Ecosystems include organisms and the nonliving components
of a particular habitat. How an organism uses resources within
the ecosystem depends upon upon its niche, or ecological role.
A food web describes how energy flows through an ecosystem.
Producers, a major niche in all ecosystems, undergo photosynthesis
to produce food. All producers are autotrophic, or "self-feeding".
In terrestrial ecosystems, producers are usually green plants.
Freshwater and marine ecosystems frequently have algae as
the dominant producers. Most of the energy captured through
photosynthesis is used for growth and maintenance of the producers.
A small amount of energy is available to feed consumers.
Consumers obtain their food from other organisms. They are
heterotrophs, or "other feeders". Herbivores
are a type of consumer that feeds directly on green plants
or algae in aquatic systems. Since herbivores take their food
directly from the producer level, they are also called primary
consumers. Carnivores feed on other animals and are
secondary or even tertiary consumers. Omnivors feed
on both plants and animals. Humans are omnivores. Decomposers,
mostly bacteria and fungi, recycle nutrients from decaying
organic material (detