3–2 Energy Flow - Tripod.com

3–2 Energy Flow Section 3–2

At the core of every organism’s interaction with the environ- Key Concepts 1 FOCUS
ment is its need for energy to power life’s processes. • Where does the energy for life
Consider, for example, the energy that ants use to carry objects Objectives
many times their size or the energy that birds use to migrate processes come from?
thousands of miles. Think about the energy that you need to get • How does energy flow 3.2.1 Identify the source of energy
out of bed in the morning! The flow of energy through an ecosys- for life processes.
tem is one of the most important factors that determines the through living systems?
system’s capacity to sustain life. • How efficient is the transfer of 3.2.2 Trace the flow of energy
through living systems.
Producers energy among organisms in
an ecosystem? 3.2.3 Evaluate the efficiency of
Without a constant input of energy, living systems cannot energy transfer among organ-
function. Sunlight is the main energy source for life Vocabulary isms in an ecosystem.
on Earth. Of all the sun’s energy that reaches Earth’s surface, autotroph • producer
only a small amount—less than 1 percent—is used by living photosynthesis Vocabulary Preview
things. This seemingly small amount is enough to produce as chemosynthesis • heterotroph
much as 3.5 kilograms of living tissue per square meter a year consumer • herbivore To help students understand related
in some tropical forests. carnivore • omnivore terms in this section, write the follow-
detritivore • decomposer ing sets of words and word parts on
In a few ecosystems, some organisms obtain energy from a food chain • food web the board.
source other than sunlight. Some types of organisms trophic level Set 1: photo-, chemo-, synthesis
rely on the energy stored in inorganic chemical com- ecological pyramid • biomass Set 2: herb-, carn-, omni-, detritus,
pounds. For instance, mineral water that flows underground or -vore
boils out of hot springs and undersea vents is loaded with Reading Strategy: Have students look up the meaning
chemical energy. Building Vocabulary As of all words and parts in a dictionary
you read, make notes about the and list them. As students read the
Only plants, some algae, and certain bacteria can capture meaning of each term in the list section and make notes about the
energy from sunlight or chemicals and use that energy to produce above and how it relates to terms, they can check the text’s defi-
food. These organisms are called autotrophs. Autotrophs use energy flow in the biosphere. nitions against this list.
energy from the environment to fuel the assembly of simple Then, draw a concept map to
inorganic compounds into complex organic molecules. These show the relationships among Reading Strategy
organic molecules combine and recombine to produce living tissue. these terms.
Because they make their own food, autotrophs, like the kelp in Students’ concept maps could be
Figure 3–4, are also called producers. Both types of producers— titled “Energy Flow” and begin with
those that capture energy from sunlight and those that capture autotrophs, or producers, which
chemical energy—are essential to the flow of energy through the make food through photosynthesis
biosphere. or chemosynthesis. Then, students
should add the various types of het-
̄ Figure 3–4 Sunlight falls on a dense kelp forest off the coast erotrophs to their concept maps and
show how the various types of organ-
of California. Kelp is an autotroph that uses energy from isms are interrelated, using the terms
the sun to produce living tissue. food chain, food web, trophic level,
and ecological pyramid.
SECTION RESOURCES
2 INSTRUCT
Print: Technology:
Producers
• Teaching Resources, Section Review 3–2 • iText, Section 3–2
• Transparencies Plus, Section 3–2 Building Science Skills
• Reading and Study Workbook A, Section 3–2
WeorSkabvoeok Measuring Have groups of students
• Adapted Reading and Study B, cut out one-square-meter pieces of
Section 3–2 heavy wrapping or butcher paper.
Tim r Next, let each group use a balance
and various common objects in the
• Lesson Plans, Section 3–2 classroom to measure out 3.5 kg of
mass, and then place the objects on
the paper square. Encourage the
groups to examine one another’s
piles of objects. Emphasize that each
pile represents the amount of living
tissue produced per square meter
each year in a tropical forest.

