Campbell Biology

certainty of paternity exists because it has been reinforced Figure 51.17  Appearance
over generations by natural selection. The intriguing relation-
ship between certainty of paternity and male parental care of zebra finches in nature.
remains an area of active research. The male zebra finch (left) is more
highly patterned and colorful than
the female zebra finch.

Sexual Selection and Mate Choice

Sexual dimorphism results from sexual selection, a form of

natural selection in which differences in reproductive success

among individuals are a consequence of differences in mating

success (see Concept 23.4). Sexual selection can take the form

of intersexual selection, in which members of one sex choose

mates on the basis of characteristics of the other sex, such as Mate choice can also be influenced by imprinting, as
courtship songs, or intrasexual selection, which involves com- revealed by experiments carried out with zebra finches
petition between members of one sex for mates. (Taeniopygia guttata). Both male and female zebra finches nor-

Mate Choice by Females  Mate preferences of females mally lack any feather crest on their head (Figure 51.17). To

may play a central role in the evolution of male behavior explore whether parental appearance affects mate preference

and anatomy through intersexual selection. Consider, for in offspring independent of any genetic influence, research-

example, the courtship behavior of stalk-eyed flies. The eyes ers provided zebra finches with artificial ornamentation. A

of these insects are at the tips of stalks, which are longer in 2.5-cm-long red feather was taped to the forehead feathers

males than in females. During courtship, a male approaches of either or both zebra finch parents when their chicks were

the female headfirst. Researchers have shown that females 8 days old, approximately 2 days before they opened their eyes.

are more likely to mate with males that have relatively long A control group of zebra finches was raised by unadorned

eyestalks. Why would females favor this seemingly arbitrary parents. When the chicks matured, they were presented with

trait? Ornaments such as long eyestalks in these flies and prospective mates that were either artificially ornamented with

bright coloration in birds correlate in general with health and a red feather or non-ornamented (Figure 51.18). Males showed

vitality. A female whose mate choice is Figure 51.18  Sexual selection influenced by imprinting. Experiments demonstrated
a healthy male is likely to produce more that female zebra finch chicks that had imprinted on artificially ornamented fathers preferred
offspring that survive to reproduce. As ornamented males as adult mates. For all experimental groups, male offspring showed no
a result, males may compete with each preference for either ornamented or non-ornamented female mates.

other in ritualized contests to attract Experimental Groups of Parental Pairs Control Group
female attention (Figure 51.16). In

faceoffs between male stalkeyed flies, Both parents Males Females Parents not
the male whose eyestalk length is ornamented ornamented ornamented ornamented

smaller usually retreats peacefully.

Figure 51.16  A face-off between male
stalk-eyed flies competing for female
attention.

Offspring Offspring

Mate preference of female offspring: Mate preference of female offspring:
ornamented male none

1150 Unit seven  Animal Form and Function

no preference. Females raised by a male parent that was not informative. Mate-choice behavior typically did not change
ornamented also showed no preference. However, females when the difference in coloration was particularly large. Mate-
raised by an ornamented male parent preferred ornamented choice copying can thus mask genetically controlled female
males as their own mates. Thus, female finches apparently preference below a certain threshold of difference, in this case
take cues from their fathers in choosing mates. for male color.

Mate-choice copying, a behavior in which individu- Mate-choice copying, a form of social learning, has also
als in a population copy the mate choice of others, has been been observed in several other fish and bird species. What is
studied in the guppy, Poecilia reticulata. When a female guppy the selective pressure for such a mechanism? One possibility is
chooses between males with no other females present, the that a female that mates with males that are attractive to other
female almost always chooses the male with more orange females increases the probability that her male offspring will
coloration. To explore if the behavior of other females could also be attractive and have high reproductive success.
influence this preference, an experiment was set up using both
living females and artificial model females (Figure 51.19). If a Male Competition for Mates  The previous examples
female guppy observed the model “courting” a male with less
extensive orange markings, she often copied the preference show how female choice can select for one best type of
of the model female. That is, the female chose the male that male in a given situation, resulting in low variation among
had been presented in association with a model female rather males. Similarly, male competition for mates can reduce
than a more orange alternative. The exceptions were also variation among males. Such competition may involve
agonistic behavior, an often-ritualized contest that deter-
Figure 51.19  Mate choice copying by female guppies mines which competitor gains access to a resource, such
(Poecilia reticulata). In the absence of other females (control as food or a mate (Figure 51.20; see also Figure 51.16).
group), female guppies generally choose males with more orange
coloration. However, when a female model is placed near one of the Despite the potential for male competition to select for
males (experimental group), female guppies often copy the apparent reduced variation, behavioral and morphological varia-
mate choice of the model, even if the male is less colorful than others. tion in males is extremely high in some vertebrate species,
Guppy females ignored the mate choice of the model only if an
alternative male had much more orange coloration. Figure 51.20  Agonistic interaction. Male eastern grey
kangaroos (Macropus giganteus) often “box” in contests that determine
Control Sample which male is most likely to mate with an available female. Typically, one
male snorts loudly and strikes the other with his forelimbs. If the male
Male guppies under attack does not retreat, the fight may escalate into grappling or
with varying the two males balancing on their tails while attempting to kick each
degrees of other with the sharp toenails of their hind feet.
coloration

Female guppies prefer males with
more orange coloration.

Experimental Sample

Female model Video: Agonistic Behavior in Wolves
in mock Video: Snake Ritual Wrestling
courtship with
less orange
male
Female guppies prefer males that
are associated with another female.

chapter 51  Animal Behavior 1151

including species of fish and deer, as well as in a wide vari- By comparing the competition between common side-
ety of invertebrates. In some species, sexual selection has blotched lizard males to the children’s game of rock-paper-
led to the evolution of alternative male mating behavior scissors, scientists devised an explanation for the cycles
and morphology. How do scientists analyze situations of variation in the lizard population. In the game, paper
where more than one mating behavior can result in suc- defeats rock, rock defeats scissors, and scissors defeats
cessful reproduction? One approach relies on the rules paper. Each hand symbol thus wins one matchup but loses
that govern games. the other. Similarly, each type of male lizard has an advan-
tage over one of the other two types. When blue-throats
Applying Game Theory are abundant, they can defend the few females in their
territories from the advances of the sneaky yellow-throat
Often, the fitness of a particular behavioral phenotype is males. However, blue-throats cannot defend their territo-
influenced by other behavioral phenotypes in the popula- ries against the hyperaggressive orange-throats. Once the
tion. In studying such situations, behavioral ecologists orange-throats become the most abundant, the larger num-
use a range of tools, including game theory. Developed by ber of females in each territory provides the opportunity
American mathematician John Nash and others to model for the yellow-throats to have greater mating success. The
human economic behavior, game theory evaluates alter- yellow-throats become more frequent, but then give way to
native strategies in situations where the outcome depends the blue-throats, whose tactic of guarding small territories
on the strategies of all the individuals involved. once again allows them the most success. Thus, follow-
ing the population over time, one sees a persistence of all
As an example of applying game theory to mating behav- three color types and a periodic shift in which type is most
ior, let’s consider the side-blotched lizard (Uta stansburiana) prevalent.
of California. Genetic variations give rise to males with
orange, blue, or yellow throats (Figure 51.21). One would Game theory provides a way to think about complex
expect that natural selection would favor one of the three evolutionary problems in which relative performance (repro-
color types, yet all three persist. Why? The answer appears ductive success relative to other phenotypes), not absolute
to lie in the fact that each throat color is associated with performance, is the key to understanding the evolution of
a different pattern of behavior: Orange-throat males are behavior. This makes game theory an important tool because
the most aggressive and defend large territories that con- the relative performance of one phenotype compared with
tain many females. Blue-throat males are also territorial others is a measure of Darwinian fitness.
but defend smaller territories and fewer females. Yellow-
throats are nonterritorial males that mimic females and Concept Check 51.3
use “sneaky” tactics to gain the chance to mate.
1. Why does the mode of fertilization correlate with the
Evidence indicates that the mating success of each male presence or absence of male parental care?
lizard type is influenced by the relative abundance of the
other types, an example of frequency-dependent selection. 2. MAKE CONNECTIONS Balancing selection can main-
In one study population, the most frequent throat coloration tain variation at a locus (see Concept 23.4). Based on the
changed over a period of several years from blue to orange foraging experiments described in this chapter, devise a
to yellow and back to blue. simple hypothesis to explain the presence of both for R
and for s alleles in natural fly populations.
Figure 51.21  Male polymorphism in the side-blotched lizard
(Uta stansburiana). An orange-throat male, left; a blue-throat male, 3. WHAT IF? Suppose an infection in a common side-
center; a yellow-throat male, right. blotched lizard population killed many more males than
females. What would be the immediate effect on male
competition for reproductive success?

For suggested answers, see Appendix A.

Concept  51.4

Genetic analyses and the concept of
inclusive fitness provide a basis for
studying the evolution of behavior

Evolution We’ll now explore issues related to Tinbergen’s
fourth question—the evolutionary history of behaviors. We
will first look at the genetic control of a behavior. Next, we

1152 Unit seven  Animal Form and Function

will examine the genetic variation underlying the evolution Figure 51.22  A pair of prairie voles (Microtus ochrogaster)
of particular behaviors. Finally, we will see how expanding huddling. Male North American prairie voles associate closely with their
the definition of fitness beyond individual survival can help mates, as shown here, and contribute substantially to the care of young.
explain “selfless” behavior.
receptor levels in the brain regulate postmating behavior,
Genetic Basis of Behavior researchers inserted the vasopressin receptor gene from
prairie voles into the genome of meadow voles. The male
In exploring the genetic basis of behavior, we’ll begin with meadow voles carrying this gene not only developed brains
the courtship behavior of the male fruit fly (see Figure 51.4). with higher levels of the vasopressin receptor but also showed
During courtship, the male fly carries out a complex series many of the same mating behaviors as male prairie voles,
of actions in response to multiple sensory stimuli. Genetic such as pair-bonding. Thus, although many genes influence
studies have revealed that a single gene called fru controls pair-bonding and parenting in male voles, a change in the
this entire courtship ritual. If the fru gene is mutated to an level of expression of the vasopressin receptor is sufficient to
inactive form, males do not court or mate with females. alter the development of these behaviors.
(The name fru is short for fruitless, reflecting the absence of
offspring from the mutant males.) Normal male and female Genetic Variation and the Evolution
flies express distinct forms of the fru gene. When females of Behavior
are genetically manipulated to express the male form of
fru, they court other females, performing the role normally Behavioral differences between closely related species, such
played by the male. as meadow and prairie voles, are common. Significant differ-
ences in behavior can also be found within a species but are
How does the fru gene control so many different actions? often less obvious. When behavioral variation between popu-
Experiments carried out cooperatively in several laborato- lations of a species correlates with variation in environmental
ries demonstrated that fru is a master regulatory gene that conditions, it may reflect natural selection.
directs the expression and activity of many genes with nar-
rower functions. Together, genes that are controlled by the Case Study: Variation in Prey Selection
fru gene bring about sex-specific development of the fly
nervous system. In effect, fru programs the fly for male court- An example of genetically based behavioral variation
ship behavior by overseeing a male-specific wiring of the within a species involves prey selection by the western
central nervous system. garter snake (Thamnophis elegans). The natural diet of this
species differs widely across its range in California. Coastal
In many cases, differences in behavior arise not from gene
inactivation, but from variation in the activity or amount of
a gene product. One striking example comes from the study
of two related species of voles, which are small, mouse-like
rodents. Male meadow voles (Microtus pennsylvanicus) are
solitary and do not form lasting relationships with mates.
Following mating, they pay little attention to their pups. In
contrast, male prairie voles (Microtus ochrogaster) form a pair-
bond with a single female after they mate (Figure 51.22).
Male prairie voles provide care for their young pups, hovering
over them, licking them, and carrying them, while acting
aggressively toward intruders.

A peptide neurotransmitter is critical for the partnering
and parental behavior of male voles. Known as antidiuretic
hormone (ADH) or vasopressin (see Concept 44.5), this
peptide is released during mating and binds to a specific
receptor in the central nervous system. When male prairie
voles are given a drug that inhibits the receptor in the brain
that detects vasopressin, the male voles fail to form pair-
bonds after mating.

The vasopressin receptor gene is much more highly
expressed in the brain of prairie voles than in the brain of
meadow voles. Testing the hypothesis that vasopressin

chapter 51  Animal Behavior 1153

Figure 51.23  Western garter snake from a coastal habitat The marked variation in behavior observed today between
eating a banana slug. Experiments indicate that the preference the coastal and inland populations may be evidence of this
of these snakes for banana slugs may be influenced more by genetics past evolutionary change.
than by environment.
Case Study: Variation in Migratory Patterns
populations feed predominantly on banana slugs (Ariolimax
californicus) (Figure 51.23). Inland populations feed on frogs, Another species suited to the study of behavioral variation
leeches, and fish, but not banana slugs. In fact, banana slugs is the blackcap (Sylvia atricapilla), a small migratory warbler.
are rare or absent in the inland habitats. Blackcaps that breed in Germany generally migrate southwest
to Spain and then south to Africa for the winter. In the 1950s,
When researchers offered banana slugs to snakes col- a few blackcaps began to spend their winters in Britain, and
lected from each wild population, most coastal snakes over time the population of blackcaps wintering in Britain
readily ate them, whereas inland snakes tended to refuse. grew to many thousands. Leg bands showed that some of
To what extent does genetic variation among snake species these birds had migrated westward from central Germany.
contribute to a fondness for banana slugs? To answer this Was this change in the pattern of migration the outcome of
question, researchers collected pregnant snakes from the natural selection? If so, the birds wintering in Britain must
wild coastal and inland populations and then housed these have a heritable difference in migratory behavior. To test
females in separate cages in the laboratory. While the off- this hypothesis, researchers at the Max Planck Institute for
spring were still very young, they were each offered a small Ornithology in Radolfzell, Germany, devised a strategy to
piece of banana slug on ten successive days. More than 60% study migratory orientation in the laboratory (Figure 51.24).
of the young snakes from coastal mothers ate banana slugs The results demonstrated that the two patterns of migration—
on eight or more of the ten days. In contrast, fewer than to the west and to the southwest—do in fact reflect genetic
20% of the young snakes from inland mothers ate a piece differences between the two populations.
of banana slug even once. Perhaps not surprisingly, banana
slugs thus appear to be a genetically acquired taste. The study of western European blackcaps indicated
that the change in their migratory behavior occurred both
How did a genetically determined difference in feeding recently and rapidly. Before the year 1950, there were no
preference come to match the snakes’ habitats so well? It known westward-migrating blackcaps in Germany. By the
turns out that the coastal and inland populations also vary 1990s, westward migrants made up 7–11% of the blackcap
with respect to their ability to recognize and respond to odor populations of Germany. Once westward migration began,
molecules produced by banana slugs. Researchers hypoth- it persisted and increased in frequency, perhaps due to the
esize that when inland snakes colonized coastal habitats widespread use of winter bird feeders in Britain, as well as
more than 10,000 years ago, some of them could recognize shorter migration distances.
banana slugs by scent. Because these snakes took advantage
of this food source, they had higher fitness than snakes in Altruism
the population that ignored the slugs. Over hundreds or
thousands of generations, the capacity to recognize the slugs We typically assume that behaviors are selfish; that is, they
as prey increased in frequency in the coastal population. benefit the individual at the expense of others, especially
competitors. For example, superior foraging ability by one
individual may leave less food for others. The problem comes
with “unselfish” behaviors. How can such behaviors arise
through natural selection? To answer this question, let’s look
more closely at some examples of unselfish behavior and
consider how they might arise.

