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Objectives
Contrast the flow of energy and chemicals in ecosystems.
Explain how trophic levels relate to food chains and food webs.
Key Terms
producer
consumer
decomposer
trophic level
food chain
herbivore
carnivore
omnivore
primary consumer
secondary consumer
tertiary consumer
detritus
food web
As small as it is, a terrarium like the one in Figure 36-1 is an ecosystem.
A terrarium includes a community of organisms such as plants, snails,
and bacteria as well as their nonliving environment—the soil, minerals,
water, and air. Just as in larger ecosystems such as forests and streams,
the terrarium illustrates two key processes of all ecosystems: energy
flow and chemical cycling.
Energy Flow and Chemical Cycling
Every organism requires energy to carry out life processes such as
growing, moving, and reproducing. Photosynthetic producers such as
plants convert the light energy from sunlight to the chemical energy of
organic compounds (Figure 36-1). Organisms called consumers obtain
chemical energy by feeding on the producers or on other consumers.
Finally, organisms called decomposers break down wastes and dead
organisms. As living things use chemical energy, they release thermal
energy in the form of heat to their surroundings. To summarize, energy
enters an ecosystem as light, is converted to chemical energy by
producers, and exits the ecosystem as heat. Energy is not recycled
within an ecosystem, but flows through it and out. Producers must
continue to receive energy as an input for the ecosystem to survive.
Figure 36-1
As in larger ecosystems, energy flows through the
terrarium ecosystem, while chemicals cycle within it.
In contrast to energy, chemicals such as carbon, oxygen, and nitrogen
can be recycled between the living and nonliving parts of ecosystems
and the biosphere. In Concept 36.3 you will read about the different
chemical cycles in more detail.
Although energy flows through an ecosystem, while chemicals can be
used again and again, the movements of both energy and chemicals are
related to patterns of feeding within the ecosystem.
Food Chains
In the desert, a grasshopper munches on a brittlebrush's bright yellow
flowers. Suddenly, a mouse seizes the grasshopper for its own meal.
These feeding relationships, from flower to grasshopper to mouse,
relate to how energy and chemicals move through the desert ecosystem.
Each of these organisms represents a feeding level, or trophic level, in
the ecosystem. The pathway of food transfer from one trophic level to
another is called a food chain.
Producers Figure 36-2 compares a terrestrial food chain and an
aquatic food chain. In both food chains, the producers make up the
trophic level that supports all other trophic levels. In terrestrial
ecosystems, plants are the main producers. In aquatic ecosystems,
phytoplankton—photosynthetic protists and bacteria—multicellular
algae, and aquatic plants are the main producers.
Figure 36-2
Each of these food chains includes five
trophic levels. The arrows indicate the
direction of food transfer between trophic
levels.
Consumers Organisms in the trophic levels above the producers are
consumers. They may be categorized according to what they eat. A
consumer (such as a horse) that eats only producers is an herbivore. A
consumer (such as a lion) that eats only other consumers is a carnivore.
And a consumer (such as a bear) that eats both producers and
consumers is an omnivore.
Consumers may also be categorized by their position in a particular
food chain. For instance, when a consumer feeds directly on producers
it is referred to as a primary consumer, or first-level consumer. In
terrestrial ecosystems, primary consumers often include insects and
birds that eat seeds and fruit, as well as grazing mammals such as
antelope and deer. In aquatic ecosystems, primary consumers include a
variety of zooplankton (mainly protists and microscopic animals such
as small shrimps) that feed on phytoplankton. Secondary consumers
(second-level consumers) eat primary consumers. On land, secondary
consumers include many small mammals and reptiles that eat insects, as
well as large carnivores that eat rodents and grazing mammals. In
aquatic ecosystems, secondary consumers are mainly small fish that eat
zooplankton. Tertiary consumers (TUR shee ehr ee)—third-level
consumers—eat secondary consumers. On land, a tertiary consumer
may be a snake eating a mouse. Some ecosystems, such as those in
Figure 36-2, can even support quaternary (fourth-level) consumers. As
you will read in Concept 36.2 the number of fourth-level consumers in
an ecosystem is usually low.
Decomposer At each trophic level, organisms produce waste and
eventually die. These wastes and remains of dead organisms are called
detritus. Decomposers are consumers that obtain energy by feeding on
and breaking down detritus. Animals that eat detritus, often called
scavengers, include earthworms, some rodents and insects, crayfish,
catfish, and vultures. But an ecosystem's main decomposers are bacteria
and fungi. These organisms, found in enormous numbers in the soil and
in the sediments at the bottom of lakes and oceans, recycle chemicals
within the ecosystem.
Diagrams of food chains, such as those in Figure 36-2, generally do not
depict the decomposers that break down the remains of the organisms
at each trophic level in the food chain. But all ecosystems do include
decomposers—their role is vital to the ongoing recycling of chemicals
in the ecosystems.
Food Webs
The feeding relationships in an ecosystem are usually more
complicated than the simple food chains you have just read about.
Since ecosystems contain many different species of animals, plants, and
other organisms, consumers have a variety of food sources. The pattern
of feeding represented by these interconnected and branching food
chains is called a food web.
Figure 36-3 shows how food chains within a food web are
interconnected. For example, the rattlesnake eats several animal species
that may also be eaten by other consumers, such as the hawk. In
addition, some consumers can feed at several different trophic levels.
The woodpecker, for instance, is a primary consumer when it eats
cactus seeds, and a secondary consumer when it eats ants or
grasshoppers. The hawk can be a secondary, tertiary, or even
quaternary consumer depending on its prey. Note that like food chains,
food web diagrams typically do not show decomposers. In the next
section you'll read more about how trophic levels in food webs relate to
energy flow in an ecosystem.
Figure 36-3
This food web
diagram shows
some of the
interconnected
food chains in the
Sonoran Desert
ecosystem. The
arrows represent
food transfer
between trophic
levels. Note that
several species
feed at more than
one trophic level.
Concept Check 36.1
1. How are the movement of energy and the movement of chemicals in
ecosystems different?
2. In the following food chain, identify the trophic levels: An owl eats a
mouse that ate berries.
3. Using Figure 36-3, identify a food chain with at least three trophic
levels in the Sonoran Desert ecosystem (other than that featured in
Figure 36-2).
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