EVOLUTION CONNECTION  Contrast the selective pressures
2019 will be closest to operating in high-density populations (those near the carrying
(A) 2.5 million. capacity, K ) versus low-density populations.
(B) 4.5 billion.
(C) 7.5 billion. 12. SCIENTIFIC INQUIRY  You are testing the hypothesis that
(D) 10.5 billion. increased population density of a particular plant species
increases the rate at which a pathogenic fungus infects
Level 2: Application/Analysis the plant. Because the fungus causes visible scars on the
leaves, you can easily determine whether a plant is infected.
6. The observation that members of a population are uniformly Design an experiment to test your hypothesis. Describe your
distributed suggests that experimental and control groups, how you would collect data,
(A) resources are distributed unevenly. and what results you would see if your hypothesis is correct.
(B) the members of the population are competing for access to
a resource. 13. SCIENCE, TECHNOLOGY, AND SOCIETY  Some people regard
(C) the members of the population are neither attracted to nor the rapid population growth of less industrialized countries as
repelled by one another. our most serious environmental problem. Others think that the
(D) the density of the population is low. population growth in industrialized countries, though smaller,
is actually a greater environmental threat. What problems result
7. According to the logistic growth equation from population growth in (a) less industrialized countries
and (b) industrialized nations? Which do you think is a greater
dN = rN (K - N) threat, and why?
dt K
14. WRITE ABOUT A THEME: Interactions  In a short essay
(A) the number of individuals added per unit time is greatest (100–150 words), identify the factor or factors in Figure 53.18
when N is close to zero. that you think may ultimately be most important for density-
dependent population regulation in humans, and explain
(B) the per capita population growth rate increases as N your reasoning.
approaches K.
15. SYNTHESIZE YOUR KNOWLEDGE 
(C) population growth is zero when N equals K.
(D) the population grows exponentially when K is small. Locusts (grasshoppers in the family Acrididae) undergo cyclic
8. During exponential growth, a population always population outbreaks, leading to massive swarms such as
(A) has a constant per capita population growth rate. this one in the Canary Islands, off the west coast of Africa.
(B) quickly reaches its carrying capacity. Of the mechanisms of density-dependent regulation shown
(C) cycles through time. in Figure 53.18, choose the two that you think most apply to
(D) loses some individuals to emigration. locust swarms, and explain why.
9. Which of the following statements about human populations For selected answers, see Appendix A.
in industrialized countries is incorrect?
(A) Birth rates and death rates are high. For additional practice questions, check out the Dynamic Study
(B) Average family size is relatively small. Modules in MasteringBiology. You can use them to study on
(C) The population has undergone the demographic your smartphone, tablet, or computer anytime, anywhere!

transition.
(D) The survivorship curve is Type I.

Level 3: Synthesis/Evaluation

10. INTERPRET THE DATA  To estimate which age cohort in a
population of females produces the most female offspring, you
need information about the number of offspring produced per
capita within that cohort and the number of individuals alive
in the cohort. Make this estimate for Belding’s ground squirrels
by multiplying the number of females alive at the start of the
year (column 2 in Table 53.1) by the average number of female
offspring produced per female (column 5 in Table 53.1). Draw a
bar graph with female age in years on the x-axis (0–1, 1–2, and
so on) and total number of female offspring produced for each
age cohort on the y-axis. Which cohort of female Belding’s
ground squirrels produces the most female young?

chapter 53  Population Ecology 1211

Community Ecology 54

Figure 54.1  Which species benefits from this interaction?

Key Concepts Communities in Motion

54.1 Community interactions are At first glance, you might think the situation looks dire for this bluestreak cleaner
wrasse, which has ventured into the mouth of the giant moray eel, a voracious preda-
classified by whether they help, tor in its coral reef habitat (Figure 54.1). With one snap of its jaw, the eel could easily
harm, or have no effect on the crush the fish and swallow it. However, the wrasse is not in danger of becoming this
species involved eel’s dinner. The much larger animal remains still, with its mouth open, and allows
the smaller fish free passage as it picks out and eats tiny parasites living inside the eel’s
54.2 Diversity and trophic structure mouth and on its skin.

characterize biological In this interaction, both organisms benefit: The cleaner wrasse gains access to a
communities supply of food, and its moray eel client is freed of parasites that might weaken it or
spread disease. There are many other examples of such mutually beneficial “cleaner”
54.3 Disturbance influences species and “client” relationships in marine habitats, such as the cleaner shrimp and eel
interaction shown below. However, other interactions between species are less benign
diversity and composition for one of the participants, and still other interactions can negatively affect the repro-
duction and survival of both species involved.
54.4 Biogeographic factors affect
In Chapter 53, you learned how individuals within a population can affect other
community diversity individuals of the same species. This chapter will examine ecological interactions
between populations of different species. A group of populations of different spe-
54.5 Pathogens alter community cies living in close enough proximity to interact is called a biological community.

structure locally and globally

When you see this blue icon, log in to MasteringBiology Get Ready for This Chapter
and go to the Study Area for digital resources.

Ecologists define the boundaries of a particular community supply on land; most terrestrial species use this resource but
to fit their research questions: They might study the com- do not usually compete for it.
munity of decomposers and other organisms living on a
rotting log, the benthic community in Lake Superior, or the Competitive Exclusion
community of trees and shrubs in Sequoia National Park in
California. What happens in a community when two species compete for
limited resources? In 1934, Russian ecologist G. F. Gause stud-
We begin this chapter by exploring the kinds of interac- ied this question using laboratory experiments with two closely
tions that occur between species in a community, such as related ciliate species, Paramecium aurelia and Paramecium
the cleaner wrasse and eel in Figure 54.1. We’ll then consider caudatum (see Figure 28.17a). He cultured the species under
several of the factors that are most significant in structuring stable conditions, adding a constant amount of food each day.
a community—in determining how many species there are, When Gause grew the two species separately, each population
which particular species are present, and the relative abun- increased rapidly in number and then leveled off at the appar-
dance of these species. Finally, we’ll apply some of the prin- ent carrying capacity of the culture (see Figure 53.11a for an
ciples of community ecology to the study of human disease. illustration of the logistic growth of a Paramecium population).
But when Gause grew the two species together, P. caudatum
Concept  54.1 became extinct in the culture. Gause inferred that P. aurelia
had a competitive edge in obtaining food. More generally, his
Community interactions results led him to conclude that two species competing for the
are classified by whether they same limiting resources cannot coexist permanently in the
help, harm, or have no effect same place. In the absence of disturbance, one species will use
on the species involved the resources more efficiently and reproduce more rapidly than
the other. Even a slight reproductive advantage will eventually
Some key relationships in the life of an organism are its inter- lead to local elimination of the inferior competitor, an outcome
actions with individuals of other species in the community. called competitive exclusion.
These interspecific interactions include competition,
predation, herbivory, parasitism, mutualism, and commen- Ecological Niches and Natural Selection
salism. In this section, we’ll define and describe each of these
interactions, grouping them according to whether they have Evolution Competition for limited resources can cause
positive (+) or negative (-) effects on the survival and repro- evolutionary change in populations. One way to examine
duction of each of the two species engaged in the interaction. how this occurs is to focus on an organism’s ecological
niche, the specific set of biotic and abiotic resources that
For example, predation is a +/- interaction, with a positive an organism uses in its environment. The niche of a tropical
effect on the predator population and a negative effect on the tree lizard, for instance, includes the temperature range it
prey population. Mutualism is a +/+ interaction because the tolerates, the size of branches on which it perches, the time
survival and reproduction of both species are increased in the of day when it is active, and the sizes and kinds of insects
presence of the other. A 0 indicates that a species is not affected it eats. Such factors define the lizard’s niche, or ecological
by the interaction in any known way. We’ll consider three role—how it fits into an ecosystem.
broad categories of ecological interactions: competition (-/-),
exploitation (+/-), and positive interactions (+/+ or +/0). We can use the niche concept to restate the principle of
competitive exclusion: Two species cannot coexist perma-
Historically, most ecological research has focused on inter- nently in a community if their niches are identical. However,
actions that have a negative effect on at least one species, such ecologically similar species can coexist in a community if one
as competition and predation. As we’ll see, however, positive or more significant differences in their niches arise through
interactions are ubiquitous and have major effects on commu- time. Evolution by natural selection can result in one of the
nity structure. species using a different set of resources or similar resources
at different times of the day or year. The differentiation of
Competition  niches that enables similar species to coexist in a community
is called resource partitioning (Figure 54.2).
Competition is a -/- interaction that occurs when indi-
viduals of different species compete for a resource that limits As a result of competition, a species’ fundamental niche,
the survival and reproduction of each species. Weeds grow- which is the niche potentially occupied by that species, is
ing in a garden compete with garden plants for soil nutrients often different from its realized niche, the portion of its fun-
and water. Lynx and foxes in the northern forests of Alaska damental niche that it actually occupies. Ecologists can iden-
and Canada compete for prey such as snowshoe hares. In tify the fundamental niche of a species by testing the range
contrast, some resources, such as oxygen, are rarely in short of conditions in which it grows and reproduces in the absence

chapter 54   Community Ecology 1213

Figure 54.2  Resource partitioning among Dominican Figure 54.3
Republic lizards. Seven species of Anolis lizards live in close proximity,
and all feed on insects and other small arthropods. However, competition Inquiry  Can a species’ niche be influenced
for food is reduced because each lizard species has a different preferred by interspecific competition?
perch, thus occupying a distinct niche.
Experiment  Ecologist Joseph Connell studied two barnacle species—
A. distichus perches A. insolitus usually perches Chthamalus stellatus and Balanus balanoides—that have a stratified
on fence posts and on shady branches. distribution on rocks along the coast of Scotland. Chthamalus is usually
other sunny surfaces. found higher on the rocks than Balanus. To determine whether the
distribution of Chthamalus is the result of interspecific competition with
Balanus, Connell removed Balanus from the rocks at several sites.

A. ricordii Chthamalus High tide
Balanus Chthamalus
realized niche

A. insolitus Ocean Balanus
A. christophei realized niche
A. aliniger
A. distichus A. cybotes Low tide
A. etheridgei
Results  Chthamalus spread into the High tide
region formerly occupied by Balanus.