In discussing selflessness, we will use the term altruism
to describe a behavior that reduces an animal’s individual fit-
ness but increases the fitness of other individuals in the popu-
lation. Consider, for example, the Belding’s ground squirrel,
which lives in the western United States and is vulnerable to
predators such as coyotes and hawks. A squirrel that sees a
predator approach often gives a high-pitched alarm call that
alerts unaware individuals to retreat to their burrows. Note
that for the squirrel that warns others, the conspicuous alarm
behavior increases the risk of being killed because it brings
attention to the caller’s location.

1154 Unit seven  Animal Form and Function

Figure 51.24 Another example of altruistic behavior occurs in honeybee
societies, in which the workers are sterile. The workers them-
Inquiry  Are differences in migratory orientation selves never reproduce, but they labor on behalf of a single
within a species genetically determined? fertile queen. Furthermore, the workers sting intruders,
a behavior that helps defend the hive but results in the
Experiment  Birds known as blackcaps that live in Germany winter death of those workers.
elsewhere. Most migrate to Spain and Africa, but a few fly to Britain,
where they find food left out by city dwellers. German scientist Peter Altruism is also observed in naked mole rats (Heterocephalus
Berthold and colleagues wondered if this change had a genetic basis. glaber), highly social rodents that live in underground cham-
To test this hypothesis, they captured blackcaps wintering in Britain bers and tunnels in southern and northeastern Africa. The
and bred them in Germany in an outdoor cage. They also collected naked mole rat, which is almost hairless and nearly blind,
young blackcaps from nests in Germany and raised them in cages. In lives in colonies of 20 to 300 individuals (Figure 51.25). Each
the autumn, the blackcaps captured in Britain and the birds raised in colony has only one reproducing female, the queen, who
cages were placed in large, glass-covered funnel cages. When the fun- mates with one to three males, called kings. The rest of the
nels were lined with carbon-coated paper and placed outside at night, colony consists of nonreproductive females and males who
the birds moved around, making marks on the paper that indicated at times sacrifice themselves to protect the queen or kings
the direction in which they were trying to “migrate.” from snakes or other predators that invade the colony.

Scratch Inclusive Fitness
marks
With these examples from ground squirrels, honeybees,
Results  The wintering adult birds captured in Britain and their and mole rats in mind, let’s return to the question of how
laboratory-raised offspring both attempted to migrate to the west. altruistic behavior arises during evolution. The easiest case
In contrast, the young birds collected from nests in southern Germany to consider is that of parents sacrificing for their offspring.
attempted to migrate to the southwest. When parents sacrifice their own well-being to produce and
aid offspring, this act actually increases the fitness of the
BRITAIN N Adults from parents because it maximizes their genetic representation
W Britain and in the population. By this logic, altruistic behavior can be
E offspring maintained by evolution even though it does not enhance
GENRMASNY the survival and reproductive success of the self-sacrificing
of British individuals.
adults
What about circumstances when individuals help others
Young W E who are not their offspring? By considering a broader
from SW group of relatives than just parents and offspring, Biologist
Germany William Hamilton found an answer. He began by proposing
that an animal could increase its genetic representation in
the next generation by helping close relatives other than its

Figure 51.25  Naked mole rats, a species of colonial
mammal that exhibits altruistic behavior. Pictured here is a queen
nursing offspring while surrounded by other members of the colony.

S

Conclusion  The young of the British blackcaps and the young
birds from Germany (the control group) were raised under similar
conditions but showed very different migratory orientations,
indicating that their migratory orientation has a genetic basis.

Data from  P. Berthold et al., Rapid microevolution of migratory behavior in a wild
bird species, Nature 360:668–690 (1992).

WHAT IF? Suppose the birds had not shown a difference in orientation in
these experiments. Could you conclude that the behavior was not genetically
based? Explain.

chapter 51  Animal Behavior 1155

own offspring. Like parents and offspring, full siblings have Figure 51.26  The coefficient of relatedness between
half their genes in common. Therefore, selection might siblings. The red band indicates a particular allele (version of
also favor helping siblings or helping one’s parents produce a gene) present on one chromosome, but not its homolog, in
more siblings. This thinking led Hamilton to the idea of parent A. Sibling 1 has inherited the allele from parent A. There
inclusive fitness, the total effect an individual has on is a probability of 1/2 that sibling 2 will also inherit this allele from
proliferating its genes by producing its own offspring and parent A. Any allele present on one chromosome of either parent
by providing aid that enables other close relatives to will behave similarly. The coefficient of relatedness between the two
produce offspring. siblings is thus 1/2, or 0.5.

Hamilton’s Rule and Kin Selection Parent A Parent B

The power of Hamilton’s hypothesis was that it provided ×
a way to measure, or quantify, the effect of altruism on fit-
ness. According to Hamilton, the three key variables in an OR
act of altruism are the benefit to the recipient, the cost to
the altruist, and the coefficient of relatedness. The benefit, 1 2 (0.5) 1 2 (0.5)
B, is the average number of extra offspring that the recipi- probability probability
ent of an altruistic act produces. The cost, C, is how many
fewer offspring the altruist produces. The coefficient of Sibling 1 Sibling 2
relatedness, r, equals the fraction of genes that, on aver-
age, are shared. Natural selection favors altruism when the WHAT IF? The coefficient of relatedness of an individual to a full (nontwin)
benefit to the recipient multiplied by the coefficient of sibling or to either parent is the same: 0.5. Does this value also hold true in
relatedness exceeds the cost to the altruist—in other words,
when rB 7 C. This statement is called Hamilton’s rule. cases of polyandry and polygyny?

To better understand Hamilton’s rule, let’s apply it to a selection that thus favors altruism by enhancing the repro-
human population in which the average individual has two ductive success of relatives is called kin selection.
children. We’ll imagine that a young man is close to drown-
ing in heavy surf, and his sister risks her life to swim out and Kin selection weakens with hereditary distance.
pull her sibling to safety. If the young man had drowned, Siblings have an r of 0.5, but between an aunt and her
his reproductive output would have been zero; but now, if niece, r = 0.25 (1/4), and between first cousins, r = 0.125
we use the average, he can father two children. The benefit (1/8). Notice that as the degree of relatedness decreases, the
to the man is thus two offspring (B = 2). What cost does his rB term in the Hamilton inequality also decreases. Would
sister incur? Let’s say that she has a 25% chance of drown- natural selection favor rescuing a cousin? Not unless the
ing in attempting the rescue. The cost of the altruistic act to surf were less treacherous. For the original conditions,
the sister is then 0.25 times 2, the number of offspring she rB = 0.125 * 2 = 0.25, which is only half the value of
would be expected to have if she had stayed on shore C (0.5). British geneticist J. B. S. Haldane appears to have
(C = 0.25 * 2 = 0.5). Finally, we note that a brother and anticipated these ideas when he jokingly stated that he
sister share half their genes on average (r = 0.5). One way to would not lay down his life for one brother, but would do
see this is in terms of the separation of homologous chromo- so for two brothers or eight cousins.
somes that occurs during meiosis of gametes (Figure 51.26;
see also Figure 13.7). If kin selection explains altruism, then the examples of
unselfish behavior we observe among diverse animal species
We can use our values of B, C, and r to evaluate whether should involve close relatives. This is apparently the case, but
natural selection would favor the altruistic act in our often in complex ways. Like most mammals, female Belding’s
imaginary scenario. For the surf rescue, rB = 0.5 * 2 = 1, ground squirrels settle close to their site of birth, whereas
whereas C = 0.5. Because rB is greater than C, Hamilton’s males settle at distant sites (Figure 51.27). Since nearly all
rule is satisfied; thus, natural selection would favor this alarm calls are given by females, they are most likely aiding
altruistic act. close relatives. In the case of worker bees, who are all sterile,
anything they do to help the entire hive benefits the only
Averaging over many individuals and generations, any permanent member who is reproductively active—the queen,
particular gene in a sister faced with the situation described who is their mother.
will be passed on to more offspring if she risks the rescue
than if she does not. Among the genes propagated in this way
may be some that contribute to altruistic behavior. Natural

1156 Unit seven  Animal Form and Function

Figure 51.27  Kin selection and altruism in Belding’sMean distance (m) called tit for tat. In the tit-for-tat strategy, an individual
ground squirrels. This graph helps explain the male-femalemoved from treats another in the same way it was treated the last time
difference in altruistic behavior of ground squirrels. Once weaned (pupsbirthplacethey met. Individuals adopting this behavior are always
are nursed for about one month), females are more likely than males altruistic, or cooperative, on the first encounter with
to live near close relatives. Alarm calls that warn these relatives increase another individual and will remain so as long as their
the inclusive fitness of the female altruist. altruism is reciprocated. When their cooperation is not
reciprocated, however, individuals employing tit for tat
300 will retaliate immediately but return to cooperative behav-
ior as soon as the other individual becomes cooperative.
Male The tit-for-tat strategy has been used to explain the few
200 apparently reciprocal altruistic interactions observed in
animals—ranging from blood sharing between nonrelated
100 vampire bats to social grooming in primates.
Female
Evolution and Human Culture
0
1 2 3 4 12 13 14 15 25 26 As animals, humans behave (and, sometimes, misbehave).
Age (months)
Just as humans vary extensively in anatomical features, we
In the case of naked mole rats, DNA analyses have shown
that all the individuals in a colony are closely related. display substantial variations in behavior. Environment
Genetically, the queen appears to be a sibling, daughter,
or mother of the kings, and the nonreproductive mole intervenes in the path from genotype to phenotype for
rats are the queen’s direct descendants or her siblings.
Therefore, when a nonreproductive individual enhances physical traits, but does so much more profoundly for
a queen’s or king’s chances of reproducing, the altruist
increases the chance that some genes identical to its own behavioral traits. Furthermore, as a consequence of our
will be passed to the next generation.
marked capacity for learning, humans are probably more
Reciprocal Altruism
able than any other animal to acquire new behaviors
Some animals occasionally behave altruistically toward
others who are not relatives. A baboon may help an unre- and skills (Figure 51.28).
lated companion in a fight, or a wolf may offer food to
another wolf even though they share no kinship. Such Some human activities have a less easily defined func-
behavior can be adaptive if the aided individual returns
the favor in the future. This sort of exchange of aid, called tion in survival and reproduction than do, for example,
reciprocal altruism, is commonly invoked to explain
altruism that occurs between unrelated humans. Reciprocal foraging or courtship. One of these activities is play, which
altruism is rare in other animals; it is limited largely to spe-
cies (such as chimpanzees) with social groups stable enough is sometimes defined as behavior that appears purposeless.
that individuals have many chances to exchange aid. It is
generally thought to occur when individuals are likely to We recognize play in children and what we think is play
meet again and when there would be negative consequences
associated with not returning favors to individuals who had in the young of other vertebrates. Behavioral biologists
been helpful in the past, a pattern of behavior that behav-
ioral ecologists refer to as “cheating.” describe “object play,” such as chimpanzees playing with

Since cheating may benefit the cheater substantially, leaves, “locomotor play,” such as the acrobatics of an ante-
how could reciprocal altruism evolve? Game theory pro-
vides a possible answer in the form of a behavioral strategy lope, and “social play,” such as the interactions and antics

of lion cubs. These categories, however,

do little to inform us about the

function of play. One idea is

that, rather than generating

specific skills or experience,

play serves as preparation

for unexpected events and

for circumstances that can-

not be controlled.

Human behavior and cul-

ture are related to evolution-

ary theory in the discipline

of sociobiology. The main

premise of sociobiology is

that certain behavioral char-

acteristics exist because they

are expressions of genes that Figure 51.28  Learning a

have been perpetuated by new behavior.

chapter 51  Animal Behavior 1157

natural selection. In his seminal 1975 book Sociobiology: including global climate change, in which individual and
The New Synthesis, E. O. Wilson speculated about the evolu- collective interests often appear to be in conflict.
tionary basis of certain kinds of social behavior. By including
a few examples from human culture, he sparked a debate that Interview with E. O. Wilson: Pioneering the field
continues today. of sociobiology

Over our recent evolutionary history, we have built up Concept Check 51.4
structured societies with governments, laws, cultural val-
ues, and religions that define what is acceptable behavior 1. Explain why geographic variation in garter snake prey
and what is not, even when unacceptable behavior might choice might indicate that the behavior evolved by
enhance an individual’s Darwinian fitness. Perhaps it is our natural selection.
social and cultural institutions that make us distinct and
that provide those qualities that at times make less appar- 2. Suppose an individual organism aids the survival and
ent the continuum between humans and other animals. reproductive success of the offspring of its sibling. How
One such quality, our considerable capacity for reciprocal might this behavior result in indirect selection for certain
altruism, will be essential as we tackle current challenges, genes carried by that individual?

3. WHAT IF? Suppose you applied Hamilton’s logic to a
situation in which one individual is past reproductive
age. Could there still be selection for an altruistic act?
For suggested answers, see Appendix A.

51 Chapter Review Go to MasteringBiology™ for Videos, Animations, Vocab Self-Quiz,
Practice Tests, and more in the Study Area.
Summary of Key Concepts
Learning, the modification of behavior as a result of experience,
can take many forms:

Concept  51.1

Discrete sensory inputs can stimulate

both simple and complex behaviors

(pp. 1138–1141) VOCAB
SELF-QUIZ
goo.gl/6u55ks
Behavior is the sum of an animal’s responses to
external and internal stimuli. In behavior studies,
proximate, or “how,” questions focus on the stimuli that trigger Imprinting
a behavior and on genetic, physiological, and anatomical mecha-
nisms underlying a behavioral act. Ultimate, or “why,” questions Forms of
address evolutionary significance. learning and
A fixed action pattern is a largely invariant behavior trig- problem solving
gered by a simple cue known as a sign stimulus. Migratory
movements involve navigation, which can be based on orien- Spatial learning
tation relative to the sun, the stars, or Earth’s magnetic field.
Animal behavior is often synchronized to the circadian cycle of
light and dark in the environment or to cues that cycle over the
seasons.
The transmission and reception of signals constitute animal
communication. Animals use visual, auditory, chemical, and Cognition

tactile signals. Chemical substances called pheromones trans-
mit species-specific information between members of a species
in behaviors ranging from foraging to courtship.

? How is migration based on circannual rhythms poorly suited for
adaptation to global climate change?

Concept  51.2 Associative learning Social learning

Learning establishes specific links between ? How do imprinting in geese and song development in sparrows differ
experience and behavior (pp. 1141–1146) with regard to the resulting behavior?

Cross-fostering studies can be used to measure the influence
of social environment and experience on behavior.

