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LESSON
Ecological Communities
Guiding Question: How do energy and nutrients move through
communities?
• Explain the difference between a producer and a
consumer.
• Explain the effect of inefficient energy transfer on
community structure.
• Describe how feeding relationships can have both
direct and indirect effects on community members.
3
Reading Strategy As you read, generate a concept map
using each of the boldface words highlighted in the lesson.
Vocabulary primary producer, photosynthesis,
chemosynthesis, consumer, cellular respiration, herbivore,
carnivore, omnivore, detritivore, decomposer, trophic level,
biomass, food chain, food web, keystone species
Life requires energy to organize matter into complex forms
such as carbohydrates, to build and maintain cellular structures, to power
interactions among species, and to power the geological forces that shape
our planet. Energy is somehow involved in nearly every biological, chemical, and physical event. So, where does all this energy come from? And
what, exactly, is energy?
Producers and Consumers
Organisms are classified as either producers or consumers
based on how they obtain energy and nutrients.
Energy is the ability to do work. It is energy that changes the position,
composition, or temperature of matter. The first law of thermodynamics states that energy cannot be created or destroyed, only changed from
one form to another. For example, solar energy is converted to thermal
energy when absorbed by a sandy beach or a dark t-shirt. Just like matter,
the total energy in the universe remains constant. However, unlike matter,
energy is not recycled in the biosphere. It moves in a one-way stream,
shaping communities in the process.
Primary Production Energy cannot be created or destroyed, but it
has to enter an ecosystem somehow. Organisms called autotrophs or
primary producers, like the plant shown in Figure 18, capture energy
from the sun or from chemicals and store it in the bonds of sugars, making energy available to the rest of the community.
5.3 LESSON PLAN PREVIEW
Differentiated Instruction Struggling students make a
Venn diagram to compare
and contrast producers and
consumers.
Inquiry Students model a
biomass pyramid.
Real World Students role-play
a scenario to balance ecosystem health and tourism needs.
5.3 RESOURCES
Outdoors Lab, Life in a Drop of Pond
Water • Real Data Online • Real Data
Math Worksheet • Lesson 5.3 Worksheets • Lesson 5.3 Assessment •
Chapter 5 Overview Presentation
GUIDING QUESTION
FOCUS Ask volunteers to make
a list of a few things they ate in
the past week on the board. Point
out that this food is either another
organism, or a combination of other
organisms. If possible, identify the
organisms that make up the foods
in the lists. Use this list to launch a
discussion on how matter and energy moves through communities.
Figure 18 Primary Producers Green
plants, like this spring pea, can capture
radiant energy from the sun and store
it in the bonds of sugar molecules.
Evolution and Community Ecology 141
Low energy,
longer
wavelength
1
Radio
waves
Infrared
Sun
10–2
10–4
10–6
Visible
light
Ultraviolet
10–8
X-rays
Wavelength (meters)
Microwaves
10–10
10–12
Gamma
rays
High energy,
shorter
wavelength
For nearly all of Earth’s ecological systems, the
sun is the ultimate source of energy. The sun releases radiation from large
portions of the electromagnetic spectrum, shown in Figure 19. Earth’s
atmosphere filters much of this out, and we see only some of this radiation as visible light. Some primary producers, such as green plants, algae,
and cyanobacteria, can turn light energy from the sun into chemical
energy in a process called photosynthesis. Photosynthesis is the process
by which primary producers use sunlight to convert carbon dioxide and
water into sugars, releasing oxygen along the way. Photosynthesis is a
complex process, but the overall reaction can be summarized with the
following equation:
▶ Energy From the Sun 10–14
6CO2 + 6H2O + the sun’s energy → C6H12O6 (sugar) + 6O2
Not all communities are powered by the sun’s
energy. On the floor of the ocean, jets of water heated by magma under
Earth’s crust gush into the icy-cold depths. In one of the more amazing
scientific discoveries of recent decades, scientists realized that these deepsea vents host entire communities of organisms. Deep-sea vents are deep
enough underwater that they completely lack sunlight. Instead, primary
producers such as bacteria use energy stored in the bonds of hydrogen
sulfide (H2S) to convert carbon dioxide and water into sugars in a process
called chemosynthesis. Chemosynthesis can be summarized as:
▶ Energy From Chemicals 6CO2 + 6H2O + 3H2S → C6H12O6 (sugar) + 3H2SO4
Figure 19 Energy From the Sun
The sun emits radiation from many
portions of the electromagnetic
spectrum. All photosynthesis is
powered by just a small portion of the
visible light that reaches Earth.
ANSWERS
Reading Checkpoint The main
difference is the source of energy.
For photosynthesis, it is the sun. For
chemosynthesis, it is energy stored in
chemical bonds.
