ECOSYSTEMS STUDY SHEET
NAME_________________________________
VOCABULARY
Ecosystem____________________________________________________________
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herbivore
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carnivore
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food chain
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decomposer __________________________________________________________
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food web
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energy pyramid ______________________________________________________
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Look for the order of events in the transfer of energy.
SCIENCE CONCEPTS
how food energy is passed from plant to animal to animal in an ecosystem
how food chains make up food webs
Energy Transfer
You read that plants make their own food through the process of photosynthesis. So do a
few other organisms, such as algae and lichens (LY•kuhnz). Plants are the main producers
in most land ecosystems.
An ecosystem (EE•koh•sis•tuhm) includes all the organisms in an area and the
environment in which they live. This is a tundra ecosystem. All the organisms shown here
are part of a tundra ecosystem. An ecosystem includes many kinds of organisms.
Some tundra animals, like caribou, eat plants and other producers. The food energy stored
in the reindeer moss is transferred to the caribou. An animal that eats plants or other
producers is an herbivore. Herbivores are also called firstlevel consumers.
Other tundra animals, such as wolves, don’t eat plants. They get their energy by eating
other animals, like caribou. Food energy stored in the caribou is transferred to the wolf. An
animal that eats mainly other animals is a carnivore. Carnivores are also called secondlevel consumers.
Reindeer moss, a
lichen, makes food
by photosynthesis.
The food energy is
stored in the
organism.
The food
energy is
stored in
the
organism.
The
caribou
gets its
energy by
eating
reindeer
moss.
Some animals, called omnivores, eat both plants and other animals. Omnivores can be
first-level or second-level consumers. The bear shown on the first page of the lesson is an
omnivore. So are most people.
When the moss, caribou,
and wolf die,
decomposers break down
their remains. Then the
reindeer moss and other
producers can take up
any remaining nutrients.
In another ecosystem, a large carnivore, such as a hawk, might eat a smaller carnivore,
such as a snake. That makes the hawk a third-level consumer. Each time something eats
something else, food energy is transferred from one organism to the next. The transfer of
food energy between organisms is called a food chain.
When plants and animals die, what happens to the food energy stored in their remains?
The remains are broken down and the food energy is used by decomposers. A
decomposer is a consumer that gets its food energy by breaking down the remains of
dead organisms. Decomposers can be animals, such as earthworms. Many decomposers
are fungi. Others are singlecelled organisms—protists or bacteria.
Decomposers use some of the nutrients as food. The rest become mixed into the soil. Then
plant roots can take up these nutrients. In this way, decomposers connect both ends of a
food chain.
You know that all the organisms in an ecosystem depend on producers to make food. Then
food energy is transferred through the ecosystem from one consumer level to another. All
along the way, decomposers get energy from the remains of dead organisms. Any
nutrients not used are returned to the soil.
After food energy is taken in by a second level
consumer, what can happen next?.
Food Webs
You know that most animals eat more than one kind of food. For example, a hawk might
eat a mouse that ate seeds. The same hawk might also eat a small snake that ate
grasshoppers and other insects. The insects, in turn, might have eaten grass. An organism,
such as the hawk, can be a part of several food chains. In this way, food chains overlap. A
food web shows the relationships among different food chains.
The producers in this prairie
ecosystem include grasses, clover,
and purple coneflowers. First-level
consumers, or herbivores, include
insects, mice, ground squirrels, and
bison. Second-level and third-level
consumers—carnivores— include
spiders, snakes, and hawks. The
decomposers that you can see are
mushrooms. What you can’t see are
the millions of single-celled
decomposers. They are in the soil,
helping recycle nutrients
If all the mosquitoes in a pond died,
what might happen next?
In this pond ecosystem, the producers include
water plants and algae. Here the first-level
consumers, or herbivores, include insects and
tadpoles. Second-level and third-level consumers
include fish. Some of the birds, such as ducks, are
herbivores, while others are carnivores. The turtle
is an omnivore, eating insects, tiny fish, and
plants. The water is full of decomposers, such as
snails, worms, and single-celled protists.
Carnivores eat consumers such as herbivores, omnivores, and other carnivores. Carnivores
also limit the number of animals below them in a food web. For example, without snakes,
the number of mice in the prairie ecosystem would keep increasing. In time, the mice
would eat all the available food. Then the mice would starve, and so would hawks, which
eat mice.
Organisms in an ecosystem depend on one another for survival. A change in the number of
one kind of organism can affect the entire ecosystem!
Energy Pyramid
Not all the food energy of plants is passed on to the herbivores that eat them. Producers
use about 90 percent of the food energy they produce for their own life processes. They
store the other 10 percent in their leaves, stems, roots, fruits, and seeds.
Animals that eat the producers get only 10 percent of the energy the producers made.
These herbivores then use for their life processes 90 percent of the energy they got from
the producers. They store the other 10 percent in their bodies.
An energy pyramid shows that each level of a food chain passes on less food energy than
the level before it. Most of the energy in each level is used at that level. Only a little
energy is passed on to the next level.
Because each level passes so little energy to the next, the first-level consumers need many
producers to support them. In the same way, the second-level consumers need many firstlevel consumers to support them. This pattern continues up to the top of the food chain.
That’s why the base of an energy pyramid is so wide. That’s also why only one or two
animals are at the top of the pyramid.
Most food chains have only three or four levels. If there were more, a huge number of
producers would be needed at the base of the pyramid! Sometimes, things in the
environment may cause the number of organisms at one level of the pyramid to change.
Then the whole food chain is affected. Suppose a drought kills most of the grasses in an
area. Then some of the first-level consumers will starve. Many second-level and third-level
consumers will go hungry, too.
Suppose people cut down a forest to provide space for houses. The second-level and thirdlevel consumers may not be able to find enough small animals to eat, so they may leave
that ecosystem. With fewer carnivores to eat them, the number of small animals will
increase over time. If there isn’t enough food for their larger numbers, many will starve.
When a change in numbers occurs at any level of a food chain, the entire chain will be
affected.
.
The owl is a third
level consumer. It
takes a lot of grass,
locusts, and snakes to
provide the owl with
the energy it needs
The snakes are secondlevel consumers. They
pass on to the owl only
10 percent of the
energy they receive
from the locusts.
The locusts are
firstl evel
consumers. They
pass on to the
snakes only 10
percent of the
energy they
receive from the
grasses.