Chthamalus
fundamental niche

of competitors. They can also test whether a potential competi- Ocean Low tide
tor limits a species’ realized niche by removing the competitor
and seeing if the first species expands into the newly available Conclusion  Interspecific competition makes the realized niche of
space. The classic experiment depicted in Figure 54.3 clearly Chthamalus much smaller than its fundamental niche.
showed that competition between two barnacle species kept
one species from occupying part of its fundamental niche. Data from  J. H. Connell, The influence of interspecific competition and other factors
on the distribution of the barnacle Chthamalus stellatus, Ecology 42:710–723 (1961).
Species can partition their niches not just in space, as
lizards and barnacles do, but in time as well. The common Instructors: A related Experimental Inquiry Tutorial
spiny mouse (Acomys cahirinus) and the golden spiny mouse can be assigned in MasteringBiology.
(A. russatus) live in rocky habitats of the Middle East and
Africa, sharing similar microhabitats and food sources. Where WHAT IF? Other observations showed that Balanus cannot survive high on
they coexist, A. cahirinus is nocturnal (active at night), while the rocks because it dries out during low tides. How would Balanus’s realized
A. russatus is diurnal (active during the day). Surprisingly, niche compare with its fundamental niche?
laboratory research showed that A. russatus is naturally noctur-
nal. To be active during the day, it must override its biological Character Displacement
clock in the presence of A. cahirinus. When researchers in Israel
removed all A. cahirinus individuals from a site in the species’ Closely related species whose populations are sometimes
natural habitat, A. russatus individuals at that site became allopatric (geographically separate; see Concept 24.2) and
nocturnal, consistent with the laboratory results. This change sometimes sympatric (geographically overlapping) pro-
vide more evidence for the importance of competition
in behavior suggests in structuring communities. In some cases, the allopatric
that competition exists populations of such species are morphologically similar and
between the species and use similar resources. By contrast, sympatric populations,
that partitioning of their which would potentially compete for resources, show dif-
active time helps them ferences in body structures and in the resources they use.
coexist. This tendency for characteristics to diverge more in sym-
patric than in allopatric populations of two species is called
The golden spiny mouse character displacement. An example of character dis-
(Acomys russatus) placement can be seen in two species of Galápagos finches,

1214 Unit eight  Ecology

Figure 54.4  Character displacement: indirect evidence Scientific Skills Exercise
of past competition. Allopatric populations of Geospiza fuliginosa
and Geospiza fortis on Los Hermanos and Daphne Islands have similar Making a Bar Graph
beak morphologies (top two graphs) and presumably eat similarly sized and a Scatter Plot
seeds. However, where the two species are sympatric on Floreana and
San Cristóbal, G. fuliginosa has a shallower, smaller beak and G. fortis Can a Native Predator Species Adapt Rapidly to an
a deeper, larger one (bottom graph), adaptations that favor eating Introduced Prey Species?  Cane toads (Bufo marinus) were
different-sized seeds. introduced to Australia in 1935 in a failed attempt to control an
insect pest. Since then, the toads have spread across northeastern
G. fuliginosa G. fortis Australia, with a population of over 200 million today. Cane toads
have glands that produce a toxin that is poisonous to snakes and
Beak other potential predators. In this exercise, you will graph and inter-
depth pret data from a two-part experiment conducted to determine
whether native Australian predators have developed resistance to
Percentages of individuals in each size class 60 Los Hermanos G. fuliginosa, the cane toad toxin.
40 allopatric
20 How the Experiment Was Done  In part 1, researchers collected
12 red-bellied black snakes (Pseudechis porphyriacus) from areas
0 where cane toads had existed for 40–60 years and another 12 from
areas free of cane toads. They recorded the percentage of snakes from
60 Daphne G. fortis, each area that ate either a freshly killed native frog (Limnodynastes
40 allopatric peronii, a species the snakes commonly eat) or a freshly killed cane
20 toad from which the toxin gland had been removed (making the
toad nonpoisonous). In part 2, researchers collected snakes from areas
0 where cane toads had been present for 5–60 years. To assess how cane
toad toxin affected the physiological activity of these snakes, they
60 Floreana, San Cristóbal Sympatric injected small amounts of the toxin into the snakes’ stomachs and
40 populations measured the snakes’ swimming speed in a small pool.

20 Data from the Experiment, Part 1

0 16 Type % of Snakes From Each Area That Ate Each Type of Prey
8 10 12 14 of Prey
Offered
Beak depth (mm) Area with Cane Toads Area with
Present for 40–60 Years No Cane Toads

INTERPRET THE DATA If the beak length of G. fortis is typically 12% Native frog 100 100
longer than the beak depth, what is the predicted beak length of G. fortis
Cane toad   0  50
individuals with the smallest beak depths observed on Floreana and San
Data from the Experiment, Part 2
Cristóbal Islands?

Number of Years  5 10 10 20 50 60 60 60 60 60

Geospiza fuliginosa and Geospiza fortis: Beak depths in these Cane Toads Had Been
species are similar in allopatric populations but have diverged
considerably in sympatric populations (Figure 54.4). Present in the Area

Exploitation % Reduction in Snake 52 19 30 30  5  5  9 11 12 22
Swimming Speed
All nonphotosynthetic organisms must eat, and all organ-
isms are at risk of being eaten. As a result, much of the drama Data from  B. L. Phillips and R. Shine, An invasive species induces rapid adaptive
in nature involves exploitation, a general term for any +/- change in a native predator: cane toads and black snakes in Australia, Proceedings
interaction in which one species benefits by feeding on the of the Royal Society B 273:1545–1550 (2006).
other species, which is harmed by the interaction. Exploitative
interactions include predation, herbivory, and parasitism. Interpret the Data

Predation 1. Make a bar graph of the data in part 1. (For additional informa-
tion about graphs, see the Scientific Skills Review in Appendix F
Predation is a +/- interaction between species in which and in the Study Area in Mastering Biology.)
one species, the predator, kills and eats the other, the prey.
Though the term predation generally elicits such images as 2. What do the data represented in the graph suggest about the
a lion attacking and eating an antelope, it applies to a wide effects of cane toads on the predatory behavior of red-bellied black
range of interactions. A rotifer (a tiny aquatic animal that is snakes in areas where the toads are and are not currently found?
smaller than many unicellular protists) that kills a protist by
eating it can also be considered a predator. Because eating and 3. Suppose a novel enzyme that deactivates the cane toad toxin
avoiding being eaten are prerequisites to reproductive success, evolved in a snake population exposed to cane toads. If the
the adaptations of both predators and prey tend to be refined researchers repeated part 1 of this study, predict how the results
through natural selection. In the Scientific Skills Exercise, would change.

4. Identify the dependent and independent variables in part 2
and make a scatter plot. What conclusion would you draw
about whether exposure to cane toads is having a selective
effect on the snakes? Explain.

5. Explain why a bar graph is appropriate for presenting the data
in part 1 and a scatter plot is appropriate for the data in part 2.

Instructors: A version of this Scientific Skills Exercise can be
assigned in MasteringBiology.

you can interpret data on the impact of natural selection for a Just as predators possess adaptations for capturing prey,
specific predator-prey interaction. potential prey have adaptations that help them avoid being
eaten. In animals, these adaptations include behavioral
Many important feeding adaptations of predators are defenses such as hiding, fleeing, and forming herds or schools.
obvious and familiar. Most predators have acute senses that Active self-defense is less common, though some large grazing
enable them to find and identify potential prey. Rattlesnakes mammals vigorously defend their young from predators.
and other pit vipers, for example, find their prey with a pair
of heat-sensing organs located between their eyes and nostrils Animals also display a variety of morphological and
(see Figure 50.7b). Owls have characteristically large eyes that physiological defensive adaptations. Mechanical or chemi-
help them see prey at night. Many predators also have adap- cal defenses protect species such as porcupines and skunks
tations such as claws, fangs, or poison that help them catch (Figure 54.5a and b). Some animals, such as the European
and subdue their food. Predators that pursue their prey are fire salamander, can synthesize toxins; others accumu-
generally fast and agile, whereas those that lie in ambush are late toxins passively from the plants they eat. Animals
often disguised in their environments. with effective chemical defenses often exhibit bright

Figure 54.5  Examples of defensive adaptations in animals. (b) Chemical
defense
(a) Mechanical
defense

▶ Porcupine ▶ Skunk
(d) Cryptic
(c) Aposematic
coloration: coloration:
warning camouflage
coloration
▶ Canyon
▶ Poison tree frog
dart frog
(f) Müllerian mimicry:
(e) Batesian mimicry: Two unpalatable
A harmless species species mimic
mimics a harmful each other.
one.

▲ Venomous green ▲ Yellow jacket
parrot snake ◀ Cuckoo bee

◀ Nonvenomous
hawkmoth larva

MAKE CONNECTIONS Explain how natural selection could increase the resemblance of a harmless
species to a distantly related harmful species. In addition to selection, what else could account for a harmless
species resembling a closely related harmful species? (See Concept 22.2.)

1216 Unit eight  Ecology

aposematic coloration, or warning coloration, such as movement of more than a dozen marine animals, including
that of poison dart frogs (Figure 54.5c). Such coloration crabs, sea stars, sea snakes, fish, and stingrays. This octopus’s
seems to be adaptive because predators often avoid brightly ability to mimic other animals enables it to approach prey—
colored prey. Cryptic coloration, or camouflage, makes for example, imitating a crab to approach another crab and
prey difficult to see (Figure 54.5d). eat it. The octopus also can defend itself from predators
through mimicry. When attacked by a damselfish, the octo-
Some prey species are protected by their resemblance to pus quickly mimics a banded sea snake, a known predator
other species. For example, in Batesian mimicry, a palat- of the damselfish.
able or harmless species mimics an unpalatable or harmful
species to which it is not closely related. The larva of the Interview with Tracy Langkilde: Studying the evolution of
hawkmoth Hemeroplanes ornatus puffs up its head and thorax novel defensive adaptations in lizards (see the interview
when disturbed, looking like the head of a small venomous before Chapter 52)
snake (Figure 54.5e). In this case, the mimicry even involves
behavior; the larva weaves its head back and forth and hisses Herbivory
like a snake. Such cases of Batesian mimicry are thought to
result from natural selection, as individuals in the harmless Ecologists use the term herbivory to refer to a exploitative
species that happen to more closely resemble the harmful (+/-) interaction in which an organism—an herbivore—eats
one are avoided by predators who have learned not to eat the parts of a plant or alga, thereby harming it. While large mam-
harmful ones. Over time, closer and closer resemblance to malian herbivores such as cattle, sheep, and water buffalo
the harmful species evolves. In Müllerian mimicry, two or may be most familiar, most herbivores are actually inverte-
more unpalatable species, such as the cuckoo bee and yellow brates, such as grasshoppers, caterpillars, and beetles. In the
jacket, resemble each other (Figure 54.5f). Presumably, the ocean, herbivores include sea urchins, some tropical fishes,
more unpalatable prey there are, the faster predators learn to and certain mammals (Figure 54.7).
avoid prey with that particular appearance. Mimicry has also
evolved in many predators. The mimic octopus Thaumoctopus Like predators, herbivores have many specialized adapta-
mimicus (Figure 54.6) can take on the appearance and tions. Many herbivorous insects have chemical sensors on
their feet that enable them to distinguish between plants
Figure 54.6  The mimic octopus. (a) After hiding six of its based on their toxicity or nutritional value. Some mammalian
tentacles in a hole in the seafloor, the octopus waves its other two herbivores, such as goats, use their sense of smell to examine
tentacles to mimic a sea snake. (b) Flattening its body and arranging plants, rejecting some and eating others. They may also eat
its arms to trail behind, the octopus mimics a flounder (a flat fish). just a specific part of a plant, such as the flowers. Many herbi-
(c) It can mimic a stingray by flattening most of its tentacles alongside vores also have specialized teeth or digestive systems adapted
its body while allowing one tentacle to extend behind it. for processing vegetation (see Concept 41.4).

(a) Mimicking a sea Unlike animals, plants cannot run away to avoid being
snake eaten. Instead, a plant’s arsenal against herbivores may
feature chemical toxins or structures such as spines and

Figure 54.7  An herbivorous marine mammal. This West
Indian manatee (Trichechus manatus) in Florida is grazing on Hydrilla,
an introduced plant species.