Photosynthesis and chemosynthesis use different energy sources, but
each uses water and carbon dioxide to produce sugars. Energy from
chemosynthesis supports many organisms including enormous clams and
tubeworms, and various species of mussels, shrimp, crabs, and fish. These
organisms have adaptations that enable them to live in the extreme hightemperature, high-pressure conditions of deep-ocean vents.
Reading
Checkpoint
hat is the primary difference between photosynthesis and
W
chemosynthesis?
Consumers Organisms that rely on other organisms for energy and
nutrients are called heterotrophs, or consumers. Consumers, like those in
Figure 20, make use of the chemical energy stored by photosynthesis or
chemosynthesis in a process called cellular respiration.
Figure 20 Consumers Most communities contain
many kinds of consumers, including a variety of herbivores,
carnivores, omnivores, detritivores, and decomposers.
(a) Herbivore
142 Lesson 3
Cellular respiration is the process by which organisms
use oxygen to release the chemical energy of sugars such as
glucose, releasing carbon dioxide and water as a byproduct.
The summary equation for cellular respiration is the exact
opposite of that for photosynthesis:
C6H12O6 (sugar) + 6O2 → 6CO2 + 6H2O + energy
Cellular respiration does not only occur in consumers—
primary producers also use cellular respiration to release
energy and nutrients they themselves have stored. In fact, the
term autotroph literally means “self feeder.” Heterotroph, on
the other hand, means “other feeder.”
(b) Carnivore
Organisms
that consume producers are known as primary consumers.
Most primary consumers, such as deer and grasshoppers, eat
plants and are called herbivores. Wolves that prey on deer are
considered secondary consumers, as are rodents and birds that
prey on grasshoppers. Tertiary consumers eat secondary consumers, and so on. Most secondary and tertiary consumers
kill and eat other animals and are called carnivores. Animals
that eat both plant and animal food are called omnivores.
▶ Herbivores, Carnivores, and Omnivores Recall that nutrients,
such as carbon, nitrogen, and phosphorus, are recycled in
ecosystems. When animals eat plants, they break down plant
tissues into their components. Then, the animals’ bodies
use the nutrients to build their own tissues. What happens
when animals die? How do nutrients re-enter the ecosystem? Luckily, ecosystems have recyclers called detritivores
and decomposers. Detritivores, such as millipedes and soil
insects, consume detritus—nonliving organic matter including leaf litter, waste products, and the dead bodies of other
community members. Large detritivores, like vultures, are
often called scavengers. Decomposers, such as fungi and
bacteria, break down nonliving matter into simpler parts
that can then be taken up and reused by primary producers.
If it were not for detritivores and decomposers,
nutrients would be lost to an ecosystem
when organisms die.
▶ Detritivores and Decomposers (c) Omnivore
(d) Detritivore
(e) Decomposer
Evolution and Community Ecology 143
Real Data
Energy Flow in Communities
Energy transfer from one trophic level to another in a
community is only about 10% efficient. In addition to
affecting community structure, this inefficiency affects
our own “energy footprints.”
1. Calculate If 1000 units of energy are available at
the producer level of the energy pyramid, about
how many units are available for first-level consumers? Second-level consumers? Third-level
consumers?
ANSWERS Real Data
1. First-level consumer: 100 units;
second-level consumer: 10 units;
third-level consumer: 1 unit
2. Very small amounts of energy are
available to organisms at higher
trophic levels
3. 10,000 units; 100,000 units;
1,000,000 units
4. Eating “down the food chain” eliminates the energy loss that occurs
as energy is transferred to higher
trophic levels.
2. Infer Use your answer to Question 1 to explain
why most communities have only about three or
four trophic levels.
3. Calculate How many units of energy would there
need to be in the first trophic level to end up with
1000 units of energy in the second? In the third? In
the fourth?
4. Apply Concepts Use your answer to Question 2 to
explain why eating “down a food chain,” vegetables
instead of meat for example, is more energy efficient.
Energy and Biomass
Inefficient energy transfer between organisms shapes the
structure of a community.
As organisms feed on one another, matter and energy move through the
community’s trophic levels. An organism’s trophic level is its rank in a
feeding hierarchy. Primary producers always make up a community’s
first trophic level. Primary, secondary, and tertiary consumers make up
the second, third, and fourth levels. In theory, a community can have any
number of trophic levels. However, the relative amounts of energy and
nutrients available at each trophic level put restrictions on a community’s
structure. Consequently, there are typically only three or four trophic
levels in any community.