The Biosphere 67

3–2 (continued) Light Energy From the Sun The best-known
Energy
Make Connections autotrophs are those that harness solar energy
Light through a process known as photosynthesis.
Chemistry On the board, write the During photosynthesis, these autotrophs use
chemical equation for photosynthesis: Carbon Energy light energy to power chemical reactions that
6CO2 ϩ 6H2O enligehrgty>C6 H12O6 ϩ 6O2 dioxide convert carbon dioxide and water into oxygen
Ask: Which element does each let- + Water Carbohydrates + Oxygen and energy-rich carbohydrates such as sugars
ter in the formulas stand for? (C for and starches. This process, shown in Figure 3–5
carbon; O, oxygen; H, hydrogen) PHOTOSYNTHESIS IN PLANTS (top), is responsible for adding oxygen to—and
Explain that the equation can be read removing carbon dioxide from—Earth’s atmos-
as “Six molecules of carbon dioxide Bacterial Cell phere. In fact, were it not for photosynthetic
and six molecules of water combine autotrophs, the air would not contain enough
in the presence of light energy to Hydrogen sulfide and oxygen oxygen for you to breathe!
yield one molecule of glucose and six combine, forming sulfur compounds.
molecules of oxygen.” Ask: Why are On land, plants are the main autotrophs.
the numbers needed in the equa- Chemical Energy In freshwater ecosystems and in the sunlit upper
tion? (Without numbers, the equation layers of the ocean, algae are the main autotrophs.
wouldn’t be balanced.) If students are Cells make carbohydrates using Photosynthetic bacteria, the most common of
not familiar with this concept, write carbon dioxide from sea water. which are the cyanobacteria (sy-an-oh-bak-
the equation on the board, and then Deep-Sea TEER-ee-uh), are important in certain wet
cross out the balanced pairs—6 car- Vent ecosystems such as tidal flats and salt marshes.
bon atoms (6C) on the left and 6
carbon atoms (C6) on the right; 18 CHEMOSYNTHESIS IN SULFUR BACTERIA Life Without Light Although plants are the
oxygen atoms (6O2 ϩ 6O) on the left
and 18 (O6 ϩ 6O2) on the right; 12 Figure 3–5 Sunlight is the main energy source most visible and best-known autotrophs, some
hydrogen atoms (6H2) on the left for life on Earth. Some types of organisms rely on autotrophs can produce food in the absence of
and 12 (H12) on the right. the energy stored in inorganic chemical com- light. Such autotrophs rely on energy within the
pounds. Plants use the energy from sunlight to carry chemical bonds of inorganic molecules such as
Consumers out the process of photosynthesis. Other autotrophs, hydrogen sulfide. When organisms use chemical
such as sulfur bacteria, use the energy stored in energy to produce carbohydrates, the process is
Build Science Skills chemical bonds for chemosynthesis. In both cases, called chemosynthesis (kee-moh-SIN-thuh-
energy-rich carbohydrates are produced. sis), as shown in Figure 3–5 (bottom). This
Classifying Divide the class into process is performed by several types of bacteria.
small groups, and provide each Surprisingly, these bacteria represent a large
group with photocopies of a wide proportion of living autotrophs. Some chemosyn-
variety of organisms, including thetic bacteria live in very remote places on
plants, multicellular algae, inverte- Earth, such as volcanic vents on the deep-ocean
brates, and vertebrates. Then, have floor and hot springs in Yellowstone Park. Others
each group sort its organisms into live in more common places, such as tidal
two piles—producers and con- marshes along the coast.
sumers—and then sort the
consumers into piles representing the What is the difference between
four subcategories of herbivores, photosynthesis and chemosynthesis?
carnivores, omnivores, and decom-
posers. Consumers

Many organisms—including animals, fungi, and
many bacteria—cannot harness energy directly
from the physical environment as autotrophs do.
The only way these organisms can acquire energy
is from other organisms. Organisms that rely on
other organisms for their energy and food supply
are called heterotrophs (HET-ur-oh-trohfs).
Heterotrophs are also called consumers.