1158 Unit seven  Animal Form and Function

Concept  51.3 2. According to Hamilton’s rule,
(A) natural selection does not favor altruistic behavior that
Selection for individual survival and causes the death of the altruist.
reproductive success can explain diverse (B) natural selection favors altruistic acts when the resulting
behaviors (pp. 1146–1152) benefit to the recipient, corrected for relatedness, exceeds
the cost to the altruist.
Controlled experiments in the laboratory can give rise to inter- (C) natural selection is more likely to favor altruistic behavior
pretable evolutionary changes in behavior. that benefits an offspring than altruistic behavior that
benefits a sibling.
An optimal foraging model is based on the idea that natural (D) the effects of kin selection are larger than the effects of
selection should favor foraging behavior that minimizes the direct natural selection on individuals.
costs of foraging and maximizes the benefits.
3. Female spotted sandpipers aggressively court males and, after
Sexual dimorphism correlates with the types of mating relation- mating, leave the clutch of young for the male to incubate. This
ship, which include monogamous and polygamous mating sequence may be repeated several times with different males until
systems. Variations in mating system and mode of fertilization no available males remain, forcing the female to incubate her last
affect certainty of paternity, which in turn has a significant clutch. Which of the following terms best describes this behavior?
influence on mating behavior and parental care. (A) polygyny
(B) polyandry
Game theory provides a way of thinking about evolution (C) promiscuity
in situations where the fitness of a particular behavioral phe- (D) certainty of paternity
notype is influenced by other behavioral phenotypes in the
population. Level 2: Application/Analysis

? In some spider species, the female eats the male immediately after 4. A region of the canary forebrain shrinks during the nonbreeding
copulation. How might you explain this behavior from an evolutionary season and enlarges when breeding season begins. This change is
probably associated with the annual
perspective? (A) addition of new syllables to a canary’s song repertoire.
(B) crystallization of subsong into adult songs.
Concept  51.4 (C) sensitive period in which canary parents imprint on new
offspring.
Genetic analyses and the concept of inclusive (D) elimination of the memorized template for songs sung the
fitness provide a basis for studying the evolution previous year.
of behavior (pp. 1152–1158)
5. Although many chimpanzees live in environments with oil
Genetic studies in insects have revealed the existence of master palm nuts, members of only a few populations use stones to
regulatory genes that control complex behaviors. Within the crack open the nuts. The likely explanation is that
underlying hierarchy, multiple genes influence specific behav- (A) the behavioral difference is caused by genetic differences
iors, such as a courtship song. Research on voles illustrates how between populations.
variation in a single gene can determine differences in complex (B) members of different populations have different nutritional
behaviors. requirements.
(C) the cultural tradition of using stones to crack nuts has
Behavioral variation within a species that corresponds to envi- arisen in only some populations.
ronmental variation may be evidence of past evolution. (D) members of different populations differ in learning ability.

Altruism can be explained by the concept of inclusive 6. Which of the following is not required for a behavioral trait to
fitness, the effect an individual has on proliferating its genes evolve by natural selection?
by producing its own offspring and by providing aid that enables (A) In each individual, the form of the behavior is determined
close relatives to reproduce. The coefficient of relatedness entirely by genes.
and Hamilton’s rule provide a way of measuring the (B) The behavior varies among individuals.
strength of the selective forces favoring altruism against the (C) An individual’s reproductive success depends in part on
potential cost of the “selfless” behavior. Kin selection how the behavior is performed.
favors altruistic behavior by enhancing the reproductive (D) Some component of the behavior is genetically inherited.
success of relatives.
Level 3: Synthesis/Evaluation
? What insight about the genetic basis of behavior emerges from
studying the effects of courtship mutations in fruit flies and of 7. DRAW IT  You are considering two optimal foraging models for
the behavior of a mussel-feeding shorebird, the oystercatcher. In
pair-bonding in voles? model A, the energetic reward increases solely with mussel size.
In model B, you take into consideration that larger mussels are
Test Your Understanding more difficult to open. Draw a graph of reward (energy benefit
on a scale of 0–10) versus mussel length (scale of 0–70 mm) for
Level 1: Knowledge/Comprehension each model. Assume that mussels under 10 mm provide no
benefit and are ignored by the birds. Also assume that mussels
1. Which of the following is true of innate start becoming difficult to open when they reach 40 mm in
behaviors? length and impossible to open when 70 mm long. Considering
(A) Their expression is only weakly influenced the graphs you have drawn, indicate what observations and
by genes. PRACTICE measurements you would want to make in this shorebird’s
TEST habitat to help determine which model is more accurate.
goo.gl/CUYGKD
(B) They occur with or without environmental
stimuli.
(C) They are expressed in most individuals in a population.
(D) They occur in invertebrates and some vertebrates but not
mammals.

chapter 51  Animal Behavior 1159

8. EVOLUTION CONNECTION  We often explain our behavior 12. SYNTHESIZE YOUR KNOWLEDGE
in terms of subjective feelings, motives, or reasons, but
evolutionary explanations are based on reproductive fitness. Acorn woodpeckers (Melanerpes formicivorus) stash acorns
Discuss the relationship between the two kinds of explanation. in storage holes they drill in trees. When these woodpeckers
For instance, is an explanation for behavior such as “falling in breed, the offspring from previous years often help with
love” incompatible with an evolutionary explanation? parental duties. Activities of these nonbreeding helpers include
incubating eggs and defending stashed acorns. Propose some
9. SCIENTIFIC INQUIRY  Scientists studying scrub jays found that questions about the proximate and ultimate causation of these
“helpers” often assist mated pairs of birds by gathering food for behaviors that a behavioral biologist could ask.
their offspring. For selected answers, see Appendix A.
(A) Propose a hypothesis to explain what advantage there
might be for the helpers to engage in this behavior instead For additional practice questions, check out the Dynamic Study
of seeking their own territories and mates. Modules in MasteringBiology. You can use them to study on
(B) Explain how you would test your hypothesis. If it is correct, your smartphone, tablet, or computer anytime, anywhere!
what results would you expect your tests to yield?

10. SCIENCE, TECHNOLOGY, AND SOCIETY  Researchers are very
interested in studying identical twins separated at birth and
raised apart. So far, the data reveal that such twins frequently
have similar personalities, mannerisms, habits, and interests.
What general question do you think researchers hope to
answer by studying such twins? Why do identical twins make
good subjects for this research? What are the potential pitfalls
of this research? What abuses might occur if the studies are
not evaluated critically? Explain your thinking.

11. WRITE ABOUT A THEME: INFORMATION  Learning is defined
as a change in behavior as a result of experience. In a short
essay (100–150 words), describe how heritable information
contributes to the acquisition of learning, using some examples
from imprinting and associative learning.

1160 Unit seven  Animal Form and Function

Unit 8 Ecology
An Interview with Tracy Langkilde
Meet the “lizard lady,”
Tracy Langkilde, Associate Much of your research is on fire ants and fence lizards.
Professor and Head of the Why study these particular organisms?
Department of Biology at
Pennsylvania State University. Interactions between species can change over time. To find out how
Winner of the Ecological such changes occur, I’m studying how communities respond to recent
Society of America’s presti- perturbations, such as the arrival of invasive fire ants. These ants
gious George Mercer Award, reached the United States in the late 1930s. They give really painful
Dr. Langkilde received a stings, and so people report them immediately. Thus, we know when
B.Sc. in Biology from James fire ants reached different regions and first encountered native fence
Cook University, Australia, lizards. With this information in hand, we can observe how fence
and a Ph.D. in Ecology and lizards from different regions have responded to fire ants over time.
Evolutionary Biology from
the University of Sydney, Have adaptations to fire ant attack evolved in fence lizard populations?
Australia. She and her students are
exploring how interactions among spe- When faced with a threat, the usual response of a fence lizard is to sit still, close its eyes,
cies can shift over time in response to and hope the threat goes away. But fire ants never go away, and just a few of them can
changes in the environment. Currently, kill an adult lizard in less than a minute. So over time, new ways of dealing with fire ant
several projects in her bustling labora- attacks have evolved in lizard populations. Instead of sitting still, the lizards shake their
tory are focused on how the arrival of bodies very rapidly, scratch at the ants with their back legs, and perform a 360-degree back
an invasive species (one that is new to flip—each of these behaviors can send fire ants flying off their bodies like little missiles.
a particular geographic region), the The lizards’ legs have also evolved, becoming longer, making these behaviors more effective.
fire ant (Solenopsis invicta), affects the
behavior, morphology, and physiology Are there practical applications of your work?
of a native species, the fence lizard
(Sceloporus undulatus). Some invasive species harm native species or cause billions of dollars of economic dam-
age each year. It can be very hard to get rid of such species. I don’t think we’re ever
“Reducing the abundance going to get rid of fire ants, for example, or cane toads in Australia. In such cases, there
of an invasive species can is tendency to not even try. My research suggests that we should attempt to reduce the
buy native populations abundance of an invasive species to the point where they don’t wipe out native species.
enough time to respond This can buy native populations enough time to respond naturally, as happened when
naturally.” adaptations to fire ant attacks evolved in fence lizard populations.

A fire ant prying up a scale on a You’ve done field work in many different parts of the world. Tell us
fence lizard’s head in order to inject about a memorable experience.
venom into the underlying skin
As an undergraduate in Australia, I worked in an area infested with deadly snakes
called taipans. After that, I was petrified of snakes. So when my Ph.D. advisor said, “Do
you want to come and work with 10,000 snakes?” I thought, “Sure, that sounds . . .
Oh my gosh, I am going to die.” But it was amazing. We were studying male courtship
behavior in the garter snake dens of Manitoba, Canada. The dens are in caves where
the snakes spend the winter. In the spring, tens of thousands of snakes emerge over a
few weeks. We’d go into a den and pick up an armful of 100 males. The males mostly
ignore you except maybe to rub their chin on you to see if you’re a female. That experi-
ence quickly got me over my snake phobia!

What do you find most rewarding about your job?

I love the variety my job offers, getting paid to travel to fascinating places, and being
my own boss. I also love helping students discover how cool it is to do research. Students
often say, “I’m really good in the field, but I’m not excited about analyzing data.” But
after collecting their own data, they can’t wait to analyze it and find out what is going
on. Working as a professor has also led to personal discoveries. For example, I was wor-
ried about juggling work demands with having a child. But I get as much done as I did
before; it just takes less time to do it. Having a child has put things in perspective—my life
is more balanced, and this has positively affected my research. I’ve taken my son to con-
ferences and into the field with me, and he loves it!

1161

An Introduction to Ecology 52
and the Biosphere

Figure 52.1  What limits the distribution of this tiny frog?

Key Concepts Discovering Ecology

52.1 Earth’s climate varies by latitude Kneeling by a stream in Papua New Guinea in 2008, Cornell University undergraduate
Michael Grundler heard a series of clicks. He first thought that the sounds must be com-
and season and is changing rapidly ing from a nearby cricket. Turning to look, he saw instead a tiny frog inflating its vocal
sac to call for a mate. Grundler would later learn that he had just discovered one of two
52.2 The distribution of terrestrial previously unknown frog species from the area, Paedophryne swiftorum and Paedophryne
amauensis (Figure 52.1). The entire genus Paedophryne is known only from the Papuan
biomes is controlled by climate Peninsula in eastern New Guinea. Adult frogs in this genus are typically only 8 mm
and disturbance (0.3 inch) long and are among the smallest adult vertebrates on Earth.

52.3 Aquatic biomes are diverse and What environmental factors limit the geographic distribution of Paedophryne? How
do variations in the frogs’ food supply or interactions with other species, such as preda-
dynamic systems that cover most tors and pathogens, affect the size of their populations? Questions like these are the sub-
of Earth ject of ecology (from the Greek oikos, home, and logos, study), the scientific study of the
interactions between organisms and the environment. (Note that here and throughout
52.4 Interactions between organisms this text, the term environment refers to other organisms as well as the physical aspects
of an organism’s surroundings.) The interactions studied by ecologists can be organized
and the environment limit the into a hierarchy that ranges in scale from single organisms to the planet (Figure 52.2).
distribution of species
In this chapter, we’ll first consider how Earth’s climate and other factors deter-
52.5 Ecological change and evolution mine the location of major life zones on land and in the oceans. We’ll then exam-
ine how ecologists investigate what controls the distribution of species. The next
affect one another over long and
short periods of time

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

1162

Figure 52.2  Exploring the Scope of Ecological Research

Ecologists work at different levels of the biological hierarchy, from individual organisms to
the planet. Here we present a sample research question for each level of the hierarchy.

Organismal Ecology

Organismal ecology, which includes the subdisciplines of physiological,
evolutionary, and behavioral ecology, is concerned with how an organism’s
structure, physiology, and behavior meet the challenges posed by its environment.
◀ How do flamingos select a mate?

Population Ecology

A population is a group of individuals of the same species living in
an area. Population ecology analyzes factors that affect population
size and how and why it changes through time.
◀ What environmental factors affect the

reproductive rate of flamingos?

Community Ecology

A community is a group of populations of different species in an area.
Community ecology examines how interactions between species, such as
predation and competition, affect community structure and organization.
◀ What factors influence the diversity of species

that interact at this African lake?

Ecosystem Ecology

An ecosystem is the community of organisms in an area and
the physical factors with which those organisms interact.
Ecosystem ecology emphasizes energy flow and chemical
cycling between organisms and the environment.
◀ What factors control photosynthetic productivity

in this aquatic ecosystem?

Landscape Ecology

A landscape (or seascape) is a mosaic of connected ecosys-
tems. Research in landscape ecology focuses on the factors
controlling exchanges of energy, materials, and organisms
across multiple ecosystems.
◀ To what extent do nutrients from terrestrial

ecosystems affect organisms in the lake?

Global Ecology

The biosphere is the global ecosystem—the sum of
all the planet’s ecosystems and landscapes. Global
ecology examines how the regional exchange of en-
ergy and materials influences the functioning and
distribution of organisms across the biosphere.
◀ How do global patterns of air circulation affect

the distribution of organisms?

1163

Figure 52.3  Exploring Global Climate Patterns

Latitudinal Variation Atmosphere 90°N (North Pole)
in Sunlight Intensity Low angle of incoming sunlight 23.5°N (Tropic of

Earth’s curved shape causes latitudinal variation in the intensity Sun overhead at equinoxes Cancer)
of sunlight. Because sunlight strikes the tropics (those regions 0° (Equator)
that lie between 23.5° north latitude and 23.5° south latitude) most 23.5°S (Tropic of
directly, more heat and light per unit of surface area are deliv-
ered there. At higher latitudes, sunlight strikes Earth at an oblique Capricorn)
angle, and thus the light energy is more diffuse on Earth’s surface. 90°S (South Pole)

Low angle of incoming sunlight Descending
dry air
Global Air Circulation absorbs
and Precipitation Patterns moisture.

Intense solar radiation near the equator initiates a global pattern of air 30°N ARID ZONE Ascending
circulation and precipitation. High temperatures in the tropics evapo- moist air
rate water from Earth’s surface and cause warm, wet air masses to rise releases
(blue arrows) and flow toward the poles. As the rising air masses expand moisture.
and cool, they release much of their water content, creating abundant
precipitation in tropical regions. The high-altitude air masses, now dry,
descend (tan arrows) toward Earth around 30° north and south, absorb-
ing moisture from the land and creating an arid climate conducive to the
development of the deserts that are common at those latitudes. Some of
the descending air then flows toward the poles. At latitudes around 60°
north and south, the air masses again rise and release abundant precipita-
tion (though less than in the tropics). Some of the cold, dry rising air then
flows to the poles, where it descends and flows back toward the equator,
absorbing moisture and creating the comparatively rainless and bitterly
cold climates of the polar regions.