Energy in Communities Although the overall amount of energy is
Figure 21 The Cost of Action
This thermogram of a car shows
temperature ranges from hot (white)
to cool (blue). A car engine, like an
organism, loses energy in the form of
heat as it converts energy from one
form to another.
conserved in any process of energy transfer, the second law of thermodynamics states that energy tends to change from a more-ordered state to
a less-ordered state. That is, systems tend to move toward increasing disorder, or entropy. The result of the second law of thermodynamics is that
no process involving energy conversion is 100% efficient. When gasoline
is burned in an automobile engine, for example, only about 14% of the
energy is used to move the automobile down the road—most of the rest is
converted to thermal energy and released as heat, as shown in Figure 21.
Thermal energy has high entropy and is very hard to capture and convert
to something else. In other words, thermal energy is generally “lost” when
released as heat.
Organisms are not that different
from car engines. They take in food through predation, herbivory, or
parasitism, and “burn it” using cellular respiration. Energy needed for life
activities is released, but in the process much of the original energy is lost
as waste heat. Due mainly to heat loss, only a small amount of the energy
consumed by an organism in one trophic level is available to be transferred to the next trophic level.
▶ Energy Transfer in Communities 144 Lesson 3
Heat
Third-level
consumer
Second-level
consumer
0.1%
Heat
1%
Heat
10%
First-level
consumer
Primary
producer
Figure 22 Pyramid of Energy This pyramid
of energy illustrates a rough rule of thumb for
the way ecological communities are structured:
Only about 10 percent of the energy contained
in any given trophic level is transferred to the
next highest level. The rest is used to power life
processes or lost as heat.
Heat
100%
Light
Energy
or
Chemical
Energy
A general rule of thumb is that each trophic
level contains just 10% of the energy of the trophic level below it,
although the actual proportion can vary greatly. So, if the primary producers represent 100 calories of a community’s energy, 10 calories (10%
of 100 calories) will be available to level two, 1 calorie (10% of 10 calories)
to level three, and 0.1 calories (10% of 1 calorie) to level four. Most communities, therefore, do not contain enough energy to support consumers
above the third or fourth trophic level. Energy transfer in a community
can be visualized as a pyramid, shown in Figure 22.
This pyramid-like pattern illustrates why eating at lower trophic
levels—eating vegetables and fruit rather than meat, for instance—decreases
a person’s ecological footprint. When we eat meat, we are taking in the
end product of far more energy consumption, per calorie of energy that
we gain, than when we eat plant products.
▶ The Ten Percent Rule ANSWERS Reading Checkpoint It is mainly
lost to the environment as heat.
Figure 23 Pyramids of Numbers
and Biomass Organisms at lower
trophic levels generally exist in far
greater numbers, with greater biomass,
than organisms at higher trophic
levels. The example shown here is
generalized; the actual shape of any
given pyramid may vary greatly.
Numbers and Biomass in Communities Similar to the amount of available energy, there are generally
fewer organisms at higher trophic levels than at lower ones. Look
at Figure 23. A mouse eats many plants in its lifetime, a snake
eats many mice, and a hawk eats many snakes. Thus, for every hawk
in a community there must be many snakes, still more mice, and a
huge number of plants. Because the difference in numbers of organisms among trophic levels tends to be large, the same pyramid-like
relationship also often holds true for biomass. A trophic level’s
biomass is the total amount of living tissue it contains. So,
although a snake weighs more than a mouse, the total snake
biomass is much less than the total biomass of mice.
Reading
Checkpoint
hat happens to energy that is not passed
W
from one trophic level to the next, or used
to power life processes?
Evolution and Community Ecology 145
ANSWERS
Figure 25 It would likely harm the
plants, because the mussels would
not be filtering phytoplankton and
clarifying the water.
Reading Checkpoint Food webs
show the many different paths that
matter and energy take as they move
through a typical community. Food
chains show only one path.
Primary producer
Food Webs and Keystone Species
Feeding relationships have both direct and indirect effects on
organisms in the community.
As energy is transferred from species on lower trophic levels to species on
higher trophic levels, it is said to pass up a food chain. A food chain is a
linear series of feeding relationships. For example, a small fish eats algae,
a larger fish eats the smaller fish, a bird eats the large fish, and an alligator
eats the bird, as shown in Figure 24.
Carnivore
Herbivore
Algae
Flagfish
Largemouth bass
Anhinga
Alligator
Figure 24 Food Chains Food chains are a linear illustration of
energy transfer through feeding relationships in a community.
Figure 25 Indirect Effects Zebra
mussels have many indirect effects
on their community—some positive
(green arrows) and some negative
(red arrows). Infer How would the
elimination of zebra mussels affect
plants in this community?