(b) Mimicking a
flounder

(c) Mimicking a
stingray

chapter 54   Community Ecology 1217

thorns. Among the plant compounds that serve as chemical Mutualism
defenses are the poison strychnine, produced by the tropical
vine Strychnos toxifera; nicotine, from the tobacco plant; and Mutualism is an interspecific interaction that benefits
tannins, from a variety of plant species. Compounds that are both species (+/+). Mutualisms are common in nature, as
not toxic to humans but may be distasteful to many herbi- illustrated by examples seen in previous chapters, including
vores are responsible for the familiar flavors of cinnamon, cellulose digestion by microorganisms in the digestive sys-
cloves, and peppermint. Certain plants produce chemicals tems of termites and ruminant mammals, animals that pol-
that cause abnormal development in some insects that linate flowers or disperse seeds, nutrient exchange between
eat them. For more examples of how plants defend them- fungi and plant roots in mycorrhizae, and photosynthesis
selves, see Make Connections Figure 39.27, “Levels of Plant by unicellular algae in corals. In some mutualisms, such as
Defenses Against Herbivores.” the acacia-ant example shown in Figure 54.8, each species
depends on the other for their survival and reproduction.
Parasitism In other mutualisms, however, both species can survive
on their own.
Parasitism is a +/- exploitative interaction in which one
organism, the parasite, derives its nourishment from Typically, both partners in a mutualism incur costs as
another organism, its host, which is harmed in the process. well as benefits. In mycorrhizae, for example, the plant
Parasites that live within the body of their host, such as tape-
worms, are called endoparasites; parasites that feed on the Figure 54.8  Mutualism between acacia trees and ants.
external surface of a host, such as ticks and lice, are called
ectoparasites. In one particular type of parasitism, para- (a) Certain species of acacia trees in Central and South America have
sitoid insects—usually small wasps—lay eggs on or in living hollow thorns (not shown) that house stinging ants of the genus
hosts. The larvae then feed on the body of the host, eventu- Pseudomyrmex. The ants feed on nectar produced by the tree and on
ally killing it. Some ecologists have estimated that at least protein-rich swellings (yellow in the photograph) at the tips of leaflets.
one-third of all species on Earth are parasites.
(b) The acacia benefits because the pugnacious ants, which attack any-
Many parasites have complex life cycles involving multi- thing that touches the tree, remove fungal spores, small herbivores,
ple hosts. The blood fluke, which currently infects approxi- and debris. They also clip vegetation that grows close to the acacia.
mately 200 million people around the world, requires two
hosts at different times in its development: humans and
freshwater snails (see Figure 33.11). Some parasites change
the behavior of their current host in ways that increase
the likelihood that the parasite will reach its next host. For
instance, crustaceans that are parasitized by acanthocepha-
lan (spiny-headed) worms leave protective cover and move
into the open, where they are more likely to be eaten by the
birds that are the second host in the worm’s life cycle.

Parasites can significantly affect the survival, reproduc-
tion, and density of their host population, either directly or
indirectly. For example, ticks that feed as ectoparasites on
moose can weaken their hosts by withdrawing blood and
causing hair breakage and loss. In their weakened condition,
the moose have a greater chance of dying from cold stress or
predation by wolves.

Positive Interactions

While nature abounds with dramatic and gory examples of
exploitative interactions, ecological communities are also
heavily influenced by positive interactions, a term that
refers to a +/+ or +/0 interaction in which at least one spe-
cies benefits and neither is harmed. Positive interactions
include mutualism and commensalism. As we’ll see, positive
interactions can affect the diversity of species found in an
ecological community.

1218 Unit eight  Ecology

often transfers carbohydrates to the fungus, while the fun- Figure 54.10  Facilitation by black rush (Juncus gerardii)
gus transfers limiting nutrients, such as phosphorus, to the in New England salt marshes. Black rush increases the number
plant. Each partner benefits, but each partner also experi- of plant species that can live in the upper middle zone of the marsh.
ences a cost: It transfers materials that it could have used to
support its own growth and metabolism. For an interaction Number of plant species 8
to be considered a mutualism, the benefits to each partner
must exceed the costs. 6

Commensalism 4

An interaction between species that benefits one of the spe- 2
cies but neither harms nor helps the other (+/0) is called
commensalism. Like mutualism, commensalisms are (a) Salt marsh with Juncus 0
common in nature. For instance, many wildflower species (foreground) (b) With Juncus Without Juncus
that grow optimally in low light levels are only found in
shaded, forest floor environments. Such shade-tolerant ecological interactions: Their effects can change over time.
“specialists” depend entirely on the trees that tower above In this case, an interaction whose effects are typically +/0
them—the trees provide their dim habitat. Yet the survival (commensalism) may at times become +/+ (mutualism).
and reproduction of the trees are not affected by these
wildflowers. Thus, these species are involved in a +/0 Positive interactions can have significant influence on
interaction in which the wildflowers benefit and the trees the structure of ecological communities. For instance, the
are not affected. black rush Juncus gerardii makes the soil more hospitable for
other plant species in some areas of New England salt marshes
In another example of a commensalism, cattle egrets feed (Figure 54.10a). Juncus helps prevent salt buildup in the soil
on insects flushed out of the grass by grazing bison, cattle, by shading the soil surface, which reduces evaporation. Juncus
horses, and other herbivores (Figure 54.9). Because the birds also prevents the salt marsh soils from becoming oxygen
increase their feeding rates when following the herbivores, they depleted as it transports oxygen to its belowground tissues.
clearly benefit from the association. Much of the time, the In one study, when  Juncus was removed from areas in the
herbivores are not affected by the birds. At times, however, upper middle intertidal zone, those areas supported 50%
the herbivores too may derive some benefit; for example, the fewer plant species (Figure 54.10b).
birds may remove and eat ticks and other ectoparasites from
their skin, or they may warn the herbivores of a predator’s Like positive interactions, competition and exploitation
approach. This example illustrates another key point about (predation, herbivory, and parasitism) also can strongly
affect the structure of ecological communities, as examples
Figure 54.9  Commensalism between cattle egrets throughout the rest of this chapter will show.
and African buffalo.
Concept Check 54.1

1. Explain how competition, predation, and mutualism
differ in their effects on the interacting populations of
two species.

2. According to the principle of competitive exclusion, what
outcome is expected when two species with identical
niches compete for a resource? Why?

3. MAKE CONNECTIONS Figure 24.14 illustrates how a
hybrid zone can change over time. Imagine that two
finch species colonize a new island and are capable of
hybridizing (mating and producing viable offspring). The
island contains two plant species, one with large seeds
and one with small seeds, growing in isolated habitats. If
the two finch species specialize in eating different plant
species, would reproductive barriers be reinforced, weak-
ened, or unchanged in this hybrid zone? Explain.

For suggested answers, see Appendix A.

chapter 54   Community Ecology 1219

Concept  54.2 the four types of trees in community 1, but unless you looked
carefully, you might see only the abundant species A in the
Diversity and trophic structure second forest. Most observers would intuitively describe com-
characterize biological communities munity 1 as the more diverse of the two communities.

Ecological communities can be characterized by certain gen- Ecologists use many tools to compare the diversity of com-
eral attributes, including how diverse they are and the feeding munities across time and space. They often calculate indexes
relationships of their species. In some cases, as you’ll read, a of diversity based on species richness and relative abundance.
few species exert strong control on a community’s structure, One widely used index is Shannon diversity (H):
particularly on the composition, relative abundance, and
diversity of its species. H = - ( pA ln pA + pB ln pB + pC ln pC + c)

Species Diversity where A, B, C . . . are the species in the community, p is the
relative abundance of each species, and ln is the natural
The species diversity of a community—the variety of dif- logarithm; the ln of each value of p can be determined using
ferent kinds of organisms that make up the community—has the “ln” key on a scientific calculator. A higher value of H
two components. One is species richness, the number of indicates a more diverse community. Let’s use this equation
different species in the community. The other is the relative to calculate the Shannon diversity index of the two com-
abundance of the different species, the proportion each munities in Figure 54.11. For community 1, p = 0.25 for
species represents of all individuals in the community. each species, so

Imagine two small forest communities, each with 100 H = -4(0.25 ln 0.25) = 1.39.
individuals distributed among four tree species (A, B, C,
and D) as follows: For community 2,

Community 1: 25A, 25B, 25C, 25D H = -[0.8 ln 0.8 + 2(0.05 ln 0.05) + 0.1 ln 0.1] = 0.71.
Community 2: 80A, 5B, 5C, 10D
The species richness is the same for both communities because These calculations confirm our intuitive description of
they both contain four species of trees, but the relative abun-
dance is very different (Figure 54.11). You would easily notice community 1 as more diverse.

Figure 54.11  Which forest is more diverse? Ecologists Determining the number and relative
would say that community 1 has greater species diversity, a measure
that includes both species richness and relative abundance. abundance of species in a community can Two samples,

A BC D be challenging. Because most species in a one species

Community 1 community are relatively rare, it may be
A: 25% B: 25% C: 25% D: 25%
hard to obtain a sample size large enough
Community 2
A: 80% B: 5% C: 5% D: 10% to be representative. It can also be dif-

1220 Unit eight  Ecology ficult to identify some of the species in

the community. If an unknown organism

cannot be identified on the basis of mor-

phology alone, it is useful to compare all or

part of its genome to a reference database

of DNA sequences from known organisms.

For example, although the two samples of

red algae shown here might appear to be

two different species, comparing their

sequences of a short standardized sec-

tion of DNA—a DNA “barcode”—to

a reference database shows that they

belong to the same species. Researchers

are increasingly using DNA sequencing for

species identification as it becomes cheaper

and as DNA sequences from more organisms are

added to comparative databases.

It can also be difficult to census the highly mobile or less

visible members of communities, such as microorganisms,

deep-sea creatures, and nocturnal species. The small size of

microorganisms makes them particularly difficult to sample,

so ecologists now commonly use molecular tools to help

determine microbial diversity (Figure 54.12).

Shannon diversity (H)Figure 54.12 Figure 54.13  Study plots
at the Cedar Creek Ecosystem
Research Method  Determining Microbial Science Reserve, site of long-term
Diversity Using Molecular Tools experiments on manipulating plant
diversity.
Application  Ecologists are increasingly using molecular tech-
niques to determine microbial diversity and richness in environmental Diversity and Community Stability
samples. One such technique produces a DNA profile for microbial
taxa based on sequence variations in the DNA that encodes the small In addition to measuring species diversity, ecologists manipu-
subunit of ribosomal RNA. Noah Fierer and Rob Jackson, then of Duke late diversity in experimental communities in nature and
University, used this method to compare the diversity of soil bacteria in the laboratory. Many experiments examine the potential
in 98 habitats across North and South America to help identify environ- benefits of diversity, including increased productivity and
mental variables associated with high bacterial diversity. stability of biological communities.
Technique  Researchers first extract and purify DNA from the
microbial community in each sample. They use the polymerase chain Researchers at the Cedar Creek Ecosystem Science
reaction (PCR; see Figure 20.8) to amplify the ribosomal DNA and Reserve, in Minnesota, have been manipulating plant
diversity in experimental communities for more than two
label it with a fluorescent dye. Restriction decades (Figure 54.13). Higher-diversity communities gen-
enzymes then cut the amplified, labeled erally are more productive and are better able to withstand
DNA into fragments of different lengths, and recover from environmental stresses, such as droughts.
which are separated by gel electrophoresis. More diverse communities are also more stable year to year
(A gel is shown here; see also Figures 20.6 in their productivity. In one decade-long experiment, for
and 20.7.) The number and abundance of instance, researchers at Cedar Creek created 168 plots, each
these fragments characterize the DNA pro- containing 1, 2, 4, 8, or 16 perennial grassland species. The
file of the sample. Based on their analysis, most diverse plots consistently produced more biomass
Fierer and Jackson calculated the Shannon (the total mass of all organisms in a habitat) than the single-
diversity (H) of each sample. They then species plots each year.
looked for a correlation between H and
several environmental variables, including Higher-diversity communities are often more resistant to
vegetation type, mean annual temperature invasive species, which are organisms that become estab-
and rainfall, and soil acidity. lished outside their native range. Scientists working in Long
Results  The diversity of bacterial communities was related almost Island Sound, off the coast of Connecticut, created communi-
exclusively to soil pH, with the Shannon diversity being highest in ties of different levels of diversity consisting of sessile marine
neutral soils and lowest in acidic soils. Amazonian rain forests, which invertebrates, including tunicates (see Figure 34.5). They then
have extremely high plant and animal diversity, had the most acidic examined how vulnerable these experimental communities
soils and the lowest bacterial diversity of the samples tested. were to invasion by an exotic tunicate. They found that the
exotic tunicate was four times more likely to survive in lower-
3.6 diversity communities than in higher-diversity ones. The
researchers concluded that relatively diverse communities
3.4 captured more of the resources available in the system, leav-
ing fewer resources for the invader and decreasing its survival.
3.2
Trophic Structure
3.0
In addition to species diversity, the structure and dynamics of a
2.8 community also depend on the feeding relationships between
organisms—the trophic structure of the community. The
2.6 transfer of food energy upward from its source in plants and
other autotrophs (primary producers) through herbivores
2.4 (primary consumers) to carnivores (secondary, tertiary, and
2.2

34 5 6 7 8 9
Soil pH

Data from  N. Fierer and R. B. Jackson, The diversity and biogeography of soil
bacterial communities, Proceedings of the National Academy of Sciences USA
103:626–631 (2006).

chapter 54   Community Ecology 1221

Figure 54.14  Examples of terrestrial and marine food Figure 54.15  An Antarctic marine food web. Arrows follow
chains. The arrows trace the transfer of food through the trophic levels the transfer of food from the producers (phytoplankton) up through
of a community when organisms feed on one another. Decomposers, the trophic levels. For simplicity, this diagram omits decomposers. At
which “feed” on the remains of organisms from all trophic levels, various times over the last two centuries, humans have also played a
are not shown here. role in the Antarctic food web as consumers of fish, krill, and whales.

Quaternary Sperm
consumers whales

Carnivore Tertiary Carnivore Baleen Smaller Elephant
Carnivore consumers Carnivore whales toothed seals
Carnivore Secondary Carnivore whales
consumers Crabeater
seals Leopard
seals

Primary Birds Fishes Squids
consumers

Herbivore Zooplankton

Primary Carnivorous
producers plankton

Plant Phytoplankton Krill Copepods

A terrestrial food chain A marine food chain Phyto-
plankton
VISUAL SKILLS Suppose the abundance of carnivores that eat zooplankton
increased greatly. Use this diagram to infer how that might affect phytoplankton VISUAL SKILLS For each organism in this food web, indicate the number
of other kinds of organism that it eats. Which two organisms are both predator
abundance. and prey for each other?

quaternary consumers) and eventually to decomposers is Figure Walkthrough
referred to as a food chain (Figure 54.14). The position an
organism occupies in a food chain is called its trophic level. zooplankton, are another important link in these food webs,
as they are in turn eaten by seals and toothed whales.
Food Webs
How are food chains linked into food webs? A given spe-
A food chain is not an isolated unit, separate from other feed- cies may weave into the web at more than one trophic level.
ing relationships in a community. Instead, a group of food In the food web shown in Figure 54.15, krill feed on phyto-
chains are linked together to form a food web. Ecologists plankton as well as on other grazing zooplankton, such as
diagram the trophic relationships of a community using copepods. Such “nonexclusive” consumers are also found in
arrows that link species according to who eats whom. In terrestrial communities. For instance, foxes are omnivores
an Antarctic pelagic community, for example, the primary whose diet includes berries and other plant materials, her-
producers are phytoplankton, which serve as food for the bivores such as mice, and other predators, such as weasels.
dominant grazing zooplankton, especially krill and cope- Humans are among the most versatile of omnivores.
pods, both of which are crustaceans (Figure 54.15). These
zooplankton species are in turn eaten by various carnivores,
including other plankton, penguins, seals, fishes, and baleen
whales. Squids, which are carnivores that feed on fish and

1222 Unit eight  Ecology

Figure 54.16  Partial food web for Chesapeake Bay estuary. Figure 54.17  Test of the energetic hypothesis for the
The sea nettle (Chrysaora quinquecirrha) and the juvenile striped bass restriction of food chain length. Researchers manipulated the
(Morone saxatilis) are the main predators of fish larvae (the bay anchovy productivity of experimental tree-hole communities in Queensland,
( Anchoa mitchilli ), along with several other species). Note that sea nettles Australia, by providing leaf litter input at three levels. Reducing
are secondary consumers when they eat zooplankton, but tertiary energy input reduced food chain length, a result consistent with
consumers when they eat fish larvae, which are themselves secondary the energetic hypothesis.
consumers of zooplankton.

Sea Juvenile Number of trophic links 5
nettle striped bass
Fish 4 Medium: 1 10 Low: 1 100
larvae natural rate natural rate
3 Productivity

2

1
0

High (control):
natural rate of

litter fall

Fish eggs Zooplankton longer in habitats characterized by higher photosynthetic
production, since the amount of energy stored in primary
Complicated food webs can be simplified in two ways for producers is greater than in habitats with lower photosyn-
easier study. First, species with similar trophic relationships thetic production.
in a given community can be grouped into broad functional
groups. In Figure 54.15, more than 100 phytoplankton spe- Ecologists tested the energetic hypothesis in experiments
cies are grouped as the primary producers in the food web. that mimicked the “tree-hole” communities found in tropical
A second way to simplify a food web is to isolate a portion of forests. Many trees have small branch scars that rot, form-
the web that interacts very little with the rest of the community. ing holes in the tree trunk. These tree holes hold water and
Figure 54.16 illustrates a partial food web for sea nettles (a type provide a habitat for tiny communities consisting of micro-
of cnidarian) and juvenile striped bass in the Chesapeake Bay organisms and insects that feed on leaf litter, as well as preda-
estuary on the Atlantic coast of the United States. tory insects. Figure 54.17 shows the results of experiments
in which researchers manipulated productivity by varying
Limits on Food Chain Length the amount of leaf litter in an experiment using artificial tree
holes (water-filled pots placed around the trees); previous
Each food chain within a food web is usually only a few links studies had shown that the communities that colonized these
long. In the Antarctic web of Figure 54.15, there are rarely pots were similar to those in natural tree holes. As predicted
more than seven links from the producers to any top-level by the energetic hypothesis, the pots with the most leaf litter,
predator, and most chains in this web have fewer links. In and hence the greatest total food supply at the producer level,
fact, most food webs studied to date have chains consisting supported the longest food chains.
of five or fewer links.
Another factor that may limit food chain length is that
Why do food chains tend to be relatively short? The most carnivores in a food chain tend to be larger at higher trophic
common explanation, known as the energetic hypothesis, levels. The size of a carnivore puts an upper limit on the size
suggests that the length of a food chain is limited by the of food it can take into its mouth. And except in a few cases,
inefficiency of energy transfer along the chain. On average, large carnivores cannot live on very small food items because
only about 10% of the energy stored in the organic matter of they cannot obtain enough food in a given time to meet their
each trophic level is converted to organic matter at the next metabolic needs. Among the exceptions are baleen whales,
trophic level (see Concept 55.3). Thus, a producer level con- huge filter feeders with adaptations that enable them to con-
sisting of 100 kg of plant material can support about 10 kg of sume enormous quantities of krill and other small organisms
herbivore biomass and 1 kg of carnivore biomass. The ener- (see Figure 41.5).
getic hypothesis predicts that food chains should be relatively
Species with a Large Impact

Certain species have an especially large impact on the struc-
ture of entire communities because they are highly abundant
or play a pivotal role in community dynamics. The impact of

chapter 54   Community Ecology 1223

these species occurs through trophic interactions and theirNumber of species avoiding herbivory or the impact of disease. The latter idea
influence on the physical environment.present could explain the high biomass attained in some environ-
ments by invasive species such as kudzu (see Figure 56.8).
Dominant species in a community are the species that Such species may not face the herbivores or parasites that
are the most abundant or that collectively have the highest would otherwise hold their populations in check.
biomass. There can be different explanations for why particu-
lar species become dominant. One hypothesis suggests that The impact of a dominant species can be discovered
dominant species are competitively superior in exploiting when it is removed from the community. For example, the
limited resources such as space, water, or nutrients. Another American chestnut was a dominant tree in deciduous forests
hypothesis is that dominant species are most successful at of eastern North America before 1910, making up more than
40% of mature trees. Then humans accidentally introduced
Figure 54.18 the fungal disease chestnut blight to New York City via nurs-
ery stock imported from Asia. Between 1910 and 1950, this
Inquiry  Is Pisaster ochraceus a keystone species? fungus killed almost all of the chestnut trees in eastern North
America. In this case, removing the dominant species had a
Experiment  In rocky intertidal communities of western North relatively small impact on some species but large effects on
America, the relatively uncommon sea star Pisaster ochraceus preys on others. Oaks, hickories, beeches, and red maples that were
mussels such as Mytilus californianus, a dominant species and strong already present in the forest increased in abundance and
competitor for space. replaced the chestnuts. No mammals or birds seemed to have
been harmed by the loss of the chestnut, but seven species of
Robert Paine, of the University of Washington, removed Pisaster from moths and butterflies that fed on the tree became extinct.
an area in the intertidal zone and examined the effect on species
richness. In contrast to dominant species, keystone species are
Results  In the absence of Pisaster, species richness declined as not usually abundant in a community. They exert strong
mussels monopolized the rock face and eliminated most other inver- control on community structure not by numerical might but
tebrates and algae. In a control area where Pisaster was not removed, by their pivotal ecological roles. Figure 54.18 highlights the
species richness changed very little. importance of a keystone species, a sea star, in maintaining
the diversity of an intertidal community.
20
15 With Pisaster (control) Still other organisms exert their influence on a commu-
10 nity not through trophic interactions but by changing their
physical environment. Species that dramatically alter their
5 Without Pisaster (experimental) environment are called ecosystem engineers or, to avoid
0 implying conscious intent, “foundation species.” A familiar
ecosystem engineer is the beaver (Figure 54.19). The effects
1963 ’64 ’65 ’66 ’67 ’68 ’69 ’70 ’71 ’72 ’73 of ecosystem engineers on other species can be positive or
Year negative, depending on the needs of the other species.

Conclusion  Pisaster acts as a keystone species, exerting an HHMI Video: Some Animals Are More Equal than
influence on the community that is not reflected in its abundance. Others: Keystone Species and Trophic Cascades

Data from  R. T. Paine, Food web complexity and species diversity, American Naturalist Figure 54.19  Beavers as ecosystem engineers. By felling
100:65–75 (1966). trees, building dams, and creating ponds, beavers can transform large
WHAT IF? Suppose that an invasive fungus killed most individuals of areas of forest into flooded wetlands.
Mytilus at these sites. Predict how species richness would be affected if
Pisaster were then removed.

1224 Unit eight  Ecology

Bottom-Up and Top-Down Controls Figure 54.20  Results of biomanipulation in a lake with
top-down control of community organization. Decreasing the
The ways in which adjacent trophic levels affect one another abundance of fish that ate zooplankton results in a decrease in the
can be useful for describing community organization. Let’s biomass of algae, improving water quality.
consider the three possible relationships between plants
(V for vegetation) and herbivores (H): Top-down Control Polluted State Restored State

V S H V d H V   4  H Fish Abundant Rare

The arrows indicate that a change in the biomass of one trophic Zooplankton Rare Abundant

level causes a change in the other trophic level. V S H means Algae Abundant Rare

that an increase in vegetation will increase the numbers or and industrial wastewater until 1976. After pollution controls
biomass of herbivores, but not vice versa. In this situation, her-
bivores are limited by vegetation, but vegetation is not limited reduced these inputs, the water quality of the lake began to

by herbivory. In contrast, V d H means that an increase in improve. By 1986, however, massive blooms of cyanobacteria

herbivore biomass will decrease the abundance of vegetation, started to occur in the lake. These blooms coincided with an
but not vice versa. A double-headed arrow indicates that each
trophic level is sensitive to changes in the biomass of the other. increase in the population of roach, a fish species that eats

Two models of community organization are common: zooplankton, which otherwise keep the cyanobacteria and

the bottom-up model and the top-down model. The V S H algae in check. To reverse these changes, ecologists removed

linkage suggests a bottom-up model, which postulates a nearly a million kilograms of fish from the lake between 1989
unidirectional influence from lower to higher trophic levels.
In this case, the presence or absence of mineral nutrients and 1993, reducing roach abundance by about 80%. At the
(N) controls plant (V) numbers, which control herbivore (H)
numbers, which in turn control predator (P) numbers. The same time, they added a fourth trophic level by stocking the

simplified bottom-up model is thus N S V S H S P. To lake with pike perch, a predatory fish Lake Vesijärvi,
that eats roach. The water became Finland
change the community structure of a bottom-up community, clear, and the last cyanobacte-
you need to alter biomass at the lower trophic levels, allowing
those changes to propagate up through the food web. If you rial bloom was in 1989.
add mineral nutrients to stimulate plant growth, then the
higher trophic levels should also increase in biomass. If you Ecologists continue to
change predator abundance, however, the effect should not
extend down to the lower trophic levels. monitor the lake for evi-

In contrast, the top-down model postulates the oppo- dence of cyanobacterial
site: Predation mainly controls community organization
because predators limit herbivores, herbivores limit plants, blooms and low oxygen avail-
and plants limit nutrient levels through nutrient uptake.
ability, but the lake has remained
The simplified top-down model, N d V d H d P, is also
clear, even though roach removal ended in 1993.
called the trophic cascade model. In a lake community with
four trophic levels, the model predicts that removing the top As these examples show, communities vary in their degree
carnivores will increase the abundance of primary carnivores,
in turn decreasing the number of herbivores, increasing phy- of bottom-up and top-down control. To manage agricultural
toplankton abundance, and decreasing concentrations of
mineral nutrients. The effects thus move down the trophic landscapes, parks, reservoirs, and fisheries, we need to under-
structure as alternating +/- effects.
stand each particular community’s dynamics.
Ecologists have applied the top-down model to improve
water quality in lakes with a high abundance of algae. This Concept Check 54.2
approach, called biomanipulation, attempts to prevent
algal blooms by altering the density of higher-level consumers. 1. What two components contribute to species diversity?
In lakes with three trophic levels, removing fish should Explain how two communities with the same number
improve water quality by increasing zooplankton density, of species can differ in species diversity.
thereby decreasing algal populations (Figure 54.20). In lakes
with four trophic levels, adding top predators should have 2. How is a food chain different from a food web?
the same effect. 3. WHAT IF? Consider a grassland with five trophic levels:

Ecologists in Finland used biomanipulation to help purify grasses, mice, snakes, raccoons, and bobcats. If you
Lake Vesijärvi, a large lake that was polluted with city sewage released additional bobcats into the grassland, how
would grass biomass change if the bottom-up model
applied? If the top-down model applied?
4. MAKE CONNECTIONS Rising atmospheric CO2 levels
lead to ocean acidification (see Figure 3.12) and higher
ocean temperatures, both of which can reduce krill abun-
dance. Predict how a drop in krill abundance might affect
other organisms in the food web shown in Figure 54.15.
Which organisms are particularly at risk? Explain.

For suggested answers, see Appendix A.

chapter 54   Community Ecology 1225

Concept  54.3 Number of taxasome grassland biomes require regular burning to maintain
their structure and species composition. Many streams and
Disturbance influences species ponds are disturbed by seasonal flooding and drying. A high
diversity and composition level of disturbance is generally the result of frequent and
intense disturbance, while low disturbance levels can result
Decades ago, most ecologists favored the traditional view from either a low frequency or low intensity of disturbance.
that biological communities are at equilibrium, a more or less
stable balance, unless seriously disturbed by human activities. The intermediate disturbance hypothesis states that
The “balance of nature” view focused on competition as a key moderate levels of disturbance foster greater species diver-
factor determining community composition and maintain- sity than do high or low levels of disturbance. High levels
ing stability in communities. Stability in this context refers to of disturbance reduce diversity by creating environmental
a community’s tendency to reach and maintain a relatively stresses that exceed the tolerances of many species or by
constant composition of species. disturbing the community so often that slow-growing or
slow-colonizing species are excluded. At the other extreme,
One of the earliest proponents of this view, F. E. Clements, low levels of disturbance can reduce species diversity by
of the Carnegie Institution of Washington, argued in the early allowing competitively dominant species to exclude less com-
1900s that the community of plants at a site had only one petitive ones. Meanwhile, intermediate levels of disturbance
stable equilibrium, a climax community controlled solely by can foster greater species diversity by opening up habitats
climate. According to Clements, biotic interactions caused the for occupation by less competitive species. Such intermedi-
species in the community to function as an integrated unit— ate disturbance levels rarely create conditions so severe that
in effect, as a superorganism. His argument was based on the they exceed the environmental tolerances or recovery rates of
observation that certain species of plants are consistently potential community members.
found together, such as the oaks, maples, birches, and beeches
in deciduous forests of the northeastern United States. The intermediate disturbance hypothesis is supported
by many terrestrial and aquatic studies. In one study, ecolo-
Other ecologists questioned whether most communi- gists in New Zealand compared the richness of invertebrates
ties were at equilibrium or functioned as integrated units. living in the beds of streams exposed to different frequen-
A. G. Tansley, of Oxford University, challenged the concept cies and intensities of flooding (Figure 54.21). When floods
of a climax community, arguing that differences in soils, occurred either very frequently or rarely, invertebrate rich-
topography, and other factors created many potential com- ness was low. Frequent floods made it difficult for some
munities that were stable within a region. H. A. Gleason, of species to become established in the streambed, while rare
the University of Chicago, saw communities not as superor- floods resulted in species being displaced by superior com-
ganisms but as chance assemblages of species found together petitors. Invertebrate richness peaked in streams that had
because they happen to have similar abiotic requirements— an intermediate frequency or intensity of flooding, as pre-
for example, for temperature, rainfall, and soil type. Gleason dicted by the hypothesis.
and other ecologists also realized that disturbance keeps
many communities from reaching a state of equilibrium in Although moderate levels of disturbance appear to
species diversity or composition. A disturbance is an event, maximize species diversity in some cases, small and large
such as a storm, fire, flood, drought, or human activity, that
changes a community by removing organisms from it or Figure 54.21  Testing the intermediate disturbance
altering resource availability. hypothesis. Researchers identified the taxa (species or genera)
of invertebrates at two locations in each of 27 New Zealand streams.
This emphasis on change in communities has led to They assessed the intensity of flooding at each location using an index
the formulation of the nonequilibrium model, which of streambed disturbance. The number of invertebrate taxa peaked
describes most communities as constantly changing after dis- where the intensity of flooding was at intermediate levels.
turbance. Even relatively stable communities can be rapidly
transformed into nonequilibrium communities. Let’s exam- 35
ine some of the ways that disturbances influence community
structure and composition. 30

Characterizing Disturbance 25

The types of disturbances and their frequency and severity 20
vary among communities. Storms disturb almost all com-
munities, even those in the oceans through the action of 15
waves. Fire is a significant disturbance; in fact, chaparral and
10
1226 Unit eight  Ecology 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0
Index of disturbance intensity (log scale)

disturbances also can have important effects on community many burned areas in the park were largely covered with
structure. Small-scale disturbances can create patches of new vegetation, suggesting that the species in this commu-
different habitats across a landscape, which help maintain nity are adapted to rapid recovery after fire (Figure 54.22b).
diversity in a community. Large-scale disturbances are also In fact, large-scale fires have periodically swept through
a natural part of many communities. Much of Yellowstone the lodgepole pine forests of Yellowstone and other north-
National Park, for example, is dominated by lodgepole ern areas for thousands of years. In contrast, more south-
pine, a tree species that requires the rejuvenating influence erly pine forests were historically affected by frequent but
of periodic fires. Lodgepole pine cones remain closed until low-intensity fires. In these forests, a century of human
exposed to intense heat. When a forest fire burns the trees, intervention to suppress small fires has allowed an unnatu-
the cones open and the seeds are released. The new genera- ral buildup of fuels in some places and elevated the risk of
tion of lodgepole pines can then thrive on nutrients released large, severe fires to which the species are not adapted.
from the burned trees and in the sunlight that is no longer
blocked by taller trees. Studies of the Yellowstone forest community and many
others indicate that they are nonequilibrium communi-
In the summer of 1988, extensive areas of Yellowstone ties, changing continually because of natural disturbances
burned during a severe drought (Figure 54.22a). By 1989, and the internal processes of growth and reproduction.
Mounting evidence suggests that nonequilibrium condi-
Figure 54.22  Recovery following a large-scale disturbance. tions are in fact the norm for most communities.
The 1988 Yellowstone National Park fires burned large areas of forests
dominated by lodgepole pines. Ecological Succession

(a) Soon after fire. While all trees in the foreground of this photograph Changes in the composition and structure of terrestrial
were killed by the fire, unburned trees can be seen in other locations. communities are most apparent after a severe disturbance,
such as a volcanic eruption or a glacier, strips away all the
(b) One year after fire. The community has begun to recover. Herbaceous existing vegetation. The disturbed area may be colonized by
plants, different from those in the former forest, cover the ground. a variety of species, which are gradually replaced by other
Interview with Monica Turner: Studying the consequences species, which are in turn replaced by still other species—a
of fire and other disturbances on ecosystems process called ecological succession. When this process
begins in a virtually lifeless area where soil has not yet
formed, such as on a new volcanic island or on the rubble
(moraine) left by a retreating glacier, it is called primary
succession.

During primary succession, the only life-forms initially
present are often prokaryotes and protists. Lichens and
mosses, which grow from windblown spores, are commonly
the first macroscopic photosynthesizers to colonize such
areas. Soil develops gradually as rocks weather and organic
matter accumulates from the decomposed remains of the
early colonizers. Once soil is present, the lichens and mosses
are usually overgrown by grasses, shrubs, and trees that
sprout from seeds blown in from nearby areas or carried in
by animals. Eventually, an area is colonized by plants that
become the community’s dominant form of vegetation.
Producing such a community through primary succession
may take hundreds or thousands of years.

Early-arriving species and later-arriving ones may be linked
by one of three key processes. The early arrivals may facilitate
the appearance of the later species by making the environ-
ment more favorable—for example, by increasing the fertility
of the soil. Alternatively, the early species may inhibit estab-
lishment of the later species, so that successful colonization
by later species occurs in spite of, rather than because of, the
activities of the early species. Finally, the early species may be
completely independent of the later species, which tolerate

chapter 54   Community Ecology 1227

Figure 54.23  Glacial retreat and primary succession at Glacier Bay, Alaska. The different
shades of blue on the map show retreat of the glacier since 1760, based on historical descriptions.

