Science High School Reviewer | Grade 4 Science | Life Science | Exploring Ecosystems | Lesson 9: Energy Flow in Ecosystems

9.1 Energy flow in ecosystems

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Every living thing needs energy. Energy in an ecosystem is
passed from one organism to another.
Energy in Plants and Animals
Sunlight is the main energy source for life on Earth.
Energy from sunlight affects both the living and nonliving
parts of an ecosystem.
In the process of photosynthesis, green plants change
sunlight energy into chemical energy. The plants use
this chemical energy to stay alive. Since plants make, or
produce, their own food, they are called producers.
Many organisms cannot make their own food, so they
eat other organisms. Organisms that eat, or consume, other
living things are called consumers. There are different types
of consumers. Consumers that get energy by eating only
plants are herbivores. Consumers that eat only animals
are carnivores. Consumers that eat both plants and
animals are omnivores. Some consumers, called
scavengers, feed on dead plants and animals. Scavengers
that eat only meat are carnivores.
Organisms have adaptations that help them in their
niche. Mountain lions and other carnivores have claws for
catching and sharp teeth for eating their prey. Deer, cows,
and some other herbivores have front teeth that rip leaves
off plants, flat back teeth to grind leaves, and a four-part
stomach to help digest the food. Scavengers such as the
turkey vulture use their sharp beaks to tear meat.

A Food Chain
The energy stored by producers can be transferred along
a food chain. In a food chain, organisms transfer energy by
eating and being eaten.
A food chain always begins with energy from sunlight.
Producers are the next link in the chain. The flow of energy
through the chain happens in one direction. In the diagram
of a food chain, arrows show the flow of energy. Arrows point
from the “eaten” to the “eater.”
In a desert ecosystem, a food chain might consist of a
producer, such as the prickly pear cactus, that is eaten by an
omnivore, such as the collared peccary.
Coyotes are predators. Predators get the energy they need by
hunting and killing prey. The collared peccary is prey for the
coyote. The coyote uses the food energy that was stored in the
prickly pear cactus and then passed to the peccary.

Small Things That Make a Big Difference
What would happen if an ecosystem had only producers and
consumers? Before long, plants would use up all the minerals
in the soil. They would die. Herbivores would then starve. If
nutrients and minerals were not replaced, new organisms
would not be able to grow.
Decomposers digest the waste and remains of dead plants
and animals. These remains still have food energy stored inside
them. Some bacteria, fungi, and insects are decomposers.
Decomposers break down dead plant and animal tissues into
minerals and nutrients that are put back into the soil, air, and
water. Living plants use these materials.
When animals eat the plants, the
minerals and nutrients are passed
from the plants to the animals.
Whatever affects the
decomposers of an ecosystem
affects the health of the
soil. The producers that
grow in the soil and all the
consumers that eat them
are affected too.

How matter flows in an ecosystem

Food web

Decay in Ecosystems
All living things eventually die and rot, or
decay. Decaying plants make up a large part of
the Okefenokee Swamp. Decay is a very important
process. Without decay, dead organisms and wastes
would pile up and interfere with the habitats of
living things.
During the process of decay, scavengers feed
on the remains of the dead organisms. Then
decomposers, such as fungi and bacteria, break
down the dead organisms and return minerals and
nutrients to the ecosystem.
Rate of Decay
Dead organisms can decay slowly or quickly. How
fast an organism decays depends on temperature.
You put food in a refrigerator because organisms
decay more slowly in colder temperatures. In warmer
temperatures, decay happens quickly.
You cover the food in your refrigerator because
organisms decay more quickly when they are in
contact with oxygen. Fungi and some bacteria need
oxygen to live and grow. In many ecosystems, green
plants are an important source of oxygen.
The amount of moisture can also affect decay.
Many decomposers grow better and work faster when
there is more moisture.
Things that were once alive change more quickly
than things that were never alive. Suppose you put a
dead insect and a pebble that are about the same size
in a hole. You put a leaf and a piece of plastic bag
that’s about the size of the leaf in the same hole.
You won’t see it happen, but the decomposers go
to work immediately. They begin breaking down the
remains of the insect and the leaf. Soon, the minerals
and nutrients in the decomposing material will return
to the soil, air, and water. But many years will pass
before the plastic bag and the pebble change.

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To understand an ecosystem, scientists look
at the relationships and roles of organisms within
a community.

Producers
Every organism in an ecosystem relies on
producers. Producers are organisms that make
their own food using the energy in sunlight.
Producers on land include green plants, such as
grasses and trees. In lakes and oceans, the main
producers are algae. Many other protists are
producers, too.