Inclusion/Special Needs Advanced Learners
To engage students’ interest in feeding relation- Point out to interested students this sentence on
ships and ecological pyramids, ask students page 68 about chemosynthetic bacteria:
about feeding relationships with which they “Surprisingly, these bacteria represent a large
may have some familiarity. For example, most proportion of living autotrophs.” Challenge stu-
students will know that birds in their neighbor- dents to find out about such bacteria, including
hood feed on either seeds and berries or small those that live in hot springs and those that live
animals such as worms and insects. Elicit from in the deep ocean around vents. Have students
students ideas about relative numbers at differ- prepare reports about what they find and pre-
ent trophic levels, energy transfer, and biomass sent them to the class when students study bac-
comparisons. teria in Chapter 19.

68 Chapter 3

There are many different types of heterotrophs. Herbivores ̆ Figure 3–6 This fungus, Feeding
obtain energy by eating only plants. Some herbivores are cows, Relationships
caterpillars, and deer. Carnivores, including snakes, dogs, growing on the forest floor, is a
and owls, eat animals. Humans, bears, crows, and other decomposer that obtains nutrients Use Visuals
omnivores eat both plants and animals. Detritivores, by breaking down dead and
(dee-TRYT-uh-vawrz), such as mites, earthworms, snails, and decaying plants and animals. It is Figure 3–7 After students have
crabs, feed on plant and animal remains and other dead matter, called a coral fungus because of its studied the figure, ask: Among the
collectively called detritus. Another important group of het- color and shape. Classifying Is organisms shown, which are
erotrophs, called decomposers, breaks down organic matter. the fungus a producer or a consumer? autotrophs and which are het-
Bacteria and fungi, such as the one in Figure 3–6, are decomposers. erotrophs? (The algae are autotrophs;
the others are heterotrophs.) Among
Feeding Relationships the heterotrophs, which are herbi-
vores and which are carnivores?
What happens to the energy in an ecosystem when one organ- (The zooplankton are herbivores; the
ism eats another? That energy moves along a one-way path. other heterotrophs are carnivores.)
What kind of heterotrophs might
Energy flows through an ecosystem in one direction, enter this food chain when the
from the sun or inorganic compounds to autotrophs shark dies and falls to the ocean
(producers) and then to various heterotrophs (consumers). floor? (Detritivores and decomposers)
The relationships between producers and consumers connect
organisms into feeding networks based on who eats whom. Build Science Skills

Food Chains The energy stored by producers can be passed Applying Concepts Show students
some acorns, sunflower seeds, or
through an ecosystem along a food chain, a series of steps in other common type of seed, and ask:
which organisms transfer energy by eating and being eaten. For Where did these seeds come from?
example, in a prairie ecosystem, a food chain might consist of a (A plant) What kind of animal might
producer, such as grass, that is fed upon by a herbivore, such as eat these seeds? (Depending on the
a grazing antelope. The herbivore is in turn fed upon by a type of seeds used, a squirrel, chick-
carnivore, such as a coyote. In this situation, the carnivore is adee, mouse, or chipmunk might eat
only two steps removed from the producer. them.) What kind of animal might
eat the animal that ate the seeds?
In some marine food chains, such as the one in Figure 3–7, (A larger carnivore such as a fox, hawk,
the producers are microscopic algae that are eaten by very small or coyote) What is the feeding rela-
organisms called zooplankton (zoh-oh-PLANK-tun). The zoo- tionship that you just described
plankton, in turn, are eaten by small fish, such as herring. The called? (A food chain) What happens
herring are eaten by squid, which are ultimately eaten by large to energy in the food chain?
fish, such as sharks. In this food chain, the top carnivore is four (Energy is transferred from the organ-
steps removed from the producer. ism being eaten to the organism doing
the eating. Some energy from lower
̄ Figure 3–7 Food chains show the one- trophic levels is lost as heat.) What
was the original source of energy
way flow of energy in an ecosystem. In this in the food chain? (The sun)
marine food chain, energy is passed from the
producers (algae) to four different groups of
consumers.

