Key Terms
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producer
consumer
food chain
primary consumer
food web
herbivore
decomposer
omnivore
secondary consumer
carnivore
trophic level
detritus
tertiary consumer
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 (on separate paper, or emailed to nyoshuff@gmail.com)
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.
4. Write a definition for each of the underlined key terms found in Concept 36.1.