Energy Flow
in
Ecosystems
Movement of Energy through Ecosystems

Everything that organisms do in
an ecosystem; running,
breathing, burrowing, growing,
requires energy.

The flow of energy is the most
important factor that controls
what kind of organisms live in an
ecosystem and how many
organisms the ecosystem can
support.
Movement of Energy through Ecosystems
Primary Energy Source

Most life on Earth depends on
photosynthetic organisms, which
capture sunlight and convert it to
chemical energy in organic
molecules. These organic
molecules are what we call food.

The rate at which organic
material is produced by
photosynthetic organisms is
called primary productivity.

Primary productivity determines
the amount of energy available in
an ecosystem.
Movement of Energy through Ecosystems
Primary Energy Source

Most organisms in an ecosystem
can be thought of as chemical
machines driven by the energy
produced in photosynthesis.

Organisms that first capture the
energy are called producers and
include plants, some kinds of
bacteria, and algae.
Movement of Energy through Ecosystems
Primary Energy Source

Producers make energy-storing
molecules.

All other organisms in an
ecosystem are consumers.

Consumers are those organisms
that consume plants or other
organisms to obtain their energy.
Movement of Energy through Ecosystems
Trophic Levels

Ecologists study how energy
moves through an ecosystem by
assigning organisms in that
ecosystem to a specific level
called a trophic level.

In the chart at right, energy
moves from one trophic level to
the next; from sunlight to
producer to primary consumer
and on down the chain.
Movement of Energy through Ecosystems
Trophic Levels

The path of energy through the
trophic levels of an ecosystem is
called a food chain.

The lowest trophic level of an
ecosystem is occupied by the
producers (plants, algae,
bacteria) which take sunlight and
convert it to food energy.

Producers use the energy of the
sun to build energy rich
carbohydrates.
Movement of Energy through Ecosystems
Trophic Levels: Primary Consumers

At the second trophic level we
have herbivores. Herbivores are
animals that eat plants or other
primary producers.

These organisms that consume
producers are called primary
consumers.

Cows and horses, as well as
caterpillars and ducks, field mice
and moose; are all examples of
primary consumers.
Movement of Energy through Ecosystems
Trophic Levels: Secondary Consumers

A herbivore must be able to break
down a plants molecules into useful
energy.

At the third trophic level we have
secondary consumers.

Secondary consumers are animals
that eat other animals.

These animals are called carnivores.

In the diagram at right, the mouse is a
primary consumer because it feeds off
the producer.
The snake is a secondary consumer
because it feeds off the mouse.

Movement of Energy through Ecosystems
Trophic Levels: Secondary Consumers

At the third trophic level (secondary
consumers), animals such as tigers,
wolves, and snakes are carnivores;
consuming other animals.

Some animals, such as bears, eat
both plants and other animals. They
are both herbivores and carnivores
together, called an omnivore.
Movement of Energy through Ecosystems
Trophic Levels: decomposers

In every ecosystem there is a special
class of consumers called detritivores.

Detritivores are organisms which
obtain their energy from organic
wastes and dead bodies produced at
all trophic levels.

They are the decomposers of the food
chain; assisting in breaking down
dead organisms and helping return
their nutrients to the Earth.

These decomposers include worms,
bacteria, fungi and some insects.
Movement of Energy through Ecosystems
Trophic Levels: decomposers

Decomposition of bodies and wastes
releases nutrients back into the
environment to be recycled by other
organisms.

The recycled nutrients provide
mineral nourishment for the producers
in the food chain.
Movement of Energy through Ecosystems
Trophic Levels: Tertiary consumers

Many ecosystems contain a fourth
trophic level made up of carnivores
that consume other carnivores called
tertiary consumers.

The hawk that eats a snake is a
tertiary consumer unless a larger bird
of prey eats the hawk.

Very rarely do ecosystems contain
more than 4 trophic levels.
Food Webs

In most ecosystems, energy does not
simply follow one path on one level
because organisms often feed at many
different levels of a food chain.

This creates a complicated
interconnected group of food chains
called a food web.

In the diagram at right, the producers
are the grass and plants and the rabbits,
squirrels, mice birds and insects. The
fox, hawk and snake are secondary
consumers, eating the rabbits and
squirrels.

The fox, hawks and snakes are all also
examples of tertiary consumers because
they are the final stop on the food web.
Only with their death and decay does
their energy return to the system.
Food Webs
Look at the food web to the right.

Which organisms in this chart are
the producers taking their energy
from sunlight?

Which are the primary consumers
that feed off the producers
plants?

Which are the secondary
producers that feed off the
primary consumers?

Which of these organisms are
tertiary consumers, being the final
steps in their food chains?
Loss of Energy in a Food Chain:

A deer browsing on grass is
taking in potential energy.
Potential energy is stored in the
chemical bonds within the
molecules of the leaves.

Some of this energy is
transformed to other forms of
energy, such as fat in an
organism.

Some of this energy aids the deer
in life processes; such as running
and breathing.

Much of the energy is dispersed
into the environment as body
heat given off by the deer.
Energy Loss

During every transfer of energy in
an ecosystem, energy is lost as
heat.

Although heat can be used to do
work (as in a steam engine), it is
generally not a useful source of
energy in biological systems.

Thus, the amount of useful
energy decreases as energy
passes through the levels of an
ecosystem.

This loss of energy limits the
number of trophic levels an
ecosystem can support.
Energy Loss in a food chain

When a plant harvests energy
from sunlight, photosynthesis
captures only about 1% of the
energy available to the leaves.

When a herbivore consumes a
plant, using the plants molecules
to make food molecules within
the herbivore, only about 10% of
the energy in the plant ends up in
the herbivores molecules. 90% is
lost in the process.

When a carnivore consumes the
herbivore, again only 10% of the
energy of the herbivore gets
passed along. Again, 90% is lost.
Energy Loss in a food chain

At each trophic level, the energy
stored by the organisms in a level
is about 1/10th of the energy
stored by the previous level.

Ecologists often illustrate the flow
of energy through ecosystems
with an energy pyramid.

An energy pyramid is a diagram
in which each trophic level is
represented by a block, and the
blocks are stacked on top of each
other, with the lowest trophic level
as the base of the pyramid.
Energy pyramid
Energy Loss in a food chain

The organisms at higher feeding
levels of an energy pyramid do
not necessarily require less
energy to live than organisms at
lower levels.

Since so much energy is lost at
each level, the amount of energy
in the producer level limits the
number of consumers the
ecosystem can support.

As a result, there usually are few
organisms at the highest level in
a food web and increasingly more
organisms as you move down the
energy pyramid to successively
lower feeding levels.
Energy pyramid