66.5°N (Arctic Circle) 0°
60°N

30°N Westerlies

Northeast trades

0°

Southeast trades

30°S Westerlies Air flowing close to Earth’s surface creates predictable global wind
patterns. As Earth rotates on its axis, land near the equator moves
60°S faster than that at the poles, deflecting the winds from the vertical
66.5°S (Antarctic Circle) paths shown above and creating the more easterly and westerly
flows shown at left. Cooling trade winds blow from east to west in
the tropics; prevailing westerlies blow from west to east in the tem-
perate zones, defined as the regions between the Tropic of Cancer
and the Arctic Circle and between the Tropic of Capricorn and the
Antarctic Circle.

1164 Unit eight  Ecology

four chapters focus on population, community, ecosystem, and water, and evaporation of water that cause dramatic
and global ecology, as we explore how ecologists apply latitudinal variations in climate. Figure 52.3 summarizes
biological knowledge to predict the global consequences Earth’s climate patterns and how they are formed.
of human activities and to conserve Earth’s biodiversity.
Regional and Local Effects on Climate
Concept  52.1
Climate varies seasonally and can be modified by other factors,
Earth’s climate varies by latitude such as large bodies of water and mountain ranges. We will
and season and is changing rapidly examine each of these factors in more detail.

The most significant influence on the distribution of organ- Seasonality
isms on land is climate, the long-term prevailing weather
conditions in a given area. Four physical factors—temperature, In middle to high latitudes, Earth’s tilted axis of rotation and
precipitation, sunlight, and wind—are key components of its annual passage around the sun cause strong seasonal cycles
climate. To set the stage for understanding how climate—and in day length, solar radiation, and temperature (Figure 52.4).
climate change—affect life on Earth, we’ll begin by examining The changing angle of the sun over the course of the year also
patterns in climate at the global, regional, and local levels. affects local environments. For example, the belts of wet and
dry air on either side of the equator move slightly northward
Global Climate Patterns and southward as the sun’s angle changes; this produces
marked wet and dry seasons around 20° north and 20° south
Global climate patterns are determined largely by the input latitude, where many tropical deciduous forests grow. In addi-
of solar energy and Earth’s movement in space. The sun tion, seasonal changes in wind patterns alter ocean currents,
warms the atmosphere, land, and water. This warming sometimes causing the upwelling of cold water from deep
establishes the temperature variations, movements of air ocean layers. This nutrient-rich water stimulates the growth of
surface-dwelling phytoplankton and the organisms that feed

Figure 52.4  Seasonal variation in sunlight intensity. Because Earth is tilted on its axis relative December solstice:
to its plane of orbit around the sun, the intensity of solar radiation varies seasonally. This variation is smallest Northern Hemisphere tilts
in the tropics and increases toward the poles. away from sun and has
shortest day and longest
March equinox: Equator faces sun directly; night; Southern Hemisphere
neither pole tilts toward sun; all regions on tilts toward sun and has
Earth experience 12 hours of daylight longest day and shortest night.
and 12 hours of darkness.

Sun

Constant tilt 60°N
of 23.5°
June solstice: Northern Hemisphere 23.5°N
tilts toward sun and has longest day
and shortest night; Southern 0° (Equator)
Hemisphere tilts away from sun and
has shortest day and longest night. 23.5°S September equinox: Equator faces sun
Figure Walkthrough directly; neither pole tilts toward sun; all
regions on Earth experience 12 hours of
daylight and 12 hours of darkness.

chapter 52  An Introduction to Ecology and the Biosphere 1165

on them. Though these upwelling zones make up only a few the water across the land (Figure 52.6). In contrast, because tem-
percent of the ocean’s area, they are the source of more than a peratures drop more quickly over land than over water at night,
quarter of all fish caught globally. air over the now warmer water rises, drawing cooler air from the
land back out over the water and replacing it with warmer air
Bodies of Water from offshore. This local moderation of climate can be limited to
the coast itself, however. In southern California and southwest-
Ocean currents influence climate along the coasts of conti- ern Australia, cool, dry ocean breezes in summer are warmed
nents by heating or cooling overlying air masses that pass when they contact the land, absorbing moisture and creating
across the land. Coastal regions are also generally wetter than a hot, arid climate just a few kilometers inland (see Figure 3.5).
inland areas at the same latitude. The cool, misty climate This climate pattern also occurs around the Mediterranean Sea,
produced by the cold California Current that flows south- which gives it the name Mediterranean climate.
ward along western North America supports a coniferous rain
forest ecosystem along much of the continent’s Pacific coast Mountains
and large redwood groves farther south. Conversely, the west
coast of northern Europe has a mild climate because the Gulf Like large bodies of water, mountains influence air flow over
Stream carries warm water from the equator to the North land. When warm, moist air approaches a mountain, the
Atlantic (Figure 52.5). As a result, northwestern Europe is air rises and cools, releasing moisture on the windward side
warmer during winter than southeastern Canada, which is of the peak (see Figure 52.6). On the leeward side, cooler,
farther south but is cooled by the Labrador Current flowing dry air descends, absorbing moisture and producing a “rain
south from the coast of Greenland. shadow.” Such leeward rain shadows determine where many
deserts are found, including the Mojave Desert of western
Because of the high specific heat of water (see Concept 3.2), North America and the Gobi Desert of Asia.
oceans and large lakes tend to moderate the climate of nearby
land. During a hot day, when land is warmer than the water, Mountains also affect the amount of sunlight reaching an
air over the land heats up and rises, drawing a cool breeze from area and thus the local temperature and rainfall. South-facing

Figure 52.5  Global circulation of surface water in the oceans. Water is warmed at the
equator and flows north and south toward the poles, where it cools. Note the similarities between
the direction of water circulation in the gyres and the direction of the trade winds in Figure 52.3.

Labrador
Current

California Current Gulf Stream North Atlantic
30°N North Pacific Subtropical Gyre

Subtropical Gyre

Equator

30°S Indian Humboldt Current South
Ocean South Pacific Atlantic
Subtropical Subtropical Gyre Subtropical
Gyre
Gyre

E. Australian Current

Antarctic Circumpolar Current

1166 Unit eight  Ecology

Figure 52.6  How large bodies of 2 Air that encounters mountains
water and mountains affect climate. flows upward, cools at higher altitudes, and
This figure illustrates what can happen on a releases water as precipitation.
hot summer day.
Leeward side 3 Less moisture is left in the air
1 Cool air flows inland from the water, of mountains reaching the leeward side, which
moderating temperatures near the shore. Mountain therefore has little precipitation.
range This rain shadow can create a
desert on the back side of the
mountain range.

Ocean

slopes in the Northern Hemisphere receive more sunlight than Global Climate Change
north-facing slopes and are therefore warmer and drier. These
physical differences influence species distributions locally. On Because climatic variables affect the geographic ranges of
many mountains in western North America, spruce and other most plants and animals, any large-scale change in Earth’s
conifers grow on the cooler north-facing slopes, but shrubby, climate profoundly affects the biosphere. In fact, such a large-
drought-resistant plants inhabit the south-facing slopes. In scale climate “experiment” is already under way: The burning
addition, every 1,000-m increase in elevation produces an of fossil fuels and deforestation are increasing the concentra-
average temperature drop of 6°C, equivalent to that produced tions of carbon dioxide and other greenhouse gases in the
by an 880-km increase in latitude. This is one reason that high- atmosphere. This has caused climate change, a directional
elevation communities near the equator, for example, can change to the global climate that lasts three decades or more
be similar to lower-elevation communities that are far from (as opposed to short-term changes in the weather). As we’ll
the equator. explore in more detail in Concept 56.4, Earth has warmed an
average of 0.9°C (1.6°F) since 1900 and is projected to warm
Microclimate 1–6°C (2–11°F) more by the year 2100. The climate is chang-
ing in other ways as well: Wind and precipitation patterns are
At an even smaller scale is the microclimate, very fine, shifting, and extreme weather events (such as droughts and
localized patterns in climatic conditions. Many features in storms) are occurring more frequently.
the environment influence microclimate by casting shade,
altering evaporation from soil, or changing wind patterns. For How will such changes in climate affect the distribution
example, forest trees often moderate the microclimate below of organisms? One way to answer this question is to look
them. Cleared areas therefore typically experience greater tem- back at the changes that have occurred since the last ice age
perature extremes than the forest interior because of greater ended. Until about 16,000 years ago, continental glaciers
solar radiation and wind currents that arise from the rapid covered much of North America and Eurasia. As the climate
heating and cooling of open land. Within a forest, low-lying warmed and the glaciers retreated, tree species distributions
ground is usually wetter than higher ground and tends to be expanded northward. A detailed record of these changes is
occupied by different tree species. A log or large stone can shel- captured in fossilized pollen deposits that have been discov-
ter organisms such as salamanders, worms, and insects, buffer- ered in lakes and ponds. These fossilized pollen data show
ing them from the extremes of temperature and moisture. that while some species moved northward rapidly, others
moved more slowly. For species that moved more slowly,
Every environment on Earth is characterized by a mosaic the expansion of their ranges lagged several thousand years
of small-scale differences in chemical and physical attributes, behind the shift in suitable habitat.
such as temperature, light, water, and nutrients. Later in this
chapter, we’ll examine how these abiotic, or nonliving, Will plants and other species be able to keep up with
factors influence the distribution and abundance of organ- the much more rapid warming projected for this century?
isms. Similarly, all of the biotic, or living, factors—the other Consider the American beech, Fagus grandifolia. Ecological
organisms that are part of an individual’s environment—also models predict that the northern limit of the beech’s range
influence the distribution and abundance of life on Earth. may move 700–900 km northward in the next century, and
its southern range limit will shift even more. The current and

chapter 52  An Introduction to Ecology and the Biosphere 1167

Figure 52.7  Current range Figure 52.8 
and predicted ranges for the The rusty-patched
American beech under two bumblebee
climate-change scenarios. (Bombus affinis).
This species has not
American beech been able to expand
(Fagus grandifolia) its range and is now
endangered.

(a) Current range (b) 4.5°C warming (c) 6.5°C warming (Figure 52.8). Overall, climate change is causing the popula-
over next century over next century tions of many species to decrease in size or even disappear (see
Figure 1.12). In the next section, we’ll continue to examine
? T he predicted range in each scenario is based on climate factors alone. the importance of climate in determining species distributions
What other factors might alter the distribution of this species? around the world.

Interview with Margaret Davis: Using fossil Concept Check 52.1
pollen to track tree species migration
1. Explain how the sun’s unequal heating of Earth’s surface
predicted geographic ranges of this species under two differ- results in deserts near 30° north and south of the equator.
ent climate-change scenarios are illustrated in Figure 52.7. If
these predictions are even approximately correct, the beech’s 2. What are some of the differences in microclimate
range must shift 7–9 km northward per year to keep pace with between an unplanted agricultural field and a nearby
the warming climate. However, since the end of the last ice stream corridor with trees?
age, the beech has moved at a rate of only 0.2 km per year.
Without human help in moving to new habitats, species such 3. WHAT IF? Changes in Earth’s climate at the end of
as the American beech may have much smaller ranges or even the last ice age happened gradually, taking centuries
become extinct. to thousands of years. If the current global warming
happens very quickly, as predicted, how may this rapid
In fact, the climate change that has already occurred has climate change affect the evolution of long-lived trees
affected the geographic ranges of hundreds of terrestrial, compared with that of annual plants, which have much
marine, and freshwater organisms. For example, as the climate shorter generation times?
has warmed, 22 of 35 European butterfly species studied have
shifted their ranges farther north by 35–240 km in recent 4. MAKE CONNECTIONS Focusing just on the effects of
decades. In western North America, nearly 200 plant species temperature, would you expect the global distribution
have moved to lower elevations, most likely in response to of C4 plants to expand or contract as Earth becomes
decreased rain and snow at higher elevations. Other research warmer? Why? (See Concept 10.4.)
shows that a Pacific diatom species, Neodenticula seminae, For suggested answers, see Appendix A.
recently colonized the Atlantic Ocean for the first time in
800,000 years. In these and many other such cases, when Concept  52.2
climate change enables or causes a species to expand its range
into a new geographic area, other organisms living there may The distribution of terrestrial biomes is
be harmed (see Figure 56.30). controlled by climate and disturbance

Furthermore, as the climate changes, some species are fac- Earth’s life is distributed on a grand scale in biomes, major
ing a shortage of suitable replacement habitat, while others life zones characterized by vegetation type in terrestrial
cannot migrate quickly enough. For example, a 2015 study biomes (or by the physical environment in aquatic biomes,
found that on average, the geographic ranges of 67 bumble- as you’ll read in Concept 52.3). What determines where
bee species in the Northern Hemisphere were shrinking: The these biomes are located?
bumblebees were retreating from the southern edges of their
distributions, but failing to expand their ranges to the north Climate and Terrestrial Biomes

1168 Unit eight  Ecology Because climate has a strong influence on the distribution of
plant species, it is a major factor in determining the locations
of terrestrial biomes (Figure 52.9). One way to highlight the
importance of climate on the distribution of biomes is to con-
struct a climograph, a plot of the annual mean temperature

Figure 52.9  The distribution of major terrestrial biomes.

