Standard LS.6
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Energy Flow in an Ecosystem: Food Chains
Scientists follow the flow of energy through an ecosystem by studying food chains. A food
chain shows how energy passes from one living thing to another living thing. Food chains can
be found in dry land habitats such as deserts, grasslands, and forests. They can also be found in
water habitats such as ponds, marshes, swamps, rivers, and oceans.
Energy enters an ecosystem through the process of photosynthesis. Photosynthesis is the process
of converting light energy into chemical energy. During photosynthesis, light energy is captured
in the leaves of plants and converted to chemical energy which is stored in the form of glucose or
sugar. All food chains begin with green plants. They are the only living things that can produce
or make their own food. As a result, green plants are called producers. Some producers include
trees, bushes, grasses, and ferns.
While producers get their energy from the sun, many organisms on Earth get their energy by
eating or consuming producers and other organisms. Energy is then passed through the
ecosystem as one organism eats and is then eaten by another organism. These organisms are
called consumers.
Consumers are classified by what they eat. First-order consumers are organisms that eat
producers. Some familiar first-order consumers include caterpillars, mice, and rabbits. Secondorder consumers are the organisms that eat the first-order consumers. Some familiar secondorder consumers include frogs, snakes, and red foxes. Third-order consumers are those that eat
second-order consumers. These may include egrets, owls, and bobcats.
As energy travels through a food chain it is gradually used up. Decomposers are organisms that
consume the remains of this energy from waste products and the bodies of dead producers and
consumers. Decomposers obtain the energy they need to survive from this organic material
while returning the remainder to the surrounding soil, water, and air. In this way materials are
recycled and made available to the ecosystem once again. Some decomposers include bacteria,
fungi, and worms.
Energy Flow in an Ecosystem: Food Webs
We have learned that within a community of living things, energy flows from one organism to
another through a food chain. A simple food chain might begin with grass (a producer) which is
eaten by a grasshopper (a first-order consumer), which is eaten by a frog (a second-order
consumer), which is eaten by an egret (a third-order consumer) which eventually dies and is
broken down by worms (decomposers).
Food chains are not always so simple, however. In an ecosystem where living and nonliving
things interact, food chains can become very complex and energy can take many different paths.
When food chains overlap they are called food webs. Let’s investigate a possible food web that
begins with an oak tree.
An oak tree produces its own food energy through the process of photosynthesis. A caterpillar
living on the tree eats the tree’s leaves. Also living on the tree is a beetle that eats the tree’s
acorns. A sparrow then eats the caterpillar and the beetle and may later be eaten by an owl or
hawk. A squirrel and a mouse eat the tree’s acorns and at night become dinner for the owl or the
snake. The snake, mouse, squirrel, and caterpillar might also fall prey to a hawk. As you can see,
in a food web many living organisms are connected to one another by the foods they eat and
what eats them.
It is important to note that in a food web, organisms may play more than one role. For example,
in the oak tree food web the hawk is a second-order consumer when he eats the mouse and a
third-order consumer when he eats the snake.
Energy Flow in an Ecosystem: Energy Pyramids
We have learned that energy flows through food chains and more complex food webs in an
ecosystem. As this energy moves from organism to organism, however, the amount of energy
available for use decreases. When an organism consumes another organism, it uses most of the
energy it receives to carry out necessary life processes. This only leaves a small amount of
energy available for the next organism in the food web. This steady decrease in energy occurs as
you move up the food web from producer to higher-order consumers. A diagram called an
energy triangle is used to show the amount of energy available at each feeding or trophic level
in an ecosystem.
Let’s use our earlier example of an oak tree to help us understand how energy flow is displayed
in an energy triangle. If the oak tree is located in your school yard, it might be surrounded by
other trees, bushes, and grasses. By knowing how many producers are present, local scientists
can measure how much energy the producers will produce in one year. Energy is measured in
units called kilocalories (kcal). Let’s say that the producers in the school yard can produce 6,000
kcal per year. This means that there are 6,000 kcal of energy available on the first trophic level
or bottom of the energy pyramid.
The 6,000 kcal of energy on the first trophic level will support a specific number of first-order
consumers on the second trophic level. As the caterpillars of the second level consume the
producers, about 90% of the energy they take in will be used to carry out their life processes.
This means that there will only be 10% of the 6,000 kcal or 600 kcal of energy remaining for use
by the next level of consumers.
The 600 kcal of available energy on the second trophic level will support a specific number of
second-order consumers on the third trophic level. As the birds on the third level consume the
caterpillars, about 90% of the energy they take in will be used to carry out their life processes.
This means that there will only be 10% of the 600 kcal or 60 kcal of energy remaining for use by
the next level of consumers.
The 60 kcal of energy available on the third trophic level will support a specific number of thirdorder consumers on the fourth tropic level. As the owls on the fourth trophic level consume the
birds, about 90% of the energy they take in will be used to carry out their life processes. This
means that there will only be 10% of the 60 kcal or 6 kcal of energy remaining for use by another
third-order consumer or decomposer.
Think:
 Using your knowledge about energy triangles, would a population’s position on a food
web affect the size of the population? Explain.
 Using your knowledge about food chains, food webs, and energy triangles, identify some
examples of interdependence in terrestrial, freshwater, and marine ecosystems.
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