PS 12 LT2 OS 5 “Understand it”
Understanding Food Webs
Worksheet/Article
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Food is energy, and energy makes life possible. All living
things must nourish themselves in order to survive. Because
many animals nourish themselves by eating plants or other
animals, a food chain is created. Food chains do not exist in
isolation, but form an intricate network of interrelationships—
a food web.
Plants are the first stage (or
trophic level) of a food chain.
Plants are called producers (or autotrophs—"self-feeders")
because they produce energy from simple raw materials,
such as sunlight and inorganic nutrients.
Trophic Level: One
strata of a food web,
where organisms are the
same number of steps
removed from the
primary producers.
As soon as an animal eats a plant, a food chain is formed.
All animals are heterotrophs ("other-feeders"), since they find energy by consuming
other living things.
As soon as an animal eats a plant, a food chain is
formed. All animals are heterotrophs ("other-feeders"), since
they find energy by consuming other living things.
Herbivores eat plants, becoming primary or first-order
consumers.
 Carnivores eat herbivores, becoming secondary or secondorder consumers.
 Other carnivores may eat the second-order carnivores, so
they are third-order consumers. Some carnivores may eat both
herbivores and other carnivores, making them both secondand third-order consumers on different food chains.
At every trophic level, decomposers (like bacteria, fungi, moss, lichen) make
use of the wastes and remains of dead organisms. They use the energy in these
decaying remains to fuel their own metabolism. They also break down organic
compounds into inorganic ones, making substances available for reuse.
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Food chains can have as few as one link, or as many as six. Very few food chains
have more than six links. Why? The food web consists of the transfer of energy from
one species to another. Most of the energy transferred is used up by the
consumer—only 10% is available for the next animal. By the sixth trophic level,
very little energy remains to be consumed.
The animal at the top trophic level of a food chain faces no predators, but these
species often face the greatest chance of extinction, since they depend on so many
other species below them on the food chain.
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But food chains show only part of the story.
PS 12 LT2 OS 5 “Understand it”
Food chains are rarely simple linear progressions
from one trophic level to the next. Instead, a single
species, such as a hawk, might be a second-order
consumer of field mice, a third-order consumer of
frogs, and a fourth-order consumer of owls. This is a
survival strategy for the hawk, so it doesn’t become
too dependent on one species. It also creates complex
interrelationships between species, and a food web is
the result.
In most food webs, the relationships are so intricate
that ecologists are still discovering the ways that
these dynamic networks work. A change in one
species will affect the species that prey on it, but it
can also trigger unexpected consequences in species
with no direct link in the web.
In a pond ecology, for example:
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4.
5.
A pyramid of energy shows where
the available energy lies in an
ecosystem. The greatest amount is
found in the plants; the least
amount in the top-level consumers.
Aquatic invertebrates and minnow eat algae
Minnows and perch eat aquatic invertebrates
Perch and bass eat minnows
Great Blue Herons and bass eat perch
Great Blue herons eat perch.
In this food chain, there is a direct positive effect of the predator on their prey. If we
increase the population size of the predator, we expect to see a decline in the
population size of their prey (because more predators are eating the prey). Similarly, if
we see a decrease in the population size of the predator, we would expect to see an
increase in the population size of the prey (because there are fewer predators eating
the prey).
However, one species can also affect the
population size of another species indirectly
through its effect on a third species. For example,
perch eat minnows, and minnows eat algae. If the
population size of perch increases, what happens
to algae? More perch means fewer minnows, and
fewer minnows means more algae. Thus, perch
have an indirect positive effect on algae. So each species is simultaneously being
affected by a number of direct and indirect interactions.
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