Small fish

Zooplankton

Squid

Algae Shark

FACTS AND FIGURES

Energy moves up the chain feed directly on the producers. Above the primary Answers to . . .
In nature, simple “straight line” food chains are consumers are secondary consumers, then tertiary
rare, primarily because few species eat or are consumers, and, in some food chains, quaternary Photosynthesis uses light
eaten by only one other species. Nevertheless, a consumers. Not many food chains extend beyond energy. Chemosynthesis uses the ener-
food chain is a useful model for studying the four consumer levels. Decomposers (also known gy stored in chemical bonds.
transfer of energy and materials in an ecosystem. as saprotrophs), detritivores, and parasites—
organisms that live in or on other organisms and Figure 3–6 A consumer
All food chains on land begin with producers obtain energy from them—can occupy any level
that use light energy to synthesize organic com- of a food chain. The Biosphere 69
pounds. Primary consumers are herbivores that

3–2 (continued)

Objective Students will be able to How is a food chain organized?
describe the organization of a sim-
ple food chain. Materials 2 wide-mouth jars, 2 pieces of flexible
screening, 2 rubber bands, 2 bean seedlings in small
Skill Focus Classifying pots or paper cups, pea aphids, ladybird beetles

Materials 2 wide-mouth jars, 2 Procedure 5. Place both jars in a sunny location. Observe the jars
pieces of flexible screening, 2 rub- each day for one week and record your observa-
ber bands, 2 bean seedlings in small 1. Place a potted bean seedling in each of the tions each day. Water the seedlings as needed.
pots or paper cups, pea aphids, two jars.
ladybird beetles Analyze and Conclude
2. Add 20 aphids to one jar and cover the jar with 1. Observing What happened to the aphids and the
Time 15 minutes for initial setup, 5 screening to prevent the aphids from escaping. Use
minutes per day for one week to a rubber band to attach the screening to the jar. seedling in the jar without the ladybird beetles? In
observe and record the jar with the ladybird beetles? How can you
3. Add 20 aphids and 4 ladybird beetles to the second explain this difference?
Advance Prep jar. Cover the second jar as you did the first one. 2. Classifying Identify each organism in the jars as
• About two weeks before students a producer or a consumer.
4. Formulating Hypotheses Record your hypothesis
do this activity, plant bean seeds in about how the presence of the ladybird beetles will
pots or paper cups. Each group will affect the survival of the aphids and the bean
need two seedlings. Plant extras in seedling. Also, record your prediction of what will
case some plants do not thrive. happen to the organisms in each jar during the
• Aphids and ladybird beetles may be next week.
collected outdoors or ordered from
a biological supply house. Ladybird Trophic originates from the Greek Food Webs In most ecosystems, feeding relationships are
beetles also may be available at word trophe, which means “food
garden centers as natural pest- or nourishment.” What do you more complex than can be shown in a food chain. Consider, for
controls. If the organisms are col- think are the original meanings example, the relationships in a salt marsh. Although some
lected outdoors, make sure they are of the words heterotroph and producers—including marsh grass and other salt-tolerant
returned to their original locations autotroph? plants—are eaten by water birds, grasshoppers, and other
at the conclusion of the activity. herbivores, most producers complete their life cycles, then die
• Because it may be difficult to and decompose. Decomposers convert the dead plant matter to
obtain pea aphids and ladybird detritus, which is eaten by detritivores, such as sandhoppers.
beetles, you may want to use crick- The detritivores are in turn eaten by smelt and other small fish.
ets and praying mantises instead. Some of those consumers will also eat detritus directly. Add
mice, larger fish, and hawks to the scenario, and feeding rela-
Safety Caution students to handle tionships can get very confusing!
organisms without harming them.
Make sure they wash their hands When the feeding relationships among the various organ-
with soap and warm water before isms in an ecosystem form a network of complex interactions,
leaving the lab. ecologists describe these relationships as a food web. A food
web links all the food chains in an ecosystem together. The food
Strategies web in Figure 3–8, for example, shows the feeding relationships
• Make the aphids and ladybird in a salt-marsh community.