30°N Tropical forest
Tropic of Savanna
Cancer Desert
Chaparral
Equator Temperate grassland
Temperate broadleaf
Tropic of forest
Capricorn Northern coniferous
forest
30°S Tundra
High mountains
Polar ice

Animation: Terrestrial Biomes

and precipitation in a particular region. Figure 52.10 is a Figure 52.10  A climograph for some major biomes
climograph for some of the biomes found in North America. in North America. The areas plotted here encompass the ranges
Notice, for instance, that the range of precipitation in north- of annual mean temperature and precipitation in the biomes.
ern coniferous and temperate forests is similar but that tem-
perate forests are generally warmer. Grasslands are typically Desert Temperate Temperate Tropical forest
drier than either kind of forest, and deserts are drier still. 30 grassland broadleaf

Factors other than mean temperature and precipitation forest
also play a role in determining where biomes exist. Some
areas in North America with a particular combination of Annual mean temperature (°C) 15 Northern
temperature and precipitation support a temperate broadleaf coniferous
forest, but other areas with similar values for these variables forest
support a coniferous forest (see the overlap in Figure 52.10).
What could explain this variation? One thing to remember 0 Arctic and
is that the climograph is based on annual averages. Often, alpine
however, the pattern of climatic variation is as important as –15 tundra
the average climate. For example, some areas may receive 0
regular precipitation throughout the year, whereas other 100 200 300 400
areas may have distinct wet and dry seasons. Annual mean precipitation (cm)

General Features of Terrestrial Biomes INTERPRET THE DATA Some arctic tundra ecosystems receive as little
rainfall as deserts but have much more dense vegetation. What climatic factor
Most terrestrial biomes are named for major physical or
climatic features and for their predominant vegetation. could cause this difference? Explain.
Temperate grasslands, for instance, are generally found
in middle latitudes, where the climate is more moderate
than in the tropics or polar regions, and are dominated by
various grass species. Each biome is also characterized by

chapter 52  An Introduction to Ecology and the Biosphere 1169

microorganisms, fungi, and animals adapted to that particular alone would support. Hurricanes and other storms create
environment. Temperate grasslands are usually more likely openings for new species in many tropical and temperate
than temperate forests to be populated by arbuscular mycor- forests and can alter forest composition. After Hurricane
rhizal fungi (see Figure 37.15) and by large grazing mammals. Katrina struck the Gulf coast of the United States in 2005,
mixed swamp forests in the area shifted toward a dominance
Although Figure 52.9 shows distinct boundaries between of baldcypress (Taxodium distichum) and water tupelo (Nyssa
the biomes, terrestrial biomes usually grade into neighboring aquatica) because these species are less susceptible to wind
biomes, sometimes over large areas. The area of intergrada- damage than other tree species found there. As a result of
tion, called an ecotone, may be wide or narrow. disturbances, biomes are often patchy, containing several
different communities in a single area.
Vertical layering of vegetation is an important feature of
terrestrial biomes. In many forests, the layers from top to In many biomes, even the dominant plants depend on peri-
bottom consist of the upper canopy, the low-tree layer, the odic disturbance. Natural wildfires are an integral component
shrub understory, the ground layer of herbaceous plants, the of grasslands, savannas, chaparral, and many coniferous forests.
forest floor (litter layer), and the root layer. Nonforest biomes Before agricultural and urban development, much of the south-
have similar, though usually less pronounced, layers. Layering eastern United States was dominated by a single conifer species,
of vegetation provides many different habitats for animals, the longleaf pine. Without periodic burning, broadleaf trees
which sometimes exist in well-defined feeding groups, from tended to replace the pines. Forest managers now use fire as a
the insectivorous birds and bats that feed above canopies to tool to help maintain many coniferous forests.
the small mammals, numerous worms, and arthropods that
search for food in the litter and root layers below. Figure 52.12 summarizes the major features of terrestrial
biomes. As you read about the characteristics of each biome,
The species composition of each kind of biome varies from remember that humans have altered much of Earth’s surface,
one location to another. For instance, in the northern conifer- replacing natural communities with urban and agricultural
ous forest (taiga) of North America, red spruce is common in ones. The central United States, for example, is classified as
the east but does not occur in most other areas, where black grassland and once contained extensive areas of tallgrass
spruce and white spruce are abundant. As Figure 52.11 shows, prairie. Very little of the original prairie remains today,
cacti living in deserts of North and South America are similar however, having been converted to farmland.
morphologically to plants called euphorbs found in African
deserts. But since cacti and euphorbs belong to different evo- Concept Check 52.2
lutionary lineages, their similarities are due to convergent
evolution rather than shared ancestry. 1. Based on the climograph in Figure 52.10, what mainly
differentiates temperate grassland from temperate
Disturbance and Terrestrial Biomes broadleaf forest?

Biomes are dynamic, and disturbance rather than stability 2. Using Figure 52.12, identify the natural biome in which
tends to be the rule. In ecological terms, disturbance is an you live, and summarize its abiotic and biotic characteris-
event such as a storm, fire, or human activity that changes tics. Do these reflect your actual surroundings? Explain.
a community, removing organisms from it and altering
resource availability. Frequent fires can kill woody plants and 3. WHAT IF? If global warming increases average tem-
keep a savanna from becoming the woodland that climate peratures on Earth by 4°C in this century, predict which
biome is most likely to replace tundra in some locations
as a result. Explain your answer.

For suggested answers, see Appendix A.

Figure 52.11  Convergent evolution in a cactus and a euphorb. Cacti in the genus
Cereus are found in the Americas; Euphorbia canariensis, a euphorb, is native to the Canary
Islands, off the northwest coast of Africa.

Euphorbia ▶
canariensis

◀ Cereus sp.

Figure 52.12  Exploring Terrestrial Biomes

Tropical Forest

Distribution Tropical
forest occurs in equatorial
and subequatorial regions.

Precipitation In tropical
rain forests, rainfall is
relatively constant, about
200–400 cm annually. In tropical
dry forests, precipitation is highly seasonal,
about 150–200 cm annually, with a six- to
seven-month dry season.

Temperature High year-round, averaging 25–29°C
with little seasonal variation.

Plants Tropical forests are vertically layered, and

competition for light is intense. Layers in rain forests

include trees that grow above a closed canopy, the

canopy trees, one or two layers of subcanopy trees, and

layers of shrubs and herbs (small, nonwoody plants).

There are generally fewer layers in tropical dry forests. A tropical rain forest in Costa Rica
Broadleaf evergreen trees are dominant in tropical rain

forests, whereas many tropical dry forest trees drop

their leaves during the dry season. Epiphytes such as bromeliads higher in tropical forests than in any other terrestrial biome. The

and orchids generally cover tropical forest trees but are less animals, including amphibians, birds and other reptiles, mammals,

abundant in dry forests. Thorny shrubs and succulent plants are and arthropods, are adapted to the vertically layered environment

common in some tropical dry forests. and are often inconspicuous.

Animals Earth’s tropical forests are home to millions of species, Human Impact Humans have long had thriving communities in
including an estimated 5–30 million still undescribed species of tropical forests. Many tropical forests are now being cut down and
insects, spiders, and other arthropods. In fact, animal diversity is converted to farmland, urban areas, and other types of land use.

Desert Temperature Temperature is variable seasonally and daily.
Maximum air temperature in hot deserts may exceed 50°C; in cold
Distribution Deserts occur in bands near 30° north and south deserts air temperature may fall below –30°C.
latitude or at other latitudes in the interior of continents (for
instance, the Gobi Desert of north-central Asia). Plants Deserts are dominated by low, widely scattered vegetation;
the proportion of bare ground is high compared with other biomes.
Precipitation The plants include succulents such as cacti or euphorbs, deeply
Precipitation is low rooted shrubs, and herbs that grow during the infrequent moist
and highly periods. Desert plant adaptations include tolerance of heat and
variable, dessication, water storage, and reduced leaf surface area. Physical
generally less defenses, such as spines, and chemical defenses, such as toxins in
than 30 cm per
year. the leaves of shrubs, are common. Many of the plants
exhibit C4 or CAM photosynthesis.

Animals Common desert animals include snakes and
lizards, scorpions, ants, beetles, migratory and resident
birds, and seed-eating rodents. Many species are
nocturnal. Water conservation is a common adapta-
tion, with some species surviving solely on water
obtained from breaking down carbohydrates in seeds.

Human Impact Long-distance transport of water and
deep groundwater wells have allowed humans to
maintain substantial populations in deserts. Urbaniza-
tion and conversion to irrigated agriculture have
reduced the natural biodiversity of some deserts.

Organ Pipe Cactus National Continued on next page
Monument, Arizona

1171

Figure 52.12  Exploring Terrestrial Biomes (continued)

Savanna

Distribution Savanna
occurs in equatorial and
subequatorial regions.

Precipitation Seasonal
rainfall averages 30–50 cm per
year. The dry season can last up
to eight or nine months.

Temperature The savanna is warm
year-round, averaging 24–29°C, but with somewhat
more seasonal variation than in tropical forests.

Plants The scattered trees found at different densities
in the savanna often are thorny and have small leaves,
an apparent adaptation to the relatively dry conditions.
Fires are common in the dry season, and the dominant
plant species are fire-adapted and tolerant of seasonal
drought. Grasses and small nonwoody plants called
forbs, which make up most of the ground cover, grow
rapidly in response to seasonal rains and are tolerant of
grazing by large mammals and other herbivores.

Animals Large plant-eating mammals, such as wilde- A savanna in Kenya

beests and zebras, and predators, including lions and Human Impact The earliest humans may have lived in savannas.
hyenas, are common inhabitants. However, the dominant Fires set by humans may help maintain this biome, though overly
herbivores are actually insects, especially termites. During seasonal frequent fires reduce tree regeneration by killing the seedlings and
droughts, grazing mammals often migrate to parts of the savanna saplings. Cattle ranching and overhunting have led to declines in
with more forage and scattered watering holes. large-mammal populations.

Chaparral Precipitation Precipitation is highly seasonal, with rainy winters
and dry summers. Annual precipitation generally falls within the
Distribution This biome occurs in midlatitude coastal regions on range of 30–50 cm.
several continents, and its many names reflect its far-flung
distribution: chaparral in North America, matorral in Spain and Temperature Fall, winter, and spring are cool, with average
temperatures in the range of 10–12°C. Average summer tempera-
Chile, garigue and maquis ture can reach 30°C, and daytime maximum temperature can
in southern France, exceed 40°C.
and fynbos in
South Africa. Plants Chaparral is dominated by shrubs and small trees, along
with many kinds of grasses and herbs. Plant diversity is high, with

many species confined to a specific, relatively small
geographic area. Adaptations of the woody plants to
drought include their tough evergreen leaves, which
reduce water loss. Adaptations to fire are also promi-
nent. Some of the shrubs produce seeds that will
germinate only after a hot fire; food reserves stored in
their fire-resistant roots enable them to resprout
quickly and use nutrients released by the fire.

Animals Native mammals include browsers, such as
deer and goats, that feed on twigs and buds of woody
vegetation, and a high diversity of small mammals.
Chaparral areas also support many species of amphib-
ians, birds and other reptiles, and insects.

Human Impact Chaparral areas have been heavily
settled and reduced through conversion to agriculture
and urbanization. Humans contribute to the fires that
sweep across the chaparral.

An area of chaparral in California

Temperate Grassland

Distribution The veldts of
South Africa, the puszta of
Hungary, the pampas of
Argentina and Uruguay, the
steppes of Russia, and the
plains and prairies of central
North America are examples of
temperate grasslands.

Precipitation Precipitation is often highly
seasonal, with relatively dry winters and wet summers.
Annual precipitation generally averages between 30 and
100 cm. Periodic drought is common.

Temperature Winters are generally cold, with average
temperatures falling below –10°C. Summers, with
average temperatures often approaching 30°C, are hot.

Plants The dominant plants are grasses and forbs,

which vary in height from a few centimeters to 2 m in

tallgrass prairie. Many grassland plants have adapta-

tions that help them survive periodic, protracted

droughts and fire. For example, grasses can sprout A grassland in Mongolia
quickly following fire. Grazing by large mammals helps

prevent establishment of woody shrubs and trees. Human Impact Deep, fertile soils make temperate grasslands ideal

Animals Native mammals include large grazers such as bison and places for agriculture, especially for growing grains. As a conse-
wild horses. Temperate grasslands are also inhabited by a wide variety quence, most grassland in North America and much of Eurasia has
of burrowing mammals, such as prairie dogs in North America. been converted to farmland. In some drier grasslands, cattle and
other grazers have turned parts of the biome into desert.

Northern Coniferous Forest Temperature Winters are usually cold; summers may be hot.
Some areas of coniferous forest in Siberia typically range in
Distribution Extending in a broad band across northern North temperature from –50°C in winter to over 20°C in summer.
America and Eurasia to the edge of the arctic tundra, the northern
coniferous forest, or taiga, is the largest terrestrial biome on Earth. Plants Northern coniferous forests are dominated by cone-bearing
trees, such as pine, spruce, fir, and hemlock, some of which depend
Precipitation Annual precipitation generally ranges from 30 to on fire to regenerate. The conical shape of many conifers prevents
70 cm, and periodic droughts are common. However, some coastal too much snow from accumulating and breaking their branches,
coniferous forests of the U.S. Pacific Northwest are temperate rain and their needle- or scale-like leaves reduce water loss. The diversity
of plants in the shrub and herb layers of these forests is lower than
forests that may receive in temperate broadleaf forests.
over 300 cm of annual
precipitation. Animals While many migratory birds nest in northern
coniferous forests, other species reside there year-round.
The mammals of this biome, which include moose,
brown bears, and Siberian tigers, are diverse. Periodic
outbreaks of insects that feed on the dominant trees
can kill vast tracts of trees.

Human Impact Although they have not been heavily
settled by human populations, northern coniferous
forests are being logged at an alarming rate, and the
old-growth stands of these trees may soon disappear.

A coniferous forest in Norway

Continued on next page
1173

Figure 52.12  Exploring Terrestrial Biomes (continued)

Temperate Broadleaf Forest

Distribution Temperate broadleaf
forest is found mainly at midlati-
tudes in the Northern Hemisphere,
with smaller areas in Chile, South
Africa, Australia, and New Zealand.

Precipitation Precipitation can
average from about 70 to over 200 cm
annually. Significant amounts fall during
all seasons, including summer rain and, in some
forests, winter snow.

Temperature Winter temperatures average 0°C.
Summers, with temperatures up to 35°C, are hot and
humid.

Plants A mature temperate broadleaf forest has

distinct vertical layers, including a closed canopy, one

or two strata of understory trees, a shrub layer, and an

herb layer. There are few epiphytes. The dominant

plants in the Northern Hemisphere are deciduous trees,

which drop their leaves before winter, when low

temperatures would reduce photosynthesis and make A temperate broadleaf forest in New Jersey
water uptake from frozen soil difficult. In Australia,
Human Impact Temperate broadleaf forest has been heavily
evergreen eucalyptus trees dominate these forests.
settled on all continents. Logging and land clearing for agriculture
Animals In the Northern Hemisphere, many mammals hibernate and urban development cleared virtually all the original deciduous
in winter, while many bird species migrate to warmer climates. forests in North America. However, owing to their capacity for
Mammals, birds, and insects make use of all the vertical layers of recovery, these forests are returning over much of their former
the forest. range.

Tundra Precipitation Precipitation averages from 20 to 60 cm annually in
arctic tundra but may exceed 100 cm in alpine tundra.
Distribution Tundra covers expansive areas of the Arctic,
amounting to 20% of Earth’s land surface. High winds and low Temperature Winters are cold, with averages in some areas below
temperatures produce similar plant communities, called alpine –30°C. Summer temperatures generally average less than 10°C.

tundra, on very high Plants The vegetation of tundra is mostly herbaceous, consisting
mountaintops at of a mixture of mosses, grasses, and forbs, along with some dwarf
all latitudes, shrubs and trees and lichens. A permanently frozen layer of soil
including the called permafrost restricts the growth of plant roots.
tropics.
Animals Large grazing musk oxen are resident, while
caribou and reindeer are migratory. Predators include
bears, wolves, and foxes. Many bird species migrate to
the tundra for summer nesting.

Human Impact Tundra is sparsely settled but has
become the focus of significant mineral and oil
extraction in recent years.