1941

1907

1 Pioneer stage 1860 2 Dryas stage
Glacier

Bay

Alaska

1760

0 5 10 15
Kilometers

4 Spruce stage 3 Alder stage

conditions created early in succession but are neither helped transitions in the vegeta- Alder root with a cluster
nor hindered by early species. tion. Because the bare soil of nodules containing
after glacial retreat is low in nitrogen-fixing bacteria
Ecologists have conducted some of the most extensive
research on primary succession at Glacier Bay in southeast- nitrogen content, almost
ern Alaska, where glaciers have retreated more than 100 km
since 1760 (Figure 54.23). By studying the communities at all the pioneer plant species
different distances from the mouth of the bay, ecologists
can examine different stages in succession. 1 The exposed begin succession with poor
glacial moraine is colonized first by pioneering species that
include liverworts, mosses, fireweed, scattered Dryas (a mat- growth and yellow leaves
forming shrub), and willows. 2 After about three decades,
Dryas dominates the plant community. 3 A few decades due to limited nitrogen
later, the area is invaded by alder, which forms dense thickets
up to 9 m tall. 4 In the next two centuries, these alder stands supply. The exceptions are
are overgrown first by Sitka spruce and later by western
hemlock and mountain hemlock. In areas of poor drainage, Dryas and alder, whose roots
the forest floor of this spruce-hemlock forest is invaded by
sphagnum moss, which holds water and acidifies the soil, host symbiotic bacteria that
eventually killing the trees. Thus, by about 300 years after
glacial retreat, the vegetation consists of sphagnum bogs on fix atmospheric nitrogen
the poorly drained flat areas and spruce-hemlock forest on
the well-drained slopes. (see photograph on this

Succession on glacial moraines is related to changes in page and Figure 37.12). Soil
soil nutrients and other environmental factors caused by
nitrogen content increases quickly during the alder stage
1228 Unit eight  Ecology
of succession and keeps increasing during the spruce stage

(Figure 54.24). By altering soil properties, pioneer plant

species can facilitate colonization by new plant species

during succession.

In contrast to primary succession, secondary succession

occurs when an existing community has been cleared by a

disturbance that leaves the soil intact, as in Yellowstone fol-

lowing the 1988 fires (see Figure 54.22). Following the dis-

turbance, the area may return to something like its original

Figure 54.24  Changes in soil nitrogen Figure 54.25  Disturbance of the ocean floor by trawling.
content during succession at Glacier Bay. These photos show the seafloor off northwestern Australia before and
after deep-sea trawlers have passed through.
60
◀ Before
50 trawling

Soil nitrogen (g/m2) 40 After ▶
trawling
30
Concept Check 54.3
20
1. Why do high and low levels of disturbance usually reduce
10 species diversity? Why does an intermediate level of
disturbance promote species diversity?
0 Dryas Alder Spruce
Pioneer 2 3 4 2. During succession, how might the early species facilitate
1 the arrival of other species?

Successional stage 3. WHAT IF? Most prairies experience regular fires, typi-
cally every few years. If these disturbances were relatively
MAKE CONNECTIONS Figure 37.12 illustrates two types of atmospheric modest, how would the species diversity of a prairie
nitrogen fixation by prokaryotes. At the earliest stages of primary succession, likely be affected if no burning occurred for 100 years?
Explain your answer.
before any plants are present at a site, which type of nitrogen fixation would For suggested answers, see Appendix A.

occur, and why? Concept  54.4

state. For instance, in a forested area that has been cleared for Biogeographic factors affect
farming and later abandoned, the earliest plants to recolonize community diversity
are often herbaceous species that grow from windblown or
animal-borne seeds. If the area has not been burned or heavily So far, we have examined relatively small-scale or local fac-
grazed, woody shrubs may in time replace most of the herba- tors that influence the diversity of communities, including
ceous species, and forest trees may eventually replace most of the effects of species interactions, dominant species, and
the shrubs. many types of disturbances. Large-scale biogeographic fac-
tors also contribute to the tremendous range of diversity
Human Disturbance observed in biological communities. The contributions of
two biogeographic factors in particular—the latitude of a
Ecological succession is a response to disturbance of the envi- community and the area it occupies—have been investigated
ronment, and the strongest disturbances today are human for more than a century.
activities. Agricultural development has disrupted what
were once the vast grasslands of the North American prairie.
Tropical rain forests are quickly disappearing as a result
of clear-cutting for lumber, cattle grazing, and farmland.
Centuries of overgrazing and agricultural disturbance have
contributed to famine in parts of Africa by turning seasonal
grasslands into vast barren areas.

Human actions have disturbed marine ecosystems as well as
terrestrial ones. The effects of ocean trawling, in which boats
drag weighted nets across the seafloor, are similar to those
of clear-cutting a forest or plowing a field. The trawls scrape
and scour corals and other life on the seafloor (Figure 54.25).
In a typical year, ships trawl an area about the size of South
America, 150 times larger than the area of forests that are
clear-cut annually.

Because disturbance by human activities is often severe, it
reduces species diversity in many communities. In Chapter 56,
we’ll take a closer look at how human-caused disturbance is
affecting the diversity of life.

chapter 54   Community Ecology 1229

Latitudinal Gradients Figure 54.26  Energy, water, and species richness.Vertebrate species richness
Vertebrate species richness in North America increases with potential(log scale) 
In the 1850s, both Charles Darwin and Alfred Wallace evapotranspiration, expressed as rainfall equivalents (mm/yr).
pointed out that plant and animal life was generally more
abundant and diverse in the tropics than in other parts 200
of the globe. Since that time, many researchers have con-
firmed this observation. One study found that a 6.6-hectare 100
(1 ha = 10,000 m2) plot in tropical Malaysia contained
711 tree species, while a 2-ha plot of deciduous forest in 50
Michigan typically contained just 10 to 15 tree species.
Many groups of animals show similar latitudinal gradients. 10 500 1,000 1,500 2,000
For instance, there are more than 200 species of ants in 0
Brazil but only 7 in Alaska.
Potential evapotranspiration (mm/yr)
Two key factors that can affect latitudinal gradients
of species richness are evolutionary history and climate. The first, and still widely used, mathematical description
Over the course of evolution, a series of speciation events of the species-area relationship was proposed a century ago:
may lead to increased species richness in a community (see
Concept 24.2). Tropical communities are generally older S = cAz
than temperate or polar communities, which have repeat-
edly “started over” after major disturbances such as glacia- where S is the number of species found in a habitat, c is a
tions. As a result, species diversity may be highest in the constant, and A is the area of the habitat. The exponent z tells
tropics simply because there has been more time for specia- you how many more species should be found in a habitat as
tion to occur in tropical communities than in temperate or its area increases. In a log-log plot of S versus A, z is the slope
polar communities. of the line through the data points. A value of z = 1 would
indicate a linear relationship between species number and
Climate is another key factor thought to affect latitudi- area, meaning that ten times as many species would be found
nal gradients of richness and diversity. In terrestrial com- in a habitat that has ten times the area.
munities, the two main climatic factors correlated with
diversity are sunlight and precipitation, both of which In the 1960s, Robert MacArthur and E. O. Wilson tested
occur at high levels in the tropics. These factors can be the predictions of the species-area relationship by examining
considered together by measuring a community’s rate of the number of animals and plants on different island chains.
evapotranspiration, the evaporation of water from soil As one example, in the Sunda Islands of Malaysia, they found
and plants. Evapotranspiration, a function of solar radia- that the number of bird species increased with island size,
tion, temperature, and water availability, is much higher with a value of z = 0.4 (Figure 54.27). These and other stud-
in hot areas with abundant rainfall than in areas with low ies have shown that z is usually between 0.2 and 0.4.
temperatures or low precipitation. Potential evapotranspiration,
a measure of potential water loss that assumes that water is Although the slopes of different species-area curves vary,
readily available, is determined by the amount of solar radi- the basic concept of diversity increasing with increasing area
ation and temperature and is highest in regions where both applies in many situations, from surveys of ant diversity in
are plentiful. The species richness of plants and animals New Guinea to studies of plant species richness on islands of
correlates with both measures, as shown for vertebrates and different sizes.
potential evapotranspiration in Figure 54.26.
Island Equilibrium Model
Area Effects
Because of their isolation and limited size, islands provide
In 1807, naturalist and explorer Alexander von Humboldt excellent opportunities for studying the biogeographic
described one of the first patterns of species richness to be factors that affect the species diversity of communities.
recognized, the species-area curve: All other factors being By “islands,” we mean not only oceanic islands, but also habi-
equal, the larger the geographic area of a community, the tat islands on land, such as lakes, mountain peaks separated by
more species it has. One explanation for this relationship lowlands, or habitat fragments—any patch surrounded by an
is that larger areas offer a greater diversity of habitats and
microhabitats. In conservation biology, developing species-
area curves for key taxa in a community helps ecologists
predict how the loss of a given area of habitat will affect the
community’s diversity.

1230 Unit eight  Ecology

Figure 54.27  Species richness and island area. The number Figure 54.28  MacArthur and Wilson’s island equilibrium
of bird species on the Sunda Islands of Malaysia increases with model. The equilibrium number of species on an island represents a
island size. The slope of the best-fit line through the data points balance between the immigration of new species (red curve) and the
(the parameter z) is about 0.4. extinction of species already there (black curve). The black triangle
shows the predicted equilibrium number of species.
1,000
Number of bird species (log scale)
Rate of immigration or extinction100Immigration

Extinction10

1 10 102 103 104 105
1 Area of island (mi2) (log scale)

environment not suitable for the “island” species. While study- Equilibrium number
ing the species-area relationship, MacArthur and Wilson also Number of species on island
developed a method for predicting the species diversity of
islands (Figure 54.28). In their approach, the number of spe- WHAT IF? Suppose rising sea levels substantially decreased the size of the
cies on an island represents a balance between the immigra- island. How would that affect (a) the population sizes of species already on the
tion of new species to the island and the extinction of species island, (b) the extinction curve shown above, and (c) the predicted equilibrium
already there. number of species?

