Food Chains and Webs
GLOSSARY
autotrophs: organisms, such as plants, that are capable of making their own food.
biological magnification: the process by which small amounts of a poisonous chemical
become increasingly concentrated as they are consumed by organisms which
occupy higher levels in an energy pyramid.
carnivore: a flesh-eating animal, such as a mountain lion or wolf.
community: a group of different organisms living together in a given area.
consumers: organisms within a community that eat producers or other consumers.
decomposers: organisms whose function is to clean up a community by digesting the
dead bodies of producers and/or consumers.
energy pyramid: a model which illustrates the loss of energy that occurs throughout
food chains and webs.
first-order consumers: organisms that eat the producers in a food chain.
food chain: describes the order in which organisms in a community feed on each other.
food web: describes all the interconnected food chains within a community.
fourth-order consumers: organisms that eat the third-order consumers in a food chain.
herbivore: an animal that only eats plants.
heterotrophs: organisms that cannot produce their own food, and must rely on other
organisms for energy.
niche: the role an organism plays within a community.
omnivore: an animal that eats both plants and animals.
photosynthesis: the process by which plants convert light energy from the sun into
chemical energy in the form of food.
producers: organisms within a community that provide food for other organisms
incapable of producing their own food.
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second-order consumers: organisms that eat the first-order consumers in a food chain.
third-order consumers: organisms that eat the second-order consumers in a food chain.
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Study Guide #1
ENERGY TRANSFER
All of the activities of life require a source of energy. This energy is obtained directly
through feeding or indirectly from the sun.
Plants are able to absorb light energy from the sun and convert it into chemical energy
through a process called
photosynthesis. The potential
energy created is stored as food in
the plants’ tissues. Organisms,
such as plants, which are able to
manufacture their own food, are
called autotrophs.
Animals and plants that are incapable of obtaining energy from light directly must obtain it
indirectly by consuming other plants or animals. Organisms that cannot produce their own
food and must obtain it from their environment are called heterotrophs.
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Study Guide #2
BIOLOGICAL COMMUNITIES
Environmental conditions in a given area support a specific group of plants and animals.
This group is called a community. For example, a tropical rain forest may be composed of
assorted insects, birds, reptiles
and a canopy of trees.
Depending on their source of
nourishment, members of a
community are categorized into
three niches or trophic levels.
Green plants, algae or bacteria
that are capable of converting light into chemical energy via photosynthesis are called
producers. The producers provide energy to the heterotrophs that cannot produce their
own food.
Consumers are organisms in the community that feed on producers or other consumers.
Mice that eat plants are consumers. Owls that eat mice are also consumers.
The third category, decomposers, feed on the dead bodies of producers and consumers.
The decomposers recycle the dead remains into nutrients, which can then be used by other
organisms. Members of this group include bacteria and fungi.
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Study Guide #3
FOOD CHAINS
On the basis of what they eat, animals are categorized into herbivores, carnivores and
omnivores. Herbivores are “vegetarians” that eat only plants. Carnivores are flesh-eaters
and omnivores eat both plants and animals.
The term food chain describes the sequence in which organisms in a community feed on
each other. Food chains always
start with autotrophic producers.
The producers are eaten by
herbivores or omnivores, which
are, called first-order consumers.
Animals that feed on the firstorder consumers are usually
carnivores. They are called
second-order consumers. Animals that devour second-order consumers are called thirdorder consumers. An herbivorous insect eating a plant is an example of a first-order
consumer eating a producer. A frog that eats the insect is a second-order consumer. A
snake that eats the frog is a third-order consumer. If yet another carnivore eats the snake
it is a fourth-order consumer. As you can see, there may be one or several carnivores in a
food chain.
A member of a community may feed at different levels in various food chains. Some
omnivores, such as humans, routinely feed at all levels.
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Study Guide #4
FOOD WEBS AND ENERGY PYRAMIDS
The food chains within a community are intricately related. Organisms may participate in
more than one chain and
often fill different levels in
each chain. The
interconnected food chains of
a community and the flow of
energy between them make
up a food web.
An energy pyramid shows the
amount of energy captured and stored at each trophic (nutritional) level of the food chain.
Every level captures about ten percent of the energy contained in the level below it. The
other ninety percent is used to fuel life activities or is unassimilated. Moving up the levels
of the pyramid, the number of organisms decreases with the amount of available energy.
Third-order consumers, such as humans, must eat large amounts of food to maintain their
energy requirements. As the
world population explodes,
humans place a growing burden
on the levels below us in the
food chain. Because of the
interdependence of all members
of the food web, stress on one
group will be detrimental to
those that depend on it as a source of energy.
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Another problem can arise when small amounts of poisonous chemicals consumed by
organisms at one level of the energy pyramid become increasingly concentrated when
ingested by organisms farther up the pyramid. This potentially deadly process is called
biological magnification.
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