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13.2
The Structure of Ecosystems
E X P E C TAT I O N S
Describe what is meant by the trophic structure of a community.
Describe the ecosystem roles of producers, consumers, and decomposers.
As mentioned in section 13.1, ecosystems contain
both biotic and abiotic components. Each
ecosystem consists of all the organisms in one or
more communities, as well as the physical and
chemical factors affecting them. The boundaries of
ecosystems are not distinct — one ecosystem may
overlap or exist inside another. There are terrestrial
(land-based) and aquatic (water-based) ecosystems,
and ecosystems that contain both land and water.
An ecosystem can be small (such as the one shown
in Figure 13.14), or as large as the biosphere — a
global ecosystem.
Figure 13.14 Small insects fall into the pitcher plant’s cup-
shaped leaves, drown, and decompose. This plant, which
typically lives in nitrogen-poor environments, extracts the
nutrients it requires from the bodies of insects. The pitcher
plant and its surroundings comprise a small ecosystem.
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Despite the wide range in sizes and types of
ecosystems, the same basic processes take place
in all of them. Two processes are particularly
important: energy flow and chemical cycling. The
proper functioning of these processes is vital to the
survival of organisms in the ecosystem and to the
integrity of the ecosystem itself.
Trophic Structure
When ecologists refer to the trophic structure of an
ecosystem or community, they are describing the
feeding relationships among its members. Each
species is assigned to a specific trophic level in
the structure, depending on its main source of
nutrition. Most ecosystems have several trophic
levels through which energy flows and chemicals
(matter) cycle.
The first (or lowest) trophic level consists of
autotrophic organisms. Autotrophs are organisms
that can make energy-rich organic molecules (such
as glucose) from the raw materials available in the
environment. They then break these “homemade”
organic molecules down during cellular respiration
to provide the energy that fuels the rest of their life
processes. Photosynthetic autotrophs use the
energy of the Sun to drive this manufacturing
process. Almost all plants, as well as some types
of protists (algae) and bacteria (cyanobacteria),
are photosynthetic autotrophs.
All organisms need energy to drive cellular
processes. They must, therefore, have a source of
organic molecules from which they can release this
energy during cellular respiration. Autotrophs,
which are at the first trophic level, produce organic
molecules; this makes the first level the most
important. Because the first trophic level supports
all life at the higher levels, autotrophs
are referred to as the primary producers in an
ecosystem. This first trophic level provides all the
potential energy required to drive the other levels
in the ecosystem.
All organisms in the trophic levels above this
one are heterotrophs. Heterotrophs are unable to
make the energy-rich molecules they need to fuel
their life processes. Instead, they must obtain these
molecules by consuming other organisms, either
autotrophs or other heterotrophs. Therefore, they
are referred to as consumers.
Herbivores that eat autotrophs are termed
primary consumers, since they are the first eaters
in the chain. On land, insects, snails, grazing
mammals, and birds and mammals that eat seeds
and fruit are the most common herbivores. In
aquatic ecosystems, this role is often taken by
heterotrophic protists, various types of small
invertebrate animals, and some species of fish.
Carnivores that eat mainly herbivores are
secondary consumers. Spiders, frogs, and insecteating birds are examples of secondary consumers.
In most ecosystems, these secondary consumers
are themselves eaten by other carnivores, which
are known as tertiary consumers (the third set
THINKING
of eaters). There may also be higher levels of
consumers above these.
The members of another consumer group, often
referred to as decomposers, obtain their energy-rich
molecules by eating leftover or waste material
derived from all the trophic levels, including the
feces of living organisms, dead bodies, or body
parts (for example, fallen leaves). Decomposers are
very important to every ecosystem. Their role is to
break large molecules (that were once part of living
organisms) down into smaller ones and return them
to the abiotic environment. Thus, decomposers
return vitally important nutrient elements such as
carbon and nitrogen to the soil and air. These
materials can then be used again by autotrophs
to make new energy-rich organic molecules.
Decomposers are an ecosystem’s recyclers, ensuring
that the biosphere’s limited supply of required
nutrients is not lost.
LAB
Shrinking Polar Bears and
Expanding Snow Geese
Background
An incredible variety of living things inhabits the biosphere.
This biotic diversity reflects the abiotic diversity of Earth.
Through the process of evolution, populations have evolved
adaptations that enhance their survival and reproductive
ability in diverse habitats. The result is the diversity of life.
You Try It
1. Describe the ecological niches of the polar bear and the
snow goose. Include diet, preferred habitat, time of
peak activity during a day, yearly activities such as
migration, and other relevant information.
2. Would you consider the polar bear to be a generalist
or a specialist? What about the snow goose? Explain
your answer.
During the last 100 years, scientists have reported
significant changes in the world’s ecosystems. While some
of these changes seem to have been beneficial for some
species, they seem to have had negative effects on others.
For example, dramatic changes have been observed in
populations of polar bears (Ursus maritimus) and lesser
snow geese (Chen caerulescens) that breed in Canada’s
arctic. Field surveys of polar bears have revealed that
populations are declining, fewer cubs are born each year,
and individual bears are smaller and weigh less than what
has been considered typical for members of this species. In
contrast, snow goose populations have tripled since 1968.