Consumers
Organisms who cannot make their
own food are called consumers. Birds,
mammals, and other consumers get
energy from the food made by other
organisms.
We can classify consumers by the
kinds of food they eat. Herbivores
(UR•buh•vorz) eat only producers.
Porcupines and most other rodents
are herbivores. So are rabbits and deer.
Some animals eat producers and
consumers. These are omnivores
(OM•nuh•vorz). Opossums, raccoons,
and bears are all omnivores.
Carnivores (KAHR•nuh•vorz) are animals
that eat herbivores and omnivores.
Ospreys and other birds of prey are
carnivores. So are cats, tigers, and lions.
Sharks are carnivores, too.
Decomposers
Some organisms break down dead
and decaying matter into wastes and
simpler substances. These organisms are
decomposers. Worms, bacteria, fungi,
and other decomposers get energy this
way. They return the substances to the
ecosystem as nutrients. In other words,
decomposers are recyclers.

What is a food chain?
Every organism needs energy to live
and grow. The energy in an ecosystem
comes from the Sun.
Look at the animals shown on these
pages. None of them can use the Sun’s
energy directly. The energy of the Sun is
stored in food. That energy passes from
one organism to another in a food chain.
The energy in a food chain moves from
producers to consumers to decomposers.
A Pond Food Chain
Algae and green plants are first in the
pond food chain. Algae capture the Sun’s
energy during photosynthesis. They store
it in their cells as sugars.
What happens when a plant eater, such
as a mayfly, eats the algae? The insect uses
oxygen to release the energy stored in the
algae it ate. It uses some of that energy to
move, grow, eat, and reproduce. It stores
some of the energy in its tissues.
A meat eater like the sunfish might
snap up the mayfly. A blue heron may
then eat the sunfish. As you can see, even
the heron gets some of the Sun’s energy
that was passed along the chain.
All the plants and animals in the pond
become food for decomposers after they
die. Bacteria and other decomposers
break down the dead tissues into simple
nutrients that other living things can use.

A Land Food Chain Land Food Chain
A food chain on land is similar to
a pond food chain. On land the food
chain usually starts with grasses,
trees, and other green plants.
In the example on the right,
the spear thistle is the producer.
The painted lady caterpillar is
the herbivore that munches on
its leaves. The Chinese mantis,
skink, and barred owl are the other
consumers, in that order.
Where are the decomposers in
both food chains? Decomposers
are often left out of food chain
diagrams. This is because they take
part in every step of every food
chain. Every time you see a food
chain, remember the importance of
decomposers.
Notice the arrows shown in the
diagrams. The direction of the arrows
is important. Each arrow in a food
chain points away from the organism
that is eaten. The arrow always points
to the organism that eats it.

What is a food web?
A food chain is a good model of how energy travels
in the form of food. However, it only shows one path.
Most ecosystems have many different food chains that
link together. A food web shows how all the food
chains in an ecosystem are connected.
Predator and Prey
Food webs show relationships between predators
(PRED•uh•tuhrz) and their prey. A predator is a
carnivore that hunts for its food. The organisms that
it hunts are called prey. In many food webs, organisms
are predators as well as prey. You can see some of these
relationships in the diagram below.

Competition Land Food Web
A food web shows that a single
organism can take part in more than
one food chain. When this happens,
competition can result. Competition
is the struggle between organisms
for food, water, and other needs.
Look at the land food web. It has
different herbivores, such as deer,
small birds, and mice. What if they
all ate the same plants? The three
populations would compete for the
food. One population might win
out. The other populations would
die unless they found a different
food or moved to a different place.
Competition is not limited to
animals. In the forest, small plants
and flowers compete with tall trees
for sunlight and nutrients.
Individuals in a population also
compete with each other. You may
have watched squirrels in a park
compete for nuts. With all this
competition, all living things on
Earth can be considered part of one
giant food web.

What is an energy
pyramid?
Plants capture energy from the
Sun. When you eat a plant, how
much of that energy do you get?
An energy pyramid is a model
that shows the amount of energy at
each level of a food web. Producers
are always at the bottom, or base,
of the pyramid. They use about 9
10
of all the energy they produce. They
store the other tenth in their cells.
When an herbivore eats a
producer, it gets the plant’s stored
energy. But that amount is only 1
10 of
the original energy from the Sun.
There are fewer and fewer
organisms at each level of an energy
pyramid. The organisms at each
level use 9
10 of the available food
energy. Only 1
10 of the energy gets
passed to the next level. With only
1
10 of the energy available to them,
fewer organisms are able to survive
on the level above. The animals at
the top get just a tiny fraction of
the original energy.