Energy in an Ecosystem
TEKS 7.5C
Diagram the flow of energy through living systems, including food chains,
food webs, and energy pyramids.
What are energy roles in an ecosystem?
A system is a group of things that are related to one another in some way and form a functioning
unit. One example of a living system is an ecosystem. An ecosystem is a community of organisms that
live in a particular area, along with their nonliving environment. Every organism in an ecosystem has a
role in the flow of energy through the living system. An organism’s energy role in an ecosystem is
determined by how it obtains food. Each organism fills the role of producer, consumer, or decomposer.
Producers An organism that can make its own food is a producer. Energy enters most ecosystems
as sunlight. Producers, including plants and algae, capture the energy of sunlight. They use the energy
to turn water and carbon dioxide into food molecules in a process called photosynthesis. Producers are
the source of all the food in an ecosystem, directly or indirectly.
Consumers Some members of an ecosystem cannot make their own food. An organism that obtains
energy by feeding on other organisms is a consumer. Animals, such as mice and hawks are consumers.
Consumers that eat only plants are herbivores. Consumers that eat only animals are carnivores.
Omnivores eat both plants and animals.
Decomposers Decomposers break down wastes and dead organisms and return the raw materials to
the ecosystem. While obtaining energy for their own needs, decomposers return simple molecules to
the environment. These molecules can be used again by other organisms. Fungi and some bacteria are
examples of decomposers.
What is a food chain?
A food chain is a series of events in which one organism eats another organism and obtains energy. A
food chain is one way to diagram the flow of energy through a living system. Arrows in the food chain
indicate the direction in which energy is flowing.
The first organism in a food chain is always a producer. In the example shown, the algae are the
producer. The producer supplies energy to the organisms that eat the algae.
The second organism in a food chain feeds on the producer and is called a first-level consumer. The
flagfish is a first-level consumer.
Next, a second-level consumer eats the first-level consumer. The second-level consumer obtains its
energy from the first-level consumer. The second-level consumer in this example is the largemouth
bass. Food chains can have third-level consumers, such as the anhinga. They also can have fourthlevel consumers, such as the alligator.
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Diagram In one ocean food chain, worms eat algae; a small fish eats the worms; and a shark eats
the small fish. In the space below, diagram the flow of energy through this food chain. When you have
finished your food-chain diagram, identify the producer, first-level consumer, second-level consumer,
and third-level consumer on the lines provided.
Producer
First-level consumer
Second-level consumer
Third-level consumer
What is a food web?
A food chain shows only one possible path along which energy can move through an ecosystem. But
most producers and consumers are part of many food chains. A more realistic way to show the flow of
energy through an ecosystem is a food web. A food web consists of many overlapping food chains in
an ecosystem. The diagram shows a food web in a woodland ecosystem.
The diagram shows that an organism may play more than one role in an ecosystem. For example, an
omnivore such as a mouse is a first-level consumer when it eats grass. But when the mouse eats a
grasshopper, it is a second-level consumer.
What is an energy pyramid?
When an organism in an ecosystem eats, it obtains energy. The organism uses some of this energy to
move, grow, reproduce, and carry out other life activities. This means that only some of the energy it
obtains will be available to the next organism in the food web.
A diagram called an energy pyramid shows the amount of energy that moves from one feeding level
to another. In the energy pyramid shown here, energy is measured in kilocalories, or kcal.
The most energy is available at the producer level of the pyramid. As you move up the pyramid, each
level has less energy available than the level below. In general, only about 10 percent of the energy at
one level of a food web is transferred to the next higher level. The other 90 percent of the energy is
used for the organism’s life processes or is lost to the environment as heat.
The organisms at higher feeding levels of an energy pyramid do not necessarily require less energy to
live than do organisms at lower levels. The amount of energy available at the producer level limits the
number of consumers that the ecosystem can support. That is why there are usually few organisms at
the highest level in a food web.
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Review What determines an organism’s energy role in an ecosystem? List the three possible energy
roles.
Compare and Contrast How are a food chain and a food web similar? How are they different?
Diagram A chipmunk eats sunflower seeds. Later a hawk eats the chipmunk. In the space below,
use that information to diagram the flow of energy through that food chain.
Diagram Use the food chain you constructed for question 4 to diagram the flow of energy through
that energy pyramid. Assume that the organisms at the lowest level contain a total of 2000
calories of energy, and that 10 percent of the energy available at one level of the food chain is
transferred to the next level. Calculate the energy available to organisms at the second level and
the highest level. Label the energy pyramid with the amount of energy available at each level.
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