In Figure 54.28, note that the immigration rate decreases rate of species extinction. The number of species at this equi-
as the number of species on the island gets larger, while the librium point is correlated with the island’s size and distance
extinction rate increases. To see why this is so, consider a from the mainland. Like any ecological equilibrium, this spe-
newly formed oceanic island that receives colonizing spe- cies equilibrium is dynamic: Although the number of species
cies from a distant mainland. At any given time, an island’s ultimately should stabilize at a constant level, immigration
immigration and extinction rates are affected by the number and extinction continue and hence the exact species compo-
of species already present. As the number of species already sition on the island may change over time.
on the island increases, the immigration rate of new species
decreases, because any individual reaching the island is less In 1967, Dan Simberloff, then a graduate student with
likely to represent a species that is not already present. At the E. O. Wilson at Harvard University, tested the island equilib-
same time, as more species inhabit an island, extinction rates rium model in an experiment on six small mangrove islands
on the island increase because of the greater likelihood of in the Florida Keys (Figure 54.29). Simberloff and Wilson
competitive exclusion. first painstakingly identified and counted all of the arthro-
pod species on each island. As predicted by the model, they
Two physical features of the island further affect immigra- found more species on islands that were larger and closer to
tion and extinction rates: its size and its distance from the the mainland. They then fumigated four of the islands with
mainland. Small islands generally have lower immigration methyl bromide to kill all of the arthropods. After about a year,
rates because potential colonizers are less likely to reach a small the arthropod species richness on these islands increased to
island than a large one. Small islands also have higher extinc-
tion rates because they generally contain fewer resources, Figure 54.29 
have less diverse habitats, and have smaller population sizes. Mangrove
Distance from the mainland is also important; a closer island island. The islands
generally has a higher immigration rate and a lower extinction that Simberloff
rate than one farther away. Arriving colonists help sustain the and Wilson studied
presence of a species on a near island and prevent its extinction. were small, each
consisting of one
MacArthur and Wilson’s model is called the island or a few mangrove
equilibrium model because an equilibrium will eventually be trees.
reached where the rate of species immigration equals the

chapter 54   Community Ecology 1231

Figure 54.30  Testing the island equilibrium model. TheNumber of species present disease-causing microorganisms, viruses, viroids, or prions.
graph shows results for one of the islands studied. The number of (Viroids and prions are infectious RNA molecules and pro-
arthropod species increased over time; by 240 days, it had reached levels teins, respectively; see Concept 19.3.) As scientists have
similar to those found on the island prior to the start of the experiment. recently come to appreciate, pathogens have universal effects
in structuring ecological communities.
40 Number of species on
island before fumigation Pathogens produce especially clear effects when they are
introduced into new habitats, as in the case of the fungus that
30 causes chestnut blight (see Concept 54.2). A pathogen can be
particularly virulent in a new habitat because new host popula-
20 tions have not had a chance to become resistant to the patho-
gen through natural selection. The invasive chestnut blight
10 fungus had far stronger effects on the American chestnut, for
instance, than it had on Asian chestnut species in the fungus’s
40 80 120 160 200 240 280 320 360 native habitat. Humans are similarly vulnerable to the effects of
Days since fumigation emerging diseases spread by our increasingly global economy.

near their pre-fumigation values (Figure 54.30). The island Pathogens and Community Structure
closest to the mainland recovered first, while the island far-
thest from the mainland was the slowest to recover. The num- The ecological importance of disease can be highlighted by how
ber of arthropod species on the remaining two islands—which pathogens have affected coral reef communities. White-band
were not fumigated and hence served as controls—remained disease, caused by an unknown pathogen, has resulted in dra-
approximately constant during the study. matic changes in the structure and composition of Caribbean
reefs. The disease kills corals by causing their tissue to slough
Over long periods, disturbances such as storms, adaptive off in a band from the base to the tip of the branches. Because
evolutionary changes, and speciation generally alter the of the disease, staghorn coral (Acropora cervicornis) has virtually
species composition and community structure on islands. disappeared from the Caribbean since the 1980s. Populations
Nonetheless, the island equilibrium model is widely applied of elkhorn coral (Acropora palmata) have also been decimated.
in ecology. Conservation biologists in particular use it when Such corals provide key habitat for lobsters as well as snappers
designing habitat reserves or establishing a starting point for and other fish species. When the corals die, they are quickly
predicting the effects of habitat loss on species diversity. overgrown by algae. Surgeonfish and other herbivores that feed
on algae come to dominate the fish community. Eventually,
Animation: Exploring Island Biogeography the corals topple because of damage from storms and other dis-
turbances. The complex, three-dimensional structure of the reef
Concept Check 54.4 disappears, and diversity plummets.

1. Describe two hypotheses that explain why species diver- Pathogens also influence community structure in terres-
sity is greater in tropical regions than in temperate and trial ecosystems. Consider sudden oak death (SOD), a recently
polar regions. discovered disease that is caused by the protist Phytophthora
ramorum (see Concept 28.6). SOD was first described in
2. Describe how an island’s size and distance from the main- California in 1995, when hikers noticed trees dying around
land affect the island’s species richness. San Francisco Bay. By 2014, it had spread more than 1,000 km,
from the central California coast to southern Oregon, and it
3. WHAT IF? Based on MacArthur and Wilson’s island had killed more than a million oaks and other trees. The loss
equilibrium model, how would you expect the richness of of the oaks has led to the decreased abundance of at least five
birds on islands to compare with the richness of snakes bird species, including the acorn woodpecker and the oak tit-
and lizards? Explain. mouse, that rely on oaks for food and habitat. Although there
For suggested answers, see Appendix A. is currently no cure for SOD, scientists recently sequenced
the genome of P. ramorum in hopes of finding a way to fight
Concept  54.5 the pathogen.

Pathogens alter community structure Human activities are transporting pathogens around
locally and globally the world at unprecedented rates. Genetic analyses using
DNA sequencing suggest that P. ramorum likely came to
Now that we have examined several important factors that North America from Europe through the horticulture trade.
structure biological communities, we’ll finish the chapter by Similarly, the pathogens that cause human diseases are
examining community interactions involving pathogens, spread by our global economy. H1N1, the virus that causes

1232 Unit eight  Ecology

“swine flu” in humans, was first detected in Veracruz, hosts for a pathogen provides information that may be used
Mexico, in early 2009. It quickly spread around the world to control the hosts most responsible for spreading diseases.
when infected individuals flew on airplanes to other coun-
tries. By 2011, this flu outbreak had a confirmed death toll of Ecologists also use their knowledge of community interac-
more than 18,000 people. The actual number may have been tions to track the spread of zoonotic diseases. One example,
significantly higher since many people with flu-like symp- avian flu, is caused by highly contagious viruses transmitted
toms who died were not tested for H1N1. through the saliva and feces of birds (see Concept 19.3). Most
of these viruses affect wild birds mildly, but they often cause
Community Ecology and Zoonotic stronger symptoms in domesticated birds, the most common
Diseases source of human infections. Since 2003, one particular viral
strain, called H5N1, has killed hundreds of millions of poul-
Three-quarters of emerging human diseases and many of the try and more than 300 people. In 2015, for example, H5N1
most devastating established diseases are caused by zoonotic was one of three strains of avian flu that spread across poul-
pathogens—those that are transferred to humans from try farms in the United States, killing more than 40 million
other animals, either through direct contact with an infected birds. While this outbreak was devastating to the poultry
animal or by means of an intermediate species, called a industry, it did not result in any human cases.
vector. The vectors that spread zoonotic diseases are often
parasites, including ticks, lice, and mosquitoes. Control programs that quarantine domestic birds or moni-
tor their transport may be ineffective if avian flu spreads
Identifying the community of hosts and vectors for a naturally through the movements of wild birds. From 2003
pathogen can help prevent illnesses such as Lyme disease, to 2006, the H5N1 strain spread rapidly from southeast Asia
which is spread by ticks. For years, scientists thought that into Europe and Africa. The most likely place for infected
the primary host for the Lyme pathogen was the white- wild birds to enter the Americas is Alaska, the entry point for
footed mouse because mice are heavily parasitized by young ducks, geese, and shorebirds that migrate across the Bering
ticks. When researchers vaccinated mice against Lyme dis- Sea from Asia every year. Ecologists are studying the spread
ease and released them into the wild, however, the number of the virus by trapping and testing migrating and resident
of infected ticks hardly changed. Further investigation in birds in Alaska (Figure 54.32).
New York revealed that two inconspicuous shrew species
were the source for more than half the infected ticks col- While our emphasis here has been on community ecology,
lected in the field (Figure 54.31). Identifying the dominant pathogens are also greatly influenced by changes in the phys-
ical environment. To control pathogens and the diseases they
Figure 54.31  Unexpected hosts of the Lyme disease cause, scientists need an ecosystem perspective—an intimate
pathogen. A combination of ecological data and genetic analyses knowledge of how the pathogens interact with other species
enabled scientists to show that more than half of ticks carrying the and with all aspects of their environment. Ecosystems are the
Lyme pathogen became infected by feeding on the northern short- subject of Chapter 55.
tailed shrew (Blarina brevicauda) or the masked shrew (Sorex cinereus).
Figure 54.32 
◀ Collecting ticks from a Tracking avian
white-footed mouse flu. A graduate
student bands a
Percent of infected ticks 35 young gyrfalcon as
that fed on host 30 part of a project to
25 monitor the spread
20 Chipmunk Short-tailed Masked Other of avian flu.
15 shrew shrew
10 Concept Check 54.5
Host species
5 1. What are pathogens?
0 2. WHAT IF? Rabies, a viral disease in mammals, is not cur-

Mouse rently found in the British Isles. If you were in charge of
disease control there, what practical approaches might
MAKE CONNECTIONS Concept 23.1 describes genetic variation between you employ to keep the rabies virus from reaching these
populations. How might genetic variation between shrew populations in islands?

different locations affect the number of infected ticks? For suggested answers, see Appendix A.

chapter 54   Community Ecology 1233

54 Chapter Review Go to MasteringBiology™ for Videos, Animations, Vocab Self-Quiz,
Practice Tests, and more in the Study Area.

Summary of Key Concepts The bottom-up model proposes a unidirectional influence
from lower to higher trophic levels, in which nutrients and other
Concept  54.1 abiotic factors primarily determine community structure. The
top-down model proposes that control of each trophic level
Community interactions are classified comes from the trophic level above, with the result that predators
by whether they help, harm, or have control herbivores, which in turn control primary producers.
no effect on the species involved
(pp. 1213–1219) VOCAB ? Based on indexes such as Shannon diversity, is a community of higher
SELF-QUIZ species richness always more diverse than a community of lower species
goo.gl/6u55ks
richness? Explain.

Interspecific interactions affect the survival Concept  54.3
and reproduction of the species that engage in them. As shown
in the table, these interactions can be grouped into three broad Disturbance influences species diversity
categories: competition, exploitation, and positive interactions. and composition (pp. 1226–1229)

Interaction Description Increasing evidence suggests that disturbance and lack of
Competition (-/-) equilibrium, rather than stability and equilibrium, are the
Two or more species compete for a resource norm for most communities. According to the intermediate
Exploitation (+/-) that is in short supply. disturbance hypothesis, moderate levels of disturbance can
foster higher species diversity than can low or high levels of
Predation One species benefits by feeding upon the disturbance.
other species, which is harmed. Exploitation
Herbivory includes the following: Ecological succession is the sequence of community and
Parasitism ecosystem changes after a disturbance. Primary succession
One species, the predator, kills and eats the occurs where no soil exists when succession begins; secondary
Positive interactions other, the prey. succession begins in an area where soil remains after a
(+/+ or 0/+) disturbance.
An herbivore eats part of a plant or alga.
Mutualism (+/+) ? Is the disturbance pictured in Figure 54.25 more likely to initiate
Commensalism The parasite derives its nourishment from a primary or secondary succession? Explain.
(+/0) second organism, its host, which is harmed.
Concept  54.4
One species benefits, while the other
species benefits or is not harmed. Positive Biogeographic factors affect community
interactions include the following: diversity (pp. 1229–1232)

Both species benefit from the interaction. Species richness generally declines along a latitudinal gradient
from the tropics to the poles. Climate influences the diversity
One species benefits, while the other is not gradient through energy (heat and light) and water. The greater
affected. age of tropical environments also may contribute to their greater
species richness.
Competitive exclusion states that two species competing for
the same resource cannot coexist permanently in the same place. Species richness is directly related to a community’s geographic
Resource partitioning is the differentiation of ecological size, a principle formalized in the species-area curve.
niches that enables species to coexist in a community.
Species richness on islands depends on island size and distance
? For each interaction listed in the table, give an example of a pair of from the mainland. The island equilibrium model maintains that
species that exhibit the interaction. species richness on an ecological island reaches an equilibrium
where new immigrations are balanced by extinctions.
Concept  54.2
? How have periods of glaciation influenced latitudinal patterns
Diversity and trophic structure characterize of diversity?
biological communities (pp. 1220–1225)
Concept  54.5
Species diversity is affected by both the number of species
in a community—its species richness—and their relative Pathogens alter community structure locally
abundance. and globally (pp. 1232–1233)

More diverse communities typically produce more biomass and Recent work has highlighted the role that pathogens play
show less year-to-year variation in growth than less diverse com- in structuring terrestrial and marine communities.
munities and are more resistant to invasion by exotic species.
Zoonotic pathogens are transferred from other animals to
Trophic structure is a key factor in community dynamics. humans and cause the largest class of emerging human diseases.
Food chains link the trophic levels from producers to top Community ecology provides the framework for identifying key
carnivores. Branching food chains and complex trophic species interactions associated with such pathogens and for help-
interactions form food webs. ing us track and control their spread.