Fish-eating
birds
One member of a community can have a direct effect on another, for
example, when one organism eats another as part of a food chain. Organisms have indirect effects, too. Consider the zebra mussel’s effects, shown
in Figure 25. The mussels’ waste products promote bacterial growth and
disease pathogens that harm native mussels and clams. Zebra mussels
can also contain high levels of toxic chemicals that can make animals at
higher trophic levels sick. On the other hand, they provide nutrients that
nourish crayfish and other invertebrate animals. The mussels also clarify
the water by filtering out phytoplankton, which are photosynthetic algae
that live in water. As a result, sunlight penetrates more deeply into the
water and plants flourish.
Food Webs Thinking in terms of food chains can be useful, but in real-
Sunlight penetrating
more deeply into water
Musseleating fish
Bacteria
and disease
pathogens
146 Lesson 3
Crayfish
and other
invertebrates
Aquatic
Native
plants
mussels
and clams
ity, ecological systems are far more complex than simple linear
chains. For one thing, most organisms have more than one
source of food! A more accurate representation of the feeding
relationships in a community is a food web. A food web is a
visual map of feeding relationships and energy flow, showing
the many paths by which energy and nutrients pass among
organisms as they consume one another. Figure 26 shows a
simplified food web from Florida’s Everglades region. Note that
even within this simplified diagram we can pick out a number
of different food chains involving different sets of species.
Reading
Checkpoint
hy are most communities best represented with a
W
food web instead of a food chain?
Decomposers, scavengers, and detritivores
Omnivore
Carnivore
Herbivore
Primary Producer
Figure 26 Food Webs Food webs illustrate the feeding relationships
in a community. In this food web from Florida’s Everglades, arrows are
drawn from one organism to another to indicate the direction of energy
flow as a result of predation or herbivory. For example, an arrow leads from
the algae to the flagfish to indicate that flagfishes consume algae. Most
food webs, like this one, are simplified to make them easier to read and
interpret.
Anhinga
Alligator
Largemouth bass
Bobcat
Pig frog
Killifish
Raccoon
Flagfish
Moorhen
Decomposers,
scavengers, and
detritivores
Grass shrimp
and worms
Everglades
crayfish
White-tailed
deer
Plant leaves,
seeds and fruits,
and algae
Evolution and Community Ecology 147
Keystone
Keystone absent
(a)
Sea otter absent
Sea otter
(keystone species)
Kelp
Sea
urchin
(b)
ANSWERS
Lesson 3 Assessment For answers
to the Lesson 3 Assessment, see page
A–7 at the back of the book.
Overgrazed
kelp
Explosion of
sea urchin
population
Figure 27 Keystone Species (a) A keystone is
the stone that holds an arch together. (b) A keystone
species, such as the sea otter, is one that greatly
influences a community’s composition and structure.
Sea otters eat sea urchins that eat kelp in marine
nearshore environments of the Pacific. When otters
are present, they keep urchin numbers down, which
allows forests of kelp to grow and provide habitat
for many other species. When otters are absent,
urchin populations increase and the kelp is devoured,
destroying habitat and reducing species diversity.
Keystone Species Ecologists have found that
in communities, some species exert greater influence than do others. A species that has strong or
wide-reaching impact on a community is called a
keystone species. Shown in Figure 27a, a keystone
is the wedge-shaped stone at the top of an arch
that holds the structure together. If the keystone
is removed, the arch will collapse. In an ecological
community, removal of a keystone species can alter
a large portion of the food web.
Consider the ecosystem shown in Figure 27b.
Sea otters, a keystone species, eat urchins, which in
turn, eat kelp. In the 1990s, sea otter populations off
the coast of Alaska declined when orcas (killer whales) ate large numbers of otters. Fewer otters meant more urchins. The increased urchin
population caused a huge decline in the kelp “forests” offshore. The kelp
had served as habitat for many animals and plants. This is an example of
a trophic cascade: Predators at high trophic levels (sea otters) indirectly
help organisms at low trophic levels (kelp) by limiting populations at
intermediate trophic levels (urchins).
3
1. Compare and Contrast Explain the difference
between a producer and a consumer. Then, explain
the differences among an herbivore, carnivore,
omnivore, detritivore, and decomposer.
2. Calculate If there are 1623 calories available at the
first trophic level, approximately how many calories of energy would be available to a third-level
consumer (fourth trophic level)?
3. Infer Describe three effects a sudden decrease of
pig frogs have might have on the community structure shown in Figure 26. (Hint: Think about what
pig frogs eat and what eats them.)
148 Lesson 3
4. Explore the BIGQUESTION Identifying a community’s keystone species is not always easy. In fact,
some ecologists think that a community can have
many keystone species or none at all. Ecologists all
agree, however, that decomposers, as a category of
consumers, have a huge impact on a community’s
structure. Write a paragraph in which you argue
that decomposers are a “keystone group.”