Dovrefjell-Sunndalsfjella National Park, Norway,
in autumn

Concept  52.3 Figure 52.13  Zonation in aquatic environments.
(a) Zonation in a lake
Aquatic biomes are diverse and
dynamic systems that cover most Littoral Limnetic
of Earth zone zone

Unlike terrestrial biomes, aquatic biomes are characterized Photic Pelagic
primarily by their physical and chemical environment. They zone zone
also show far less latitudinal variation, with all types found Benthic
across the globe. For example, marine biomes generally have zone Aphotic
salt concentrations that average 3%, whereas freshwater zone
biomes are usually characterized by a salt concentration of
less than 0.1%. The lake environment is generally
classified on the basis of three physical criteria: light penetration
The oceans make up the largest marine biome, covering (photic and aphotic zones), distance from shore and water depth
about 75% of Earth’s surface. Because of their vast size, they (littoral and limnetic zones), and whether the environment is open
greatly impact the biosphere. Water evaporated from the water (pelagic zone) or on the bottom (benthic zone).
oceans provides most of the planet’s rainfall. Marine algae
and photosynthetic bacteria supply much of the world’s (b) Marine zonation Oceanic zone
oxygen and consume large amounts of atmospheric carbon
dioxide. Ocean temperatures have a major effect on global Intertidal zone
climate and wind patterns (see Figure 52.3), and along Neritic
with large lakes, oceans tend to moderate the climate of zone
nearby land.
0 Photic
Freshwater biomes are closely linked to the soils and biotic 200 m zone
components of the surrounding terrestrial biome. The partic-
ular characteristics of a freshwater biome are also influenced Continental
by the patterns and speed of water flow and the climate to shelf
which the biome is exposed.
Benthic Pelagic
Zonation in Aquatic Biomes zone zone
Aphotic
Many aquatic biomes are physically and chemically strati- zone
fied (layered), vertically and horizontally, as illustrated for
both a lake and a marine environment in Figure 52.13. 2,000–
Light is absorbed by water and by photosynthetic organ- 6,000 m
isms, so its intensity decreases rapidly with depth. The upper
photic zone is the region where there is sufficient light Abyssal
for photosynthesis, while the lower aphotic zone is the zone
region where little light penetrates. The photic and apho- Like lakes, the marine environment is
tic zones together make up the pelagic zone. Deep in the generally classified on the basis of light penetration (photic and
aphotic zone lies the abyssal zone, the part of the ocean aphotic zones), distance from shore and water depth (intertidal,
2,000–6,000 m below the surface. At the bottom of all of neritic, and oceanic zones), and whether the environment is open
these aquatic zones, deep or shallow, is the benthic zone. water (pelagic zone) or on the bottom (benthic and abyssal zones).
Made up of sand and organic and inorganic sediments, the
benthic zone is occupied by communities of organisms layer of abrupt temperature change called a thermocline
collectively called the benthos. A major source of food for separates the more uniformly warm upper layer from more
many benthic species is dead organic matter called detritus, uniformly cold deeper waters. Lakes tend to be particu-
which “rains” down from the productive surface waters of larly layered with respect to temperature, especially during
the photic zone. summer and winter, but many temperate lakes undergo a
semiannual mixing of their waters as a result of changing
Thermal energy from sunlight warms surface waters to
whatever depth the sunlight penetrates, but the deeper waters
remain quite cold. In the ocean and in most lakes, a narrow

chapter 52  An Introduction to Ecology and the Biosphere 1175

Figure 52.14  Seasonal turnover in lakes with winter ice cover. Seasonal turnover
causes the lake waters to be well oxygenated at all depths in spring and autumn; in winter and
summer, when the lake is stratified by temperature, the oxygen concentration decreases with depth.

Winter Spring Summer Autumn

2° 0° 4° 8°18°22° 4°
4°C 4°C
4°C 4°C
Thermocline

1 In winter, the coldest water in 2 In spring, the surface water 3 In summer, the lake regains a 4 In autumn, as surface water
the lake (0°C) lies just below the warms to 4°C and mixes with distinctive thermal profile, with cools rapidly, it sinks beneath
surface ice; water becomes the layers below, eliminating warm surface water separated the underlying layers, remixing
progressively warmer at deeper thermal stratification. Spring from cold bottom water by a the water until the surface
levels of the lake, typically 4°C winds help mix the water, narrow vertical zone of abrupt begins to freeze and the winter
at the bottom. bringing oxygen to the bottom temperature change, called a temperature profile is
and nutrients to the surface. thermocline. reestablished.

temperature profiles (Figure 52.14). This turnover, as it is Concept  52.4
called, sends oxygenated water from a lake’s surface to the
bottom and brings nutrient-rich water from the bottom to Interactions between organisms
the surface in both spring and autumn. and the environment limit the
distribution of species
In both freshwater and marine environments, communi-
ties are distributed according to water depth, degree of light Species distributions are a consequence of both ecological
penetration, distance from shore, and whether they are factors and evolutionary history. Consider kangaroos, which
found in open water or near the bottom. Marine communi- are found in Australia and nowhere else in the world. Fossil
ties, in particular, illustrate the limitations on species distri- evidence indicates that kangaroos and their close relatives
bution that result from these abiotic factors. Plankton and originated in Australia, roughly 5 million years ago. By that
many fish species live in the relatively shallow photic zone time, Australia had moved close to its present location (by
(see Figure 52.13b). Because water absorbs light so well and continental drift; see Concept 25.4), and it was not connected
the ocean is so deep, most of the ocean volume is dark (the to other landmasses. Thus, kangaroos occur only in Australia
aphotic zone) and harbors relatively little life. in part because of an accident of history: The kangaroo lin-
eage originated there at a point in time when the continent
Figure 52.15 explores the main characteristics of Earth’s was geographically isolated.
major aquatic biomes.
But ecological factors are also important. To date, kangaroos
Concept Check 52.3 have not dispersed (on their own) to other continents; hence,
they are restricted to the continent on which they originated.
1. Why are phytoplankton, and not benthic algae or rooted And within Australia, kangaroos are found in some habitats
aquatic plants, the dominant photosynthetic organisms but not in others. The red kangaroo, for example, occurs in the
of the oceanic pelagic zone? (See Figure 52.15.) arid grasslands of central Australia, but not in the tall, open for-
ests of eastern Australia. Moreover, kangaroos are not unusual
2. MAKE CONNECTIONS Many organisms living in estuar- in this respect—all species are found in some habitats but not
ies experience freshwater and saltwater conditions each others. Hence, ecologists ask not only where species occur,
day with the rising and falling of tides. Explain how but also why species occur where they do: What ecological
these changing conditions challenge the survival of these factors—biotic and abiotic—determine their distribution?
organisms (see Concept 44.1).
In many cases, both biotic and abiotic factors affect the
3. MAKE CONNECTIONS As noted in Figure 52.15, the distribution of a species, as is true of the saguaro cactus
addition of nutrients to a lake can cause an algal bloom. (Carnegiea gigantea). Saguaros are found almost exclusively
When these algae die, complex molecules in their bodies
are broken down by decomposers using aerobic respira-
tion. Explain why this would reduce the lake’s oxygen
levels (see Concept 9.1).
For suggested answers, see Appendix A.

1176 Unit eight  Ecology

Figure 52.15  Exploring Aquatic Biomes

Lakes

Physical Environment Standing bodies of
water range from ponds a few square meters
in area to lakes covering thousands of square
kilometers. Light decreases with depth,
creating stratification. Temperate lakes may
have a seasonal thermocline; tropical lowland
lakes have a thermocline year-round.

Chemical Environment The salinity, An oligotrophic lake in Jasper
oxygen concentration, and nutrient content National Park, Alberta
differ greatly among lakes and can vary with
season. Oligotrophic lakes tend to be littoral zone, the shallow, well-lit waters
nutrient-poor and generally oxygen-rich; close to shore. Farther from shore, where water
eutrophic lakes are nutrient-rich and often is too deep to support rooted aquatic plants,
depleted of oxygen in the deepest zone in the limnetic zone is inhabited by a variety
summer and if covered with ice in winter. of phytoplankton, including cyanobacteria.
The amount of decomposable organic matter
in bottom sediments is low in oligotrophic Heterotrophs In the limnetic zone, small A eutrophic lake in the Okavango
lakes and high in eutrophic lakes; high rates drifting heterotrophs, or zooplankton, graze Delta, Botswana
of decomposition in deeper layers of eutro- on the phytoplankton. The benthic zone is
phic lakes cause periodic oxygen depletion. inhabited by assorted invertebrates whose Human Impact Runoff from fertilized land
species composition depends partly on oxygen and dumping of wastes lead to nutrient
Geologic Features Oligotrophic lakes tend to levels. Fishes live in all zones with sufficient enrichment, which can produce large
have less surface area relative to their depth oxygen. numbers of algae (an algal “bloom”)
than eutrophic lakes. Oligotrophic lakes may oxygen depletion, and fish kills.
become more eutrophic over time as runoff
adds sediments and nutrients.

Photosynthetic Organisms Rooted and
floating aquatic plants in lakes live in the

Wetlands

Physical Environment A wetland is a low in dissolved oxygen. Wetlands have a flows back and forth because of rising lake
habitat that is inundated by water at least high capacity to filter dissolved nutrients and levels or tidal action. Thus, fringe wetlands
some of the time and that supports plants chemical pollutants. include both freshwater and marine biomes.
adapted to water-saturated soil. Some
wetlands are inundated at all times, whereas Geologic Features Basin wetlands develop Photosynthetic Organisms Wetlands are
others flood infrequently. in shallow basins, ranging from upland among the most productive biomes on
depressions to filled-in lakes and ponds. Earth. Their water-saturated soils favor the
Chemical Environment Because of high Riverine wetlands develop along shallow and growth of plants such as pond lilies and
organic production by plants and decompo- periodically flooded banks of rivers and cattails, many sedges, bald cypress, and black
sition by microbes and other organisms, streams. Fringe wetlands occur along the spruce, which have adaptations enabling
both the water and the soils are periodically coasts of large lakes and seas, where water them to grow in water or in soil that is
periodically anaerobic owing to the presence
of unaerated water. Woody plants dominate
the vegetation of swamps, while bogs are
dominated by sphagnum mosses.

Heterotrophs Wetlands are home to a
diverse community of invertebrates, birds,
and many other organisms. Herbivores, from
crustaceans and aquatic insect larvae to
muskrats, consume algae, detritus, and
plants. Carnivores are also varied and may
include dragonflies, otters, frogs, alligators,
and herons.

Human Impact Wetlands help purify water
and reduce peak flooding. Draining and
filling have destroyed up to 90% of wetlands
in Europe.

A basin wetland in the United Kingdom
Continued on next page

Figure 52.15  Exploring Aquatic Biomes (continued)

Streams and Rivers

Physical Environment The most prominent
physical characteristic of streams and rivers is
the speed and volume of their flow. Head-
water streams are generally cold, clear, swift,
and turbulent. Farther downstream, where
numerous tributaries may have joined,
forming a river, the water is generally warmer
and more turbid because of suspended
sediment. Streams and rivers are stratified
into vertical zones.

Chemical Environment The salt and A headwater stream in Washington The Loire River in France, far from its headwaters
nutrient content of streams and rivers
increases from the headwaters to the mouth. Photosynthetic Organisms Headwater Human Impact Municipal, agricultural, and
Headwaters are generally rich in oxygen. streams that flow through grasslands or industrial pollution degrade water quality and
Downstream water may also contain deserts may be rich in phytoplankton or kill aquatic organisms. Damming and flood
substantial oxygen, except where there has rooted aquatic plants. control impair the natural functioning of
been organic enrichment. A large fraction of stream and river ecosystems and threaten
the organic matter in rivers consists of Heterotrophs A great diversity of fishes and migratory species such as salmon.
dissolved or highly fragmented material that invertebrates inhabit unpolluted rivers and
is carried by the current from forested streams, distributed according to, and
streams. throughout, the vertical zones. In streams
flowing through temperate or tropical
Geologic Features Headwater stream forests, organic matter from terrestrial
channels are often narrow, have a rocky vegetation is the primary source of food for
bottom, and alternate between shallow aquatic consumers.
sections and deeper pools. The downstream
stretches of rivers are generally wide and
meandering. River bottoms are often silty
from sediments deposited over long periods
of time.

Estuaries

Physical Environment An estuary is a mixes little with the lower-density river water varies with the rise and fall of the tides.
transition area between river and sea. at the surface. Nutrients from the river make estuaries, like
Seawater flows up the estuary channel during wetlands, among the most productive
a rising tide and flows back down during the Chemical Environment Salinity varies biomes.
falling tide. Often, higher-density seawater spatially within estuaries, from nearly that of
occupies the bottom of the channel and fresh water to that of seawater. Salinity also Geologic Features Estuarine flow patterns
combined with the sediments carried by river
and tidal waters create a complex network of
tidal channels, islands, natural levees, and
mudflats.

Photosynthetic Organisms Saltmarsh
grasses and algae, including phytoplankton,
are the major producers in estuaries.

Heterotrophs Estuaries support an
abundance of worms, oysters, crabs, and
many fish species that humans consume.
Many marine invertebrates and fishes use
estuaries as a breeding ground or migrate
through them to freshwater habitats
upstream. Estuaries are also crucial feeding
areas for waterfowl and some marine
mammals.

Human Impact Filling, dredging, and
pollution from upstream have disrupted
estuaries worldwide.

An estuary in southern Spain

Intertidal Zones intertidal zones exposed to vigorous
wave action generally lack attached
Physical Environment An intertidal plants or algae, while sandy intertidal
zone is periodically submerged and zones in protected bays or lagoons
exposed by the tides, twice daily on most often support rich beds of seagrass
marine shores. Upper zones experience and algae.
longer exposures to air and greater
variations in temperature and salinity. Heterotrophs Many of the animals in
Changes in physical conditions from the rocky intertidal environments have
upper to the lower intertidal zones limit structural adaptations that enable them
the distributions of many organisms to attach to the hard substrate. The
to particular strata, as shown in the composition, density, and diversity of
photograph. animals change markedly from the
upper to the lower intertidal zones.
Chemical Environment Oxygen and Many of the animals in sandy or
nutrient levels are generally high and are muddy intertidal zones, such as worms,
renewed with each turn of the tides. clams, and predatory crustaceans, bury
themselves and feed as the tides bring
Geologic Features The substrates of sources of food. Other common
intertidal zones, which are generally either animals are sponges, sea anemones,
rocky or sandy, select for particular echinoderms, and small fishes.
behavior and anatomy among intertidal
organisms. The configuration of bays or Human Impact Oil pollution has
coastlines influences the magnitude of disrupted many intertidal areas. The
tides and the relative exposure of construction of rock walls and barriers
intertidal organisms to wave action. to reduce erosion from waves and storm
surges has disrupted this zone in some
Photosynthetic Organisms A high locations.
diversity and biomass of attached marine
algae inhabit rocky intertidal zones, A rocky intertidal zone on the Oregon coast
especially in the lower zone. Sandy

Oceanic Pelagic Zone oceans, producing a surge of phytoplankton fishes graze on photosynthetic plankton. The
growth. Because of this biome’s large size, oceanic pelagic zone also includes numerous
Physical Environment The oceanic photosynthetic plankton account for about free-swimming animals, such as large squids,
pelagic zone is a vast realm of open blue half of the photosynthetic activity on Earth. fishes, sea turtles, and marine mammals.
water, constantly mixed by wind driven Human Impact Overfishing has depleted
oceanic currents. Because of higher water Heterotrophs The most abundant hetero- fish stocks in all Earth’s oceans; marine life
clarity, the photic zone extends to greater trophs in this biome are zooplankton. These has also been harmed by pollution, ocean
depths than in coastal marine waters. protists, worms, copepods, shrimp-like krill, acidification, and global warming.
jellies, and small larvae of invertebrates and
Chemical Environment Oxygen levels are Continued on next page
generally high. Nutrient concentrations are
generally lower than in coastal waters. 1179
Because they are thermally stratified
year-round, some tropical areas of the
oceanic pelagic zone have lower nutrient
concentrations than temperate oceans.
Turnover between fall and spring renews
nutrients in the photic zones of temperate
and high-latitude ocean areas.

Geologic Features This biome covers
approximately 70% of Earth’s surface and
has an average depth of nearly 4,000 m. The
deepest point in the ocean is more than
10,000 m beneath the surface.