Dominant species are the most abundant species in a com- ? Suppose a pathogen attacks a keystone species. Explain how this could
munity. Keystone species are usually less abundant species alter the structure of a community.
that exert a disproportionate influence on community structure.
Ecosystem engineers influence community structure through
their effects on the physical environment.

1234 Unit eight  Ecology

Test Your Understanding Level 3: Synthesis/Evaluation

Level 1: Knowledge/Comprehension 10. DRAW IT  In the Chesapeake Bay estuary, the blue crab is an
omnivore that eats eelgrass and other primary producers as well
1. The feeding relationships among the species in a PRACTICE as clams. It is also a cannibal. In turn, the crabs are eaten by
community determine the community’s TEST humans and by the endangered Kemp’s Ridley sea turtle. Based
(A) secondary succession. on this information, draw a food web that includes the blue
(B) ecological niche. goo.gl/CUYGKD crab. Assuming that the top-down model holds for this system,
(C) species richness. describe what would happen to the abundance of eelgrass if
(D) trophic structure. humans stopped eating blue crabs.

2. The principle of competitive exclusion states that 11. EVOLUTION CONNECTION  Explain why adaptations of
(A) two species cannot coexist in the same habitat. particular organisms to interspecific competition may not
(B) competition between two species always causes extinction necessarily represent instances of character displacement. What
or emigration of one species. would a researcher have to demonstrate about two competing
(C) two species that have exactly the same niche cannot coexist species to make a convincing case for character displacement?
in a community.
(D) two species will stop reproducing until one species leaves 12. SCIENTIFIC INQUIRY  An ecologist studying desert plants
the habitat. performed the following experiment. She staked out two
identical plots, containing sagebrush plants and small annual
3. Based on the intermediate disturbance hypothesis, wildflowers. She found the same five wildflower species in
a community’s species diversity is increased by roughly equal numbers on both plots. She then enclosed one
(A) frequent massive disturbance. plot with a fence to keep out kangaroo rats, the most common
(B) stable conditions with no disturbance. grain-eaters of the area. After two years, four of the wildflower
(C) moderate levels of disturbance. species were no longer present in the fenced plot, but one
(D) human intervention to eliminate disturbance. species had become much more abundant. The control plot
had not changed in species diversity. Using the principles of
4. According to the island equilibrium model, species richness community ecology, propose a hypothesis to explain her results.
would be greatest on an island that is What additional evidence would support your hypothesis?
(A) large and remote.
(B) small and remote. 13. WRITE ABOUT A THEME: INTERACTIONS  In Batesian
(C) large and close to a mainland. mimicry, a palatable species gains protection by mimicking
(D) small and close to a mainland. an unpalatable one. Imagine that individuals of a palatable,
brightly colored fly species are blown to three remote islands.
Level 2: Application/Analysis The first island has no predators of that species; the second has
predators but no similarly colored, unpalatable species; and the
5. Predators that are keystone species can maintain species third has both predators and a similarly colored, unpalatable
diversity in a community if they species. In a short essay (100–150 words), predict what might
(A) competitively exclude other predators. happen to the coloration of the palatable species on each island
(B) prey on the community’s dominant species. through time if coloration is a genetically controlled trait.
(C) reduce the number of disruptions in the community. Explain your predictions.
(D) prey only on the least abundant species in the community.
14. SYNTHESIZE YOUR KNOWLEDGE 
6. Food chains are sometimes short because
(A) only a single species of herbivore feeds on each plant Describe two types of ecological interactions that appear to be
species. occurring between the three species shown in this photo. What
(B) local extinction of a species causes extinction of the other morphological adaptation can be seen in the species that is at
species in its food chain. the highest trophic level in this scene?
(C) most of the energy in a trophic level is lost as energy passes For selected answers, see Appendix A.
to the next higher level.
(D) most producers are inedible. For additional practice questions, check out the Dynamic Study
Modules in MasteringBiology. You can use them to study on
7. Which of the following could qualify as a top-down control on your smartphone, tablet, or computer anytime, anywhere!
a grassland community?
(A) limitation of plant biomass by rainfall amount
(B) influence of temperature on competition among plants
(C) influence of soil nutrients on the abundance of grasses
versus wildflowers
(D) effect of grazing intensity by bison on plant species
diversity

8. The most plausible hypothesis to explain why species richness
is higher in tropical than in temperate regions is that
(A) tropical communities are younger.
(B) tropical regions generally have more available water and
higher levels of solar radiation.
(C) higher temperatures cause more rapid speciation.
(D) diversity increases as evapotranspiration decreases.

9. Community 1 contains 100 individuals distributed among
four species: 5A, 5B, 85C, and 5D. Community 2 contains
100 individuals distributed among three species: 30A, 40B,
and 30C. Calculate the Shannon diversity (H) for each
community. Which community is more diverse?

chapter 54   Community Ecology 1235

Superset 55

Ecosystems and
Restoration Ecology

Figure 55.1  How can foxes transform a grassland into tundra?

Key Concepts Transformed to Tundra

55.1 Physical laws govern energy flow The arctic fox (Vulpes lagopus) is a predator native to arctic regions of North America,
Europe, and Asia (Figure 55.1). Valued for its fur, it was introduced onto hundreds
and chemical cycling in ecosystems of subarctic islands between Alaska and Russia around 1900 in an effort to establish
populations that could be easily harvested. The fox’s introduction had a surprising
55.2 Energy and other limiting factors effect: It transformed many habitats on the islands from grassland to tundra.

control primary production in How did the presence of foxes transform the islands’ vegetation from one biome
ecosystems to another? The foxes fed voraciously on the islands’ seabirds, decreasing their den-
sity almost 100-fold compared to that on fox-free islands. Fewer seabirds meant less
55.3 Energy transfer between trophic bird guano (waste), a primary source of essential nutrients for plants on the islands.
Researchers suspected that the scarcity of nutrients reduced the growth of nutrient-
levels is typically only 10% hungry grasses, favoring instead the slower-growing forbs (nonwoody plants other
efficient than grasses) and shrubs typical of tundra. To test this explanation, the scientists
added fertilizer to plots of tundra on one of the fox-infested islands. Three years later,
55.4 Biological and geochemical the fertilized plots had reverted back to grassland.

processes cycle nutrients and Each of these “fox islands” and the community of organisms on it is an exam-
water in ecosystems ple of an ecosystem, the sum of all the organisms living in a given area and the
abiotic factors with which they interact. An ecosystem can encompass a large area,
55.5 Restoration ecologists return such as a lake, forest, or island, or a microcosm, such as the space under a fallen log
or a small desert spring (Figure 55.2). As with populations and communities, the
degraded ecosystems to a more boundaries of ecosystems are not always discrete. Many ecologists view the entire
natural state

Arctic terns, major guano generators

When you see this blue icon, log in to MasteringBiology Get Ready for This Chapter
and go to the Study Area for digital resources.

these transformations within organelles and cells and mea-
sure the amounts of energy and chemical compounds that
cross the cells’ boundaries. Ecosystem ecologists do the same
thing, except in their case the “cell” is an entire ecosystem.
By determining trophic levels of feeding relationships (see
Concept 54.2) and studying how organisms interact with
their physical environment, ecologists can follow the
transformations of energy in an ecosystem and map the
movements of chemical elements.

Figure 55.2  A desert spring ecosystem. Conservation of Energy

biosphere as a global ecosystem, a composite of all the local To study energy flow and chemical cycling, ecosystem ecolo-
ecosystems on Earth. gists use approaches based on laws of physics and chemistry.
The first law of thermodynamics states that energy cannot
An ecosystem, regardless of its size, has two key emergent be created or destroyed but only transferred or transformed
properties: energy flow and chemical cycling. Energy enters (see Concept 8.1). Plants and other photosynthetic organ-
most ecosystems as sunlight. This light energy is converted isms convert solar energy to chemical energy, but the total
to chemical energy by autotrophs, passed to heterotrophs in amount of energy does not change: The amount of energy
the organic compounds of food, and dissipated as heat. As stored in organic molecules must equal the total solar energy
for chemical cycling, elements such as carbon and nitrogen intercepted by the plant minus the amounts reflected and
are passed between the biotic and abiotic components of the dissipated as heat. Ecosystem ecologists often measure energy
ecosystem. Photosynthetic and chemosynthetic organisms transfers within and across ecosystems, in part to understand
take up these elements in inorganic form from the air, soil, how many organisms a habitat can support and the amount
and water and incorporate them into their biomass, some of of food humans can harvest from a site.
which is consumed by animals. The elements are returned
in inorganic form to the environment by the metabolism The second law of thermodynamics states that every
of organisms and by decomposers that break down organic exchange of energy increases the entropy of the universe.
wastes and dead organisms. One implication of this law is that energy conversions are
inefficient. Some energy is always lost as heat. As a result,
Both energy and chemicals are transformed in ecosystems each unit of energy that enters an ecosystem eventually exits
through photosynthesis and feeding relationships. But unlike as heat. Thus, energy flows through ecosystems—it does not
chemicals, energy cannot be recycled. An ecosystem must be cycle within them for long periods of time. Because energy
powered by a continuous influx of energy from an external flowing through ecosystems is ultimately lost as heat, most
source—in most cases, the sun. As we’ll see, energy flows ecosystems would vanish if the sun were not continuously
through ecosystems, whereas chemicals cycle within them. providing energy to Earth.

Ecosystem processes yield resources critical to human Conservation of Mass
survival and welfare, ranging from the food we eat to the
oxygen we breathe. In this chapter, we’ll explore the dynam- Matter, like energy, cannot be created or destroyed. This law of
ics of energy flow and chemical cycling, emphasizing the conservation of mass is as important for ecosystems as are
results of ecosystem experiments. We’ll also consider how the laws of thermodynamics. Because mass is conserved, we can
human activities have affected energy flow and chemical determine how much of a chemical element cycles within an
cycling. Finally, we’ll examine the growing science of restora- ecosystem or is gained or lost by that ecosystem over time.
tion ecology, which focuses on returning degraded ecosys-
tems to a more natural state. Unlike energy, chemical elements are continually recycled
within ecosystems. For example, a carbon atom in CO2 might
Concept  55.1 be released from the soil by a decomposer, taken up by a blade
of grass through photosynthesis, consumed by a grazing ani-
Physical laws govern energy flow mal, and returned to the soil in the animal’s waste.
and chemical cycling in ecosystems
In addition to cycling within ecosystems, elements can
Cells transform energy and matter, subject to the laws of also be gained or lost by an ecosystem. For example, a forest
thermodynamics (see Concept 8.1). Cell biologists study gains mineral nutrients—the essential elements that plants
obtain from soil—that enter as dust or as solutes dissolved
in rainwater or leached from rocks in the ground. Nitrogen
is also supplied through the biological process of nitrogen
fixation (see Figure 37.12). In terms of losses, some elements

chapter 55  Ecosystems and Restoration Ecology 1237