beetles available to students in a Trophic Levels Each step in a food chain or food web is
central distribution center.
• You may want to let students called a trophic (TRAHF-ik) level. Producers make up the
examine the aphids with magnifiers first trophic level. Consumers make up the second, third, or
before they place them in the jars. higher trophic levels. Each consumer depends on the trophic
level below it for energy.
Expected Outcome See Analyze
and Conclude number 1 below. What is a food web?

Analyze and Conclude FACTS AND FIGURES
1. In the jar without ladybird bee-
tles, the uncontrolled aphids Two types of food webs dead organisms. This process releases simple inor-
harmed (or perhaps killed) the There are two basic types of food webs: grazing ganic molecules such as mineral salts, carbon,
seedling. In the jar with ladybird food webs and detrital food webs. A grazing food nitrogen, phosphorous, and potassium, making
beetles, the seedling was less dam- web begins with photosynthesizing plants, algae, these nutrients available for reuse by producers
aged and survived longer. The or phytoplankton. A detrital food web begins with and, eventually, all other organisms in the ecosys-
ladybird beetles helped protect the decomposers and detritivores. It is the detrital type tem. Without decomposers and detritivores, such
seedling by eating some of the of food web that enables nutrients to be recycled essential elements would remain in animal wastes
aphids that were feeding on it. in ecosystems. and dead organisms.

2. The seedlings are producers; the Decomposers and detritivores obtain energy by
aphids and ladybird beetles are breaking down organic wastes and the remains of
consumers.

70 Chapter 3

FIGURE 3–8 FOOD WEB IN A SALT MARSH Marsh hawk Use Visuals

This illustration of a food web shows some Figure 3–8 To help students deal
of the feeding relationships in a salt marsh. with the complexity of the food web,
Interpreting Graphics What does the call on different students in turn to
marsh hawk feed on? name the organisms in one food
chain, beginning with a producer
Top-level and working upward to the final con-
Carnivores sumer. For example, students might
identify a food web including algae,
Heron zooplankton, plankton-eating fishes,
and heron.
Plankton-eating fishes Clapper rail Shrew
(omnivore) Build Science Skills
Harvest mouse
First-level (omnivore) Making Models Obtain at least 25
Carnivores Grasshopper pictures of organisms—producers
and different-level consumers—that
Ribbed could be found in an ecosystem
mussel other than the one shown in Figure
3–8. Tape the pictures in random
Sandhopper order on the classroom walls, desk-
tops, and other surfaces. Give each
Zooplankton student a small ball of colored yarn
and several small pieces of masking
Herbivores tape. Then, let four or five students at
a time connect pictures with yarn to
Detritus show different food chains. Students
may crisscross the room with the
Marsh grass yarn so the food web becomes quite
Decomposers complex. When every student has
had a turn, let the class examine the
results. Ask: What is the name for
this pattern of feeding relation-
ships? (A food web) How is a food
web different from a food chain?
(A food web contains many overlap-
ping food chains, so it is much more
complex than a single food chain.)

Hetero- means “other, different,” and
auto- means “self.” Thus, heterotroph
refers to an organism that feeds on
other organisms, and autotroph refers
to one that produces its own food.

Marsh grass Pickleweed
Producers
Algae

TEACHER TO TEACHER

I have students make a food-web poster for a The posters may be drawn free-hand, or stu- Answers to . . .
particular ecosystem or biome. The food web dents may cut and paste pictures from
must contain at least five food chains consisting magazines or computer printouts. I usually have A food web is the net-
of a producer, a primary consumer, and a sec- students explain their posters to the class in oral work of feeding relationships in an
ondary consumer. Each consumer must be presentations. ecosystem.
labeled as an herbivore, carnivore, omnivore, or
decomposer. At least one predator-prey relation- —LouEllen Parker Brademan Figure 3–8 Birds and small mammals
ship must be shown. Five abiotic factors also Teacher
must be included and labeled. Potomac Senior High School The Biosphere 71
Dumfries, Virginia

3–2 (continued) Energy Pyramid Biomass Pyramid

Ecological Pyramids Shows the relative amount of energy available at each Represents the amount of 50 grams of
trophic level. Organisms use about 10 percent of this living organic matter at each human tissue
Make Connections energy for life processes. The rest is lost as heat. trophic level. Typically, the
greatest biomass is at the 500 grams
Mathematics Draw students’ atten- T 0.1% Third-level consumers base of the pyramid. of chicken
tion to the energy pyramid in Figure
3–9. Explain that the amount of ener- Light or A 1% Second-level consumers 5000 grams
gy available in food is measured in chemical E 10% First-level consumers of grain
calories. One calorie is the amount of energy
energy needed to raise the tempera-
ture of 1 gram of water 1°C. H
Scientists usually refer to the energy 100% Producers
content of food in units of kilocalo-
ries. One kilocalorie equals 1000 N S TA Ecological Pyramids
calories. A kilocalorie is also
expressed as a Calorie, with a capital For: Links on energy The amount of energy or matter in an ecosystem can be repre-
C. Then, pose the following problem: pyramids sented by an ecological pyramid. An ecological pyramid is a
Suppose that the base of this ener- diagram that shows the relative amounts of energy or matter
gy pyramid consists of plants that Visit: www.SciLinks.org contained within each trophic level in a food chain or food web.
contain 450,000 Calories of food Web Code: cbn-2032 Ecologists recognize three different types of ecological pyramids:
energy. If all the plants were eaten energy pyramids, biomass pyramids, and pyramids of numbers.
by mice and insects, how much Figure 3–9 shows an example of each type.
food energy would be available to
those first-level consumers? Energy Pyramid Theoretically, there is no limit to the
(45,000 Calories) If all the mice and
insects were eaten by snakes, how number of trophic levels that a food chain can support. But
much food energy would be avail- there is one hitch. Only part of the energy that is stored in one
able to the snakes? (4500 Calories) trophic level is passed on to the next level. This is because
If all the snakes were eaten by a organisms use much of the energy that they consume for life
hawk, how much food energy processes, such as respiration, movement, and reproduction.
would be available to the hawk? Some of the remaining energy is released into the environment
(450 Calories) How much food ener- as heat. Only about 10 percent of the energy available
gy would the hawk use for its body within one trophic level is transferred to organisms at
processes and lose as heat? (405 the next trophic level. For instance, one tenth of the solar
Calories—90 percent of 450) How energy captured by grasses ends up stored in the tissues of cows
much food energy would be stored and other grazers. Only one tenth of that energy—10 percent of
in the hawk’s body? (45 Calories) 10 percent, or 1 percent total—is transferred to the humans
that eat the cows. Thus, the more levels that exist between a
Build Science Skills producer and a top-level consumer in an ecosystem, the less
energy that remains from the original amount.
Applying Concepts Point out the
exception described in the text of a Biomass Pyramid The total amount of living tissue within
numbers pyramid. Ask: What would
be the shape of a numbers pyra- a given trophic level is called biomass. Biomass is usually
mid for the forest? (The pyramid’s expressed in terms of grams of organic matter per unit area. A
base, representing the trees, would be biomass pyramid represents the amount of potential food
much smaller than the second section, available for each trophic level in an ecosystem.
representing the insects that feed on
the trees.)

BIOLOGY UPDATE

N S TA The rule of 10 More recent studies have demonstrated that
The textbook’s discussion of energy pyramids energy efficiency varies between trophic levels in
Download a worksheet states that only about 10 percent of the energy a food chain and between different food chains.
on energy pyramids for students to available at each trophic level in a food chain is In fact, these recent studies have yielded approxi-
complete, and find additional teacher transferred to organisms at the next higher mations of energy efficiency ranging from a low
support from NSTA SciLinks. trophic level. This “rule of 10,“ which was based of 0.05 percent to a high of 20 percent.
on early studies of aquatic ecosystems, is useful as
72 Chapter 3 a general approximation. However, it does not
apply uniformly to all food chains.

Pyramid of Numbers 3 ASSESS

Shows the relative number Evaluate Understanding
of individual organisms at
each trophic level. Have each student draw and label a
food web for a specific ecosystem of
Figure 3–9 Ecological pyramids show the his or her choice. Tell students that
decreasing amounts of energy, living tissue, or the web should contain at least four
number of organisms at successive feeding levels. food chains and that each food chain
The pyramid is divided into sections that represent should consist of at least three
each trophic level. Because each trophic level organisms.
harvests only about one tenth of the energy
from the level below, it can support only about Reteach
one tenth the amount of living tissue.
Display a list of organisms that would
Pyramid of Numbers Ecological pyramids can also be be found in a specific ecosystem. Call
on students in turn to identify each
based on the numbers of individual organisms at each trophic organism as a producer or a con-
level. For some ecosystems, such as the meadow shown in sumer. Write P or C next to each
Figure 3–9 above, the shape of the pyramid of numbers is the organism’s name. Then, have stu-
same as that of the energy and biomass pyramids. This, however, dents further classify each consumer
is not always the case. In most forests, for example, there are as an herbivore, a carnivore, or an
fewer producers than there are consumers. A single tree has a omnivore; write H, C, or O next to
large amount of energy and biomass, but it is only one organism. each name. As a final step, have stu-
Many insects live in the tree, but they have less energy and dents in turn link together any three
biomass. Thus, a pyramid of numbers for a forest ecosystem organisms—a producer, an herbivore,
would not resemble a typical pyramid at all! and a carnivore—in a food chain.

3–2 Section Assessment Students may choose to describe
any of the several food chains
1. Key Concept What are 4. Explain the relationships in this Descriptive Writing shown in Figure 3–8. A typical
the two main forms of energy food chain: omnivore, herbivore, choice might begin with marsh
that power living systems? and autotroph. Refer to Figure 3– 8, which grass as the producer. The marsh
shows a food web in a salt grass is eaten by the grasshopper,
2. Key Concept Briefly 5. Critical Thinking marsh. Choose one of the which is eaten by the harvest
describe the flow of energy among Calculating Draw an energy food chains within this web. mouse, which is eaten by the
organisms in an ecosystem. pyramid for a five-step food Then, write a paragraph marsh hawk. All three animals
chain. If 100 percent of the describing the feeding should be identified as consumers.
3. Key Concept What energy is available at the first relationships among the Students might suggest that any
proportion of energy is trans- trophic level, what percentage of organisms in the food chain. or all of these organisms eventual-
ferred from one trophic level to the total energy is available at the Hint: Use the terms producers, ly die and are consumed by
the next in an ecosystem? highest trophic level? consumers, and decomposers decomposers.
in your description.
If your class subscribes to the iText,
use it to review the Key Concepts in
Section 3–2.

3–2 Section Assessment 4. The autotroph is the producer, and it is
eaten by the herbivore. The herbivore is
1. Solar energy is harnessed by autotrophs that then eaten by the omnivore.
conduct photosynthesis. Chemical energy—
the energy within the chemical bonds of 5. Students’ pyramids should show 100 per-
inorganic molecules—is harnessed by cent of the energy available at the first
autotrophs that conduct chemosynthesis. (producer) level, 10 percent at the second
level, 1 percent at the third level, 0.1 per-
2. Students should describe a one-way flow of cent at the fourth level, and 0.01 percent at
energy from autotrophs (producers) to con- the fifth level.
sumers—first herbivores, and then carnivores
and/or omnivores.

3. In general, about 10 percent

The Biosphere 73