7.5 C notes on Energy Flow through living systems
The energy flow through living systems begins with sunlight and photosynthesis. Energy flows
in one direction as it goes through photosynthetic organisms, including green plants and algae to
herbivores then carnivores and decomposers. Autotrophs, or producers, capture radiant energy
from the Sun to make sugars. Herbivores, or primary consumers, eat producers to provide them
with their food. Carnivores, or secondary consumers, eat the primary consumers as their source
of energy. Omnivores, such as humans, are capable of functioning as primary and secondary
consumers. All consumers are heterotrophic, as they cannot make their own food using the Sun,
but depend upon other organisms for their energy needs.
One way of representing the energy flow through the living components of an ecosystem is
through the use of a food chain. A food chain diagrams the transformation of energy as it flows
from the Sun to producers and then through a series of organisms, which feed upon each other.
Grass –> Grasshoppers –> Mice –> Foxes
In the example of a diagrammed food chain above, the arrows in the food chain point to the
organisms that are doing the eating. The arrows in the food chain represent the energy flow
through the ecosystem.
The energy flow is usually more intricate than illustrated by just a single food chain. Instead, a
food web is used to show a series of interrelated food chains, which provides a more accurate
picture of the feeding relationships in an ecosystem.
Feeding Relationships in an Ecosystem
Energy flow in a food web, as in the example above, also starts with the producers and goes
through the various levels of consumers as in a food chain. Again, the arrows indicate the energy
flow in an ecosystem showing the interconnected relationships among living organisms.
An energy pyramid is a diagram showing the energy amount that moves from one feeding or
trophic level to another in a food chain or food web. The most available energy is located at the
producer or autotroph level. This level is diagrammed at the pyramid base, indicating the largest
amount of available energy. The next level, primary consumers or heterotrophs, is diagrammed
directly above the producer level. This location indicates there is less available energy than the
producer level. As each level is diagrammed, there is less energy available going up the pyramid.
Energy Pyramid
On average, each feeding level diagrammed in an energy pyramid only contains about 10% of
the energy as the one below it. The other 90% of the energy is transformed as heat to the
environment or used for the organism’s life processes. There is so little available energy from
one level to the next that most food webs only have three or four feeding levels. There is not
enough energy to support many feeding levels. Organisms at higher feeding levels of an energy
pyramid do not necessarily require less to live than organisms at lower levels. Since most energy
is transferred to other energy forms at each level, the energy amount in the producer level limits
the number of consumers an ecosystem supports. The height of the pyramid is limited by this
transfer of energy.