through ecosystems in one direction, typically from the Sun,
through photosynthetic organisms including green plants and
algae, to herbivores to carnivores and decomposers.
Green plants and algae are called autotrophs or producer
organisms, as they capture solar energy to make sugars in the
process of photosynthesis. Herbivores or primary
consumers use the producer organisms to provide them with
their food. Carnivores are secondary
consumers as they eat the primary consumers as their
source of food. Some organisms are capable
of functioning as primary consumers (eating plant material)
and as secondary consumers (eating animal
material). These organisms are called omnivores.
Humans are examples of omnivores. All consumers
are examples of heterotrophic organisms, as they can
not make their own food using the sun, but depend upon the
ingestion of other organisms for their nutrition.
If an ecosystem is to be self-sustaining it must
contain a flow of energy. One way of
representing the flow of energy through the living
components of an ecosystem is through the use of a food
chain. A food chain indicates the
transfer of energy from producers through a series of
organisms which feed upon each other.
A Food Chain
Note that the arrows in the food chain
point to the organisms which are doing the
eating. Thus the arrows in the food chain
represent the flow of energy through the ecosystem.
|The algae and floating plants
are the producers in this food chain.
The aquatic crustaceans are the primary consumers
which eat the producers.
Fish are secondary
consumers eating the primary consumers.
chain may also contain third level or other consumers as
indicated by the raccoons in this food chain.
In a natural
community, the flow of energy and materials is much more
complicated than illustrated by any one food chain.
A food web is a series of interrelated food chains
which provides a more accurate picture of the feeding
relationships in an ecosystem, as more than one thing will
usually eat a particular species.
A Food Web
|Energy flow in a food web also starts with the producer
organisms through the various levels of consumer organisms
as in a food chain.
energy pyramid provides a means of describing the
feeding and energy relationships within a food chain or
Each step of an energy pyramid shows that some
energy is stored in newly made structures of the
organism which eats the preceding one. The
pyramid also shows that much of the energy is lost when
one organism in a food chain eats another.
Most of this energy which is lost goes into the
environment as heat energy.
While a continuous
input of energy from sunlight keeps the process going,
the height of energy pyramids (and therefore the length
of food chains) is limited by this loss of energy.
An Energy Pyramid
The picture at the
left is an energy pyramid. Producer organisms
represent the greatest amount of living tissue or biomass
at the bottom of the pyramid. The organisms
which occupy the rest of the pyramid belong to the feeding
levels indicated in each step. On
average, each feeding level only contains 10% of the energy
as the one below it, with the energy that is lost mostly
being transformed to heat.