Photosynthetic Organisms The dominant
photosynthetic organisms are phytoplank-
ton, including photosynthetic bacteria, that
drift with the oceanic currents. Spring
turnover renews nutrients in temperate

Open ocean near Iceland

Figure 52.15  Exploring Aquatic Biomes (continued)

Coral Reefs

Physical Environment Coral reefs are A coral reef in the Red Sea Human Impact Collecting of coral skeletons
formed largely from the calcium carbonate and overfishing have reduced populations of
skeletons of corals. Shallow reef-building Heterotrophs Corals, a diverse group of corals and reef fishes. Global warming and
corals live in the photic zone of relatively cnidarians, are themselves the predomi- pollution may be contributing to large-scale
stable tropical marine environments with nant animals on coral reefs. However, coral death. Development of coastal mangroves
high water clarity, primarily near islands and fish and invertebrate diversity is for aquaculture has also reduced spawning
along the edge of some continents. They are exceptionally high. Overall animal grounds for many species of reef fishes.
sensitive to temperatures below about diversity on coral reefs rivals that of
18–20°C and above 30°C. Deep-sea coral tropical forests.
reefs, found between 200 and 1,500 m deep,
are less known than their shallow counter-
parts but harbor as much diversity as many
shallow reefs do.

Chemical Environment Corals require high
oxygen levels and are excluded by high
inputs of fresh water and nutrients.

Geologic Features Corals require a solid
substrate for attachment. A typical coral reef
begins as a fringing reef on a young, high
island, forming an offshore barrier reef later in
the history of the island and becoming a coral
atoll as the older island submerges.

Photosynthetic Organisms Unicellular
algae live within the tissues of the corals,
forming a mutualistic relationship that
provides the corals with organic molecules.
Diverse multicellular red and green algae
growing on the reef also contribute substan-
tial amounts of photosynthesis.

Marine Benthic Zone present at sufficient concentrations to In these dark, hot environments, the food
support diverse animal life. producers are chemoautotrophic prokaryotes
Physical Environment The marine that obtain energy by oxidizing H2S formed
benthic zone consists of the seafloor below Geologic Features Soft sediments cover by a reaction of the hot water with dissolved
the surface waters of the coastal, or neritic, most of the benthic zone. However, there are sulfate (SO42– ).
zone and the offshore pelagic zone. Except areas of rocky substrate on reefs, submarine
for shallow, near-coastal areas, the marine mountains, and new oceanic crust. Heterotrophs Neritic benthic communities
benthic zone receives no sunlight. Water include numerous invertebrates and fishes.
temperature declines with depth, while Autotrophs Photosynthetic organisms, Beyond the photic zone, most consumers
pressure increases. As a result, organisms in mainly seaweeds and filamentous algae, are depend entirely on organic matter raining
the very deep benthic, or abyssal, zone are limited to shallow benthic areas with down from above. Among the animals of the
adapted to continuous cold (about 3°C) and sufficient light to support them. Unique deep-sea hydrothermal vent communities are
very high water pressure. assemblages of organisms live near deep-sea giant tube worms (pictured at left), some
hydrothermal vents on mid-ocean ridges. more than 1 m long. They are nourished by
Chemical Environment Except in areas of chemoautotrophic prokaryotes that live as
organic enrichment, oxygen is usually symbionts within their bodies. Many other
invertebrates, including arthropods and
echinoderms, are also abundant around the
hydrothermal vents.

Human Impact Overfishing has decimated
important benthic fish populations, such as
the cod of the Grand Banks off Newfound-
land. Dumping of organic wastes has created
oxygen-deprived benthic areas.

A deep-sea hydrothermal vent community

Video: Tubeworms

in the Sonoran Desert of the southwestern United CALIFORNIA ARIZONA
States and northwestern Mexico (Figure 52.16).
To the north, their range is limited by an abiotic BAJA
factor: temperature. Saguaros tolerate freezing tem- CALIFORNIA
peratures only briefly, typically for less than a day, and
generally cannot survive at temperatures below -4°C SONORA
(25°F). For the same reason, saguaros are rarely found
at elevations above 1,200 m (4,000 feet). 100 km BAJA GULF
CALIFORNIA OF
However, temperature alone does not fully Sonoran desert CALIFORNIA
explain the distribution of saguaros, including Saguaro cacti observed SUR
why they are missing from the western portion of
the Sonoran Desert. Water availability is critical Figure 52.16  Distribution
because seedling survival typically requires con-
secutive years of moist conditions, something that of the saguaro cactus in North
may occur only a few times each century. Biotic fac- America. Freezing temperatures
tors almost certainly influence their distribution as strongly limit where saguaros are found,
well. Mice and grazers such as goats eat the seedlings, but other abiotic and biotic factors are
and bats pollinate the large, white flowers that open at also important.
night. Saguaros are also vulnerable to a deadly bacterial
disease. Thus, for the saguaro, as for most other species,
ecologists need to consider multiple factors and alterna-
tive hypotheses when attempting to explain the distribution
of a species.

To see how ecologists might arrive at such an explana-
tion, let’s examine the ecological factors highlighted by
the questions in the flowchart in Figure 52.17.

Dispersal and Distribution Natural Range Expansions
and Adaptive Radiation
One factor that contributes greatly to the global distribution
of organisms is dispersal, the movement of individuals or The importance of dispersal is most evident when organisms
gametes away from their area of origin or from centers of high reach an area where they did not exist previously, called a range
population density. For example, while land-bound kangaroos expansion. For instance, 200 years ago, the cattle egret (Bubulcus
have not reached Africa under their own power, other organ- ibis) was found only in Africa and southwestern Europe. But
isms that disperse more readily, such as some birds, have. The in the late 1800s, some of these birds managed to cross the
dispersal of organisms is critical to understanding the role of Atlantic Ocean and colonize northeastern South America.
geographic isolation in evolution (see Concept 24.2) as well From there, cattle egrets gradually spread southward and also
as the patterns of species distribution we see today, including northward through Central America and into North America,
that of the Pacific diatom discussed earlier in this chapter.

Figure 52.17  Flowchart of factors limiting geographic distribution. An ecologist
studying factors limiting a species’ distribution might consider questions like these. As suggested
by the arrows leading from the “yes” responses, the ecologist would answer all of these questions
because more than one factor can limit a species’ distribution.

Yes Area inaccessible Yes Predation, parasitism, Chemical
or insufficient time competition, disease factors

Why is species Does dispersal No Do biotic factors No Do abiotic factors Water, oxygen, salinity, pH,
X absent from limit its (other species) limit its soil nutrients, etc.
an area? Temperature, light, soil
distribution? limit its distribution? Physical structure, fire, moisture, etc.
distribution? factors

? How might the importance of various abiotic factors differ for aquatic and terrestrial ecosystems?

chapter 52  An Introduction to Ecology and the Biosphere 1181

1966 Current Figure 52.19  Effects of feeding by sea urchins on seaweed
1970 distribution. Seaweed abundance in areas from which the long-spined
1965 sea urchin (Centrostephanus rodgersii ) had been removed was much
higher than seaweed abundance in adjacent control sites from which the
urchin was not removed.

100

1960

1961 1943 Seaweed cover (%) 80
1958 1937 Urchins removed
1951
60
1956
40

Figure 52.18  1970 20 Control (urchins present)
0
Dispersal of the cattle August February August February
1982 1983 1983 1984
egret in the Americas.
Native to Africa, cattle
egrets were first reported
in South America in 1877.

reaching Florida by 1960 (Figure 52.18). Today they have Biotic Factors
breeding populations as far west as the Pacific coast of the
United States and as far north as southern Canada. Our next question is whether biotic factors—other species—
limit the distribution of a species. Often, the ability of a species
In rare cases, such long-distance dispersal can lead to adap- to survive and reproduce is reduced by its interactions with
tive radiation, the rapid evolution of an ancestral species into other species, such as predators (organisms that kill their prey)
new species that fill many ecological niches. The diversity of or herbivores (organisms that eat plants or algae). Figure 52.19
Hawaiian silverswords is one example of adaptive radiation describes a specific case in which an herbivore, the long-spined
that was possible only with the long-distance dispersal of an sea urchin (Centrostephanus rodgersii), has the potential to limit
ancestral tarweed from North America (see Figure 25.22). the distribution of a food species. In certain marine ecosys-
tems, there is often an inverse relationship between the abun-
Natural range expansions clearly show the influence of dance of sea urchins and seaweeds (multicellular algae, such as
dispersal on distribution. However, opportunities to observe kelp). Where urchins that graze on seaweeds and other algae
such dispersal directly are rare, so ecologists often turn to are common, large stands of seaweeds do not become estab-
experimental methods to better understand the role of lished. As shown in Figure 52.19, Australian researchers tested
dispersal in limiting the distribution of species. whether C. rodgersii is a biotic factor limiting seaweed distribu-
tion. When this urchin was removed from experimental plots,
Species Transplants seaweed cover increased dramatically, showing that C. rodgersii
limited the distribution of seaweeds.
To determine if dispersal is a key factor limiting the distribu-
tion of a species, ecologists may observe the results of inten- In addition to predation and herbivory, the presence or
tional or accidental transplants of the species to areas where it absence of pollinators, food resources, parasites, pathogens, and
was previously absent. For a transplant to be successful, some competing organisms can act as a biotic limitation on species
of the organisms must not only survive in the new area but distribution. Such biotic limitations are common in nature.
also reproduce there sustainably. If a transplant is successful,
then we can conclude that the potential range of the species is Abiotic Factors
larger than its actual range; in other words, the species could
live in certain areas where it currently does not. The last question in the flowchart in Figure 52.17 considers
whether abiotic factors, such as temperature, water, oxygen,
Species introduced to new geographic locations can disrupt salinity, sunlight, or soil, might be limiting a species’ distribu-
the communities and ecosystems to which they have been tion. If the physical conditions at a site do not allow a species
introduced (see Concept 56.1). Consequently, ecologists rarely to survive and reproduce, then the species will not be found
move species to new regions. Instead, they document the out- there. Throughout this discussion, keep in mind that most
come when a species has been transplanted for other purposes, abiotic factors vary substantially over space and time. Daily
as when a predator is introduced to control a pest species, or and annual fluctuations of abiotic factors may either blur or
when a species has been moved accidentally to a new region.

1182 Unit eight  Ecology

accentuate regional distinctions. Furthermore, organisms can Figure 52.20  A sea urchin’s expanding range. Since the
avoid some stressful conditions temporarily through behaviors 1950s, water temperatures along the coast of Tasmania have increased,
such as dormancy or hibernation (see Concept 40.4). allowing the sea urchin C. rodgersii to expand its range to the south. The
bold orange type indicates the years when C. rodgersii was first observed
Temperature
colonizing those locations. Once its population has
Environmental temperature is an important factor in the been established in a new location, C. rodgersii
distribution of organisms because of its effect on biological has eliminated the local kelp community.
processes. Cells may rupture if the water they contain freezes
(at temperatures below 0°C), and the proteins of most organ- AUSTRALIA 1 Southern
isms denature at temperatures above 45°C. Organisms typi- TASMANIA portion of
cally function best within a specific range of environmental C. rodgersii
temperature. Temperatures outside that range may force native range
some animals to expend energy regulating their internal
temperature, as mammals and birds do (see Figure 40.17). AUSTRALIA East Australian
Extraordinary adaptations enable certain organisms, such Current
as thermophilic prokaryotes, to live outside the temperature
range habitable by other life. 1960s
2 Expanded range
As mentioned earlier, climate change has already caused of C. rodgersii;
hundreds of species to alter their geographic ranges. A shift kelp communities
in the range of one species can also have profound effects
on the distribution of other species. Consider how rising sea 1978 decimated
temperatures have affected the geographic range of the sea
urchin C. rodgersii. Since 1950, water temperatures along the TASMANIA 1980s
coast of Tasmania, an island south of mainland Australia, have
increased from 11.5°C to 12.5°C. This has enabled C. rodgersii— 1980s
whose larvae fail to develop properly if temperatures drop
below 12°C—to expand its range to the south (Figure 52.20). 1980s
The urchin is a voracious consumer of kelp and other algae. As
a result, algal communities that once harbored a rich diversity 2005
of other species have been completely destroyed in regions
where the urchin has become well established (denoted by a 3 Continuation of expanded range;
solid orange line in Figure 52.20). kelp communities threatened
100 km
Water and Oxygen
many flooded wetlands, the surface waters of streams and
The dramatic variation in water availability among habitats rivers tend to be well oxygenated because of rapid exchange
is another important factor in species distribution. Species of gases with the atmosphere.
living at the seashore or in tidal wetlands can desiccate (dry
out) as the tide recedes. Terrestrial organisms face a nearly Salinity
constant threat of desiccation, and the distribution of terres-
trial species reflects their ability to obtain and conserve water. The salt concentration of water in the environment affects
Many amphibians, such as the tiny frog in Figure 52.1, are the water balance of organisms through osmosis. Most
particularly vulnerable to drying because they use their moist, aquatic organisms are restricted to either freshwater or salt-
delicate skin for gas exchange. Desert organisms exhibit a water habitats by their limited ability to osmoregulate (see
variety of adaptations for acquiring and conserving water in Concept 44.1). Although most terrestrial organisms can
dry environments, as described in Concept 44.4. excrete excess salts from specialized glands or in feces or
urine, high-salinity habitats typically have few species of
Water affects oxygen availability in aquatic environments plants or animals. In the Scientific Skills Exercise, you can
and in flooded soils, where the slow diffusion of oxygen in interpret data from an experiment investigating the influ-
water can limit cellular respiration and other physiological ence of salinity on plant distributions.
processes. Oxygen concentrations can be particularly low
in deep ocean and deep lake waters as well as in sediments Salmon that migrate between freshwater streams and the
where organic matter is abundant. Flooded wetland soils ocean use both behavioral and physiological mechanisms to
may also have low oxygen content. Mangroves and other osmoregulate. They balance their salt content by adjusting the
trees have specialized roots that project above the water and amount of water they drink and by switching their gills from
help the root system obtain oxygen (see Figure 35.4). Unlike taking up salt in fresh water to excreting salt in the ocean.

Sunlight

Sunlight provides the energy that drives most ecosystems, and
too little sunlight can limit the distribution of photosynthetic
species. In forests, shading by leaves makes competition for
light especially intense, particularly for seedlings growing on
the forest floor. In aquatic environments, every meter of water
depth absorbs about 45% of the red light and about 2% of the

chapter 52  An Introduction to Ecology and the Biosphere 1183

Scientific Skills Exercise

Making a Bar Graph and a Line Graph
to Interpret Data

How Do Salinity and Competition Affect the Distribution
of Plants in an Estuary?  Field observations show that Spartina
patens (salt marsh hay) is a dominant plant in salt marshes and Typha
angustifolia (cattail) is a dominant plant in freshwater marshes. In
this exercise, you will graph and interpret data from an experiment
that examined the influence of an abiotic factor, salinity, and a biotic
factor, competition, on the growth of these two species.

How the Experiment Was Done  Researchers planted S. patens ▲ Spartina patens ▲ Typha angustifolia
and T. angustifolia in salt marshes and freshwater marshes with and
without neighboring plants. After two growing seasons (1.5 years), Interpret The Data
they measured the biomass of each species in each treatment. The
researchers also grew both species in a greenhouse at six salinity 1. Make a bar graph of the data from the field experiment. (For
levels and measured the biomass at each level after eight weeks. additional information about graphs, see the Scientific Skills
Review in Appendix F.) What do these data indicate about the
Data from the Field Experiment  (averages of 16 replicate samples) salinity tolerances of S. patens and T. angustifolia?

Average Biomass (g/100 cm2) 2. What do the data from the field experiment indicate about the
effect of competition on the growth of these two species? Which
Spartina patens Typha angustifolia species was limited more by competition?

Salt Freshwater Salt Freshwater 3. Make a line graph of the data from the greenhouse experiment.
Marshes Marshes Marshes Marshes Decide which values constitute the dependent and independent
variables, and use these values to set up the axes of your graph.
With neighbors  8  3 0 18
Without neighbors 4. (a) In the field, S. patens is typically absent from freshwater
10 20 0 33 marshes. Based on the data, does this appear to be due to salinity
or competition? Explain your answer. (b) T. angustifolia does not
Data from the Greenhouse Experiment grow in salt marshes. Does this appear to be due to salinity
or competition? Explain your answer.
Salinity (parts per thousand)  0 20 40 60 80 100
Instructors: A version of this Scientific Skills Exercise can be
% maximum biomass (S. patens) 77 40 29 17  9   0 assigned in MasteringBiology.

% maximum biomass (T. angustifolia) 80 20 10  0  0   0

Data from  C. M. Crain et al., Physical and biotic drivers of plant distribution across
estuarine salinity gradients, Ecology 85:2539–2549 (2004).

blue light passing through it. As a result, most photosynthesis the solubility of toxins and nutrients. Soil phosphorus, for
occurs relatively near the water surface. instance, is relatively insoluble in basic soils and precipitates
into forms unavailable to plants.
Too much light can also limit the survival of organisms. In
some ecosystems, such as deserts, high light levels can increase In a river, the composition of rocks and soil that make up
temperature stress if animals and plants are unable to avoid the substrate (riverbed) can affect water chemistry, which
the light or to cool themselves through evaporation (see in turn influences the resident organisms. In freshwater and
Figure 40.12). At high elevations, the sun’s rays are more likely
to damage DNA and proteins because the atmosphere is thin- Figure 52.21  Alpine tree line in Banff National Park,
ner, absorbing less ultraviolet (UV) radiation. Damage from Canada. Organisms living at high elevations are exposed not only to
UV radiation, combined with other abiotic stresses, prevents high levels of ultraviolet radiation but also to freezing temperatures,
trees from surviving above a certain elevation, resulting in the moisture deficits, and strong winds. Above the tree line, the
appearance of a tree line on mountain slopes (Figure 52.21). combination of such factors restricts the growth and survival of trees.

Rocks and Soil

In terrestrial environments, the pH, mineral composition,
and physical structure of rocks and soil limit the distribution
of plants and thus of the animals that feed on them, contrib-
uting to the patchiness of terrestrial ecosystems. The pH of
soil can limit the distribution of organisms directly, through
extreme acidic or basic conditions, or indirectly, by affecting

1184 Unit eight  Ecology

marine environments, the structure of the substrate deter- Figure 52.22  Reciprocal effects of ecological and
mines the organisms that can attach to it or burrow into it. evolutionary change. An ecological change, such as the expansion
of a predator’s range, can alter the selective pressures faced by prey
Concept Check 52.4 populations. This could cause evolutionary change, such as an increase
in the frequency of a new defensive mechanism in a prey population;
1. Give examples of human actions that could expand that change, in turn, could alter the outcome of ecological interactions.
a species’ distribution by changing its (a) dispersal or
(b) biotic interactions. Ecological
change
2. WHAT IF? You suspect that deer are restricting
the distribution of a tree species by preferentially Alters outcome Alters selective
eating the seedlings of the tree. How might you test of ecological pressures in
this hypothesis? interactions population

3. MAKE CONNECTIONS Hawaiian silverswords under- Evolutionary
went a remarkable adaptive radiation after their ances- change
tor reached Hawaii, while the islands were still young
(see Figure 25.22). Would you expect the cattle egret example, Trinidadian guppy (Poecilia reticulata) populations
to undergo a similar adaptive radiation in the Americas evolve rapidly when predators are removed: guppy color
(see Figure 52.18)? Explain. patterns change (see the Chapter 22 Scientific Skills Exercise)
and guppies produce fewer but larger offspring. In turn, the
For suggested answers, see Appendix A. evolution of larger body sizes altered the availability of nitro-
gen in these stream ecosystems (Figure 52.23). Larger fish
Concept  52.5 excrete more nitrogen than do smaller fish, and nitrogenous
wastes contribute to the growth of primary producers such
Ecological change and evolution as algae. Overall, this and other studies show that ecological
affect one another over long change and evolution have the potential to exert rapid feed-
and short periods of time back effects on each other. This additional layer of complex-
ity must be considered when predicting how human actions
Biologists have long recognized that ecological interac- or other events will affect the natural world.
tions can cause evolutionary change, and vice versa
(Figure 52.22). The history of life includes many examples Figure 52.23  An Estimated total nitrogen 60
of these reciprocal effects occurring over long periods of excretion rate for population
time. Consider the origin and diversification of plants. example of reciprocal 40
As described in Concept 29.3, the evolutionary origin of [μg N/(m2 • hr)]
plants altered the chemical cycling of carbon, leading to eco-evolutionary 20
the removal of large quantities of carbon dioxide from the effects. In low-predation
atmosphere. As the adaptive radiation of plants continued ponds, evolution in the 0 Guppies Guppies
over time, the appearance of new plant species provided guppy (Poecilia reticulata)
new habitats and new sources of food for insects and other leads to larger body sizes.
animals. In turn, the availability of new habitats and new Populations of these
food sources stimulated bursts of speciation in animals, larger fish excrete more
leading to further ecological changes. Here, as in many nitrogen than do the
other such examples, ecological and evolutionary changes smaller fish found in
had ongoing and major effects upon one another. high-predation ponds.

The interplay between ecological and evolutionary from high- from low-
change illustrated by the origin of plants occurred over predation predation
millions of years. Reciprocal “eco-evolutionary” effects ponds ponds
occurring over centuries or thousands of years are also well
documented, as in the examples of the mosquitofish and Concept Check 52.5
the apple maggot fly discussed in Concept 24.2. But are such
joint effects common over much shorter periods of time? 1. Describe a scenario showing how ecological change and
As we’ve seen in previous chapters, ecological change can evolution can affect one another.
cause evolutionary change over the course of a few years
to decades; examples include beak length evolution in 2. MAKE CONNECTIONS Commercial fisheries target
soapberry bugs (see Figure 22.13) and the formation of new older, larger cod fish, causing cod that reproduce at a
sunflower species (see Figure 24.18). younger age and smaller size to be favored by natural
selection. Younger, smaller cod have fewer offspring than
Recent studies show that the causation can run both do older, larger cod. Predict how evolution in response
ways: Rapid evolution can also cause ecological change. For to fishing would affect the ability of a cod population
to recover from overfishing. What other reciprocal eco-
evolutionary effects might occur? (See Concept 23.3.)
For suggested answers, see Appendix A.

chapter 52  An Introduction to Ecology and the Biosphere 1185

52 Chapter Review Go to MasteringBiology™ for Videos, Animations, Vocab Self-Quiz,
Practice Tests, and more in the Study Area.
Summary of Key Concepts
Concept  52.4
Concept  52.1
Interactions between organisms and
Earth’s climate varies by latitude the environment limit the distribution
of species (pp. 1176–1185)
and season and is changing rapidly
Ecologists want to know not only where species occur but also why
(pp. 1165–1168) VOCAB those species occur where they do.
SELF-QUIZ
goo.gl/6u55ks
Global climate patterns are largely determined
by the input of solar energy and Earth’s revolution Yes Area inaccessible or
around the sun. Does dispersal limit its distribution? insufficient time
No
The changing angle of the sun over the year, bodies of water, and
mountains exert seasonal, regional, and local effects on climate.
Fine-scale differences in certain abiotic (nonliving) factors, such
as sunlight and temperature, determine microclimate. Do biotic factors (other species) Yes Predation, parasitism,
Increasing greenhouse gas concentrations in the air are warming limit its distribution? competition, disease

Earth and altering the distributions of many species. Some species No Chemical factors:
will not be able to shift their ranges quickly enough to reach suit- Do abiotic factors limit its distribution? Water, oxygen, salinity, pH,
able habitat in the future. soil nutrients, etc.
Physical factors:
? Suppose global air circulation suddenly reversed, with moist air ascending Temperature, light, soil
at 30° north and south latitude and descending at the equator. At what structure, fire, moisture, etc.

latitude would you most likely find deserts in this scenario?

Concept  52.2 Visual Skills If you were an ecologist studying the chemical and
physical limits to the distributions of species, how might you rearrange the
The distribution of terrestrial biomes is controlled
by climate and disturbance (pp. 1168–1174) flowchart preceding this question?

Climographs show that temperature and precipitation are Concept  52.5
correlated with biomes. Because other factors also play roles
in biome location, biomes overlap. Ecological change and evolution affect
one another over long and short periods
Terrestrial biomes are often named for major physical or climatic of time (p. 1185)
factors and for their predominant vegetation. Vertical layering is
an important feature of terrestrial biomes. Ecological interactions can cause evolutionary change, as when
predators cause natural selection in a prey population.
Disturbance, both natural and human-induced, influences
the type of vegetation found in biomes. Humans have altered Likewise, an evolutionary change, such as an increase in the
much of Earth’s surface, replacing the natural terrestrial com- frequency of a new defensive mechanism in a prey population,
munities described and depicted in Figure 52.12 with urban can alter the outcome of ecological interactions.
and agricultural ones.
? Suppose humans introduced a species to a new continent where it had
The pattern of climatic variation is as important as the average few predators or parasites. How might this lead to eco-evolutionary
climate in determining where biomes occur.
feedback effects?
? In what ways are disturbances important for savanna ecosystems
and the plants in them?

Concept  52.3 Test Your Understanding

Aquatic biomes are diverse and dynamic systems Level 1: Knowledge/Comprehension
that cover most of Earth (pp. 1175–1176)

Aquatic biomes are characterized primarily by their physical envi- 1. Which of the following areas of study focuses on PRACTICE
ronment rather than by climate and are often layered with regard to the exchange of energy, organisms, and materials TEST
light penetration, temperature, and community structure. Marine between ecosystems?
biomes have a higher salt concentration than freshwater biomes. (A) organismal ecology goo.gl/CUYGKD
(B) landscape ecology
In the ocean and in most lakes, an abrupt temperature change (C) ecosystem ecology
called a thermocline separates a more uniformly warm upper (D) community ecology
layer from more uniformly cold deeper waters.
2. Which lake zone would be absent in a very shallow lake?
Many temperate lakes undergo a turnover, or mixing of water in (A) benthic zone
spring and fall, that sends deep, nutrient-rich water to the surface (B) aphotic zone
and shallow, oxygen-rich water to deeper layers. (C) pelagic zone
(D) littoral zone
? In which aquatic biomes might you find an aphotic zone?

1186 Unit eight  Ecology

Level 2: Application/Analysis 10. EVOLUTION CONNECTION  Discuss how the distribution of a
species can be affected both by its evolutionary history and by
3. Which of the following is characteristic of most terrestrial ecological factors. Could ongoing evolutionary change also
biomes? affect its distribution? Explain.
(A) a distribution predicted almost entirely by rock and soil
patterns 11. SCIENTIFIC INQUIRY  Jens Clausen and colleagues, at the
(B) clear boundaries between adjacent biomes Carnegie Institution of Washington, studied how the size of
(C) vegetation demonstrating vertical layering yarrow plants (Achillea lanulosa) growing on the slopes of the
(D) cold winter months Sierra Nevada varied with elevation. They found that plants
from low elevations were generally taller than plants from high
4. The oceans affect the biosphere in all of the following ways elevations, as shown in the diagram.
except
(A) producing a substantial amount of the biosphere’s oxygen. Altitude (m) Mean height (cm) 75
(B) removing carbon dioxide from the atmosphere.
(C) moderating the climate of terrestrial biomes. 50
(D) regulating the pH of freshwater biomes and terrestrial
groundwater. 25

5. Which statement about dispersal is false? 0
(A) Dispersal is a common component of the life cycles of
plants and animals. 3,000
(B) Colonization of devastated areas after floods or volcanic
eruptions depends on dispersal. 2,000 Sierra Nevada Great Basin
(C) Dispersal occurs only on an evolutionary time scale. 1,000 Plateau
(D) The ability to disperse can expand the geographic distribu-
tion of a species. 0

6. When climbing a mountain, we can observe transitions in Seed collection sites
biological communities that are analogous to the changes
(A) in biomes at different latitudes. Data from  J. Clausen et al., Experimental studies on the nature of species.
(B) in different depths in the ocean. III. Environmental responses of climatic races of Achillea, Carnegie Institution
(C) in a community through different seasons. of Washington Publication No. 581 (1948).
(D) in an ecosystem as it evolves over time.
Clausen and colleagues proposed two hypotheses to explain
7. Suppose that the number of bird species is determined mainly this variation within a species: (1) There are genetic differences
by the number of vertical strata found in the environment. between populations of plants found at different elevations.
If so, in which of the following biomes would you find the (2) The species has developmental flexibility and can assume
greatest number of bird species? tall or short growth forms, depending on local abiotic factors.
(A) tropical rain forest If you had seeds from yarrow plants found at low and high
(B) savanna elevations, how would you test these hypotheses?
(C) desert 12. WRITE ABOUT A THEME: INTERACTIONS  Global warming is
(D) temperate broadleaf forest occurring rapidly in arctic marine and terrestrial ecosystems.
The reflective white snow and ice cover are melting quickly
Level 3: Synthesis/Evaluation and extensively, uncovering darker-colored ocean water, plants,
and rocks. In a short essay (100–150 words), explain how this
8. WHAT IF?  If the direction of Earth’s rotation reversed, the process might exemplify positive feedback.
most predictable effect would be 13. SYNTHESIZE YOUR KNOWLEDGE 
(A) a big change in the length of the year.
(B) winds blowing from west to east along the equator. If you were to hike up Mount
(C) a loss of seasonal variation at high latitudes. Kilimanjaro in Tanzania, you
(D) the elimination of ocean currents. would pass through several
habitats, including savanna at
9. INTERPRET THE DATA  After examining Figure 52.19, you the base, forest on the slopes,
decide to study feeding relationships among sea otters, sea and alpine tundra near the
urchins, and kelp. You know that sea otters prey on sea urchins top. Explain how such diverse
and that urchins eat kelp. At four coastal sites, you measure habitats can be found at one
kelp abundance. Then you spend one day at each site and mark location near the equator.
whether otters are present or absent every 5 minutes during
the day. Graph kelp abundance (on the y-axis) versus otter
density (on the x-axis), using the data below. Then formulate a
hypothesis to explain any pattern you observe.

Otter Density Kelp Abundance
Site (# sightings per day) (% cover)
For selected answers, see Appendix A.
1 98 75
For additional practice questions, check out the Dynamic Study
2 18 15 Modules in MasteringBiology. You can use them to study on
your smartphone, tablet, or computer anytime, anywhere!
3 85 60

4 36 25

chapter 52  An Introduction to Ecology and the Biosphere 1187