Their numbers are so high that the feeding activity of these
geese is causing substantial damage to the habitats where
they and other species breed and overwinter.
3. To which trophic levels do the polar bear and snow
goose belong?
Why do you think these changes are happening? In
completing this assignment, you will draw on your
knowledge of the nature of ecology and evolution. You
might choose to work on one of these two species, and
then compare your findings with those of students who
worked on the other species.
7. How might changes that currently appear to be
occurring in Earth’s climate affect the future evolution
of each of these species?
4. Describe the feeding, sensory, and locomotory
adaptations that improve the ability of each species to
survive and reproduce in its habitat and niche. What
features allow each species to cope with environmental
stresses (for example, temperature or moisture
extremes) in its habitat?
5. Draw a map illustrating the approximate range of each
species.
6. Find out as much as you can about the evolutionary
history of these species. What other species are they
most closely related to? When might they have
appeared as a species?
8. Estimate what chance each of these species has for
surviving for the next 100 years. Does either face
possible extinction? Why or why not?
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BIO FACT
Some types of bacteria use energy derived from breaking
the chemical bonds in hydrogen sulfide molecules to form
the organic molecules they use as food. This type of
autotrophic food production is called chemosynthesis. It is
quite rare and occurs in some very unusual environments,
including ocean depths of 2500 m. At these depths there
is no light and very little oxygen. In addition, hot magma
from Earth’s molten core escapes to superheat the
surrounding water.
may eat plant parts (fruits and seeds) and
grasshoppers. In fact, many organisms, including
humans, are referred to as omnivores because they
eat plants, animals, and other types of organisms.
In other words, they are both primary and higherlevel consumers. The result is that the feeding
relationships in most ecosystems form complex
food webs, rather than simple food chains.
Figure 13.16 illustrates a food web.
In the Thinking Lab on page 441, you explored
how communities are structured and the specific
roles individuals can take within a community.
The next section will show you how energy flows
through ecosystems.
Quaternary
consumers
carnivore
carnivore
Some unusual marine organisms live near undersea
vents off the coast of British Columbia. These organisms
include tube worms, limpets, and palm worms.
Tertiary
consumers
carnivore
ELECTRONIC LEARNING PARTNER
Your Electronic Learning Partner has animation clips
that will enhance your understanding of deep-sea vent
communities.
Secondary
consumers
carnivore
Food Chains and Webs
The trophic structure determines the route taken by
the energy and matter (chemical elements) contained
in food as it moves through an ecosystem. Food is
transferred from primary producers to primary
consumers and then to secondary consumers along a
pathway referred to as a food chain (see Figure 13.15).
In reality, few ecosystems are so simple that they
consist of only a single, unbranched food chain.
More commonly, many species may feed on a
single species below themselves, while at the same
time a single upper-level consumer species may eat
many different species below itself. In addition,
organisms may eat individuals from two or more
levels. The hawk shown in Figure 13.15 may eat
mice, grasshoppers, and snakes, while the mouse
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carnivore
carnivore
Primary
consumers
zooplankton
herbivore
Primary
producers
plant
Terrestrial food chain
phytoplankton
Marine food chain
Figure 13.15 Terrestrial and aquatic communities contain
different species, but can have the same overall trophic
structure. “Plankton” is a general term referring to small
aquatic animals and protists. What distinguishes the two
types of plankton?
wastes and
dead organisms
Tertiary and
secondary
consumers
Secondary
and primary
consumers
Primary
consumers
Primary
producers
Decomposers
(plants, algae,
cyanobacteria)
(bacteria, fungi,
certain animals)
Figure 13.16 A simplified food web. There are many other species that feed with
or on the ones shown in the diagram. Which species eat organisms on more than
one trophic level?
SECTION
REVIEW
1.
K/U Explain why some ecosystems can support
highly complex food webs while others can support
only relatively simple ones.
2.
Draw a chart to illustrate a food web that could
be found in a typical pond ecosystem (show at least
three distinct trophic levels). Identify the organisms
and label each trophic level.
3.
4.
5.
K/U Explain why autotrophs rather than
decomposers occupy the lowest level of a food chain.
6.
K/U Could photosynthetic producers exist in the
absence of consumers in an ecosystem? Explain
your answer.
7.
K/U Describe the typical energy sources that
decomposers rely on in an aquatic ecosystem
(such as a pond or lake).
8.
K/U Describe the difference between a food chain
and a food web. Which is more realistic in its
depiction of what actually exists in nature?
9.
K/U Explain why producer organisms that live
deep below the surface of Earth’s oceans rely on
chemosynthesis rather than photosynthesis to
manufacture high-energy food molecules.
C
K/U Explain how the same species can occupy
more than one trophic level within the same food
web. Explain how this type of ecological interaction
can enhance the stability of a food web.
K/U Describe the types of biotic and abiotic factors
that can lead to the collapse of a food web in an
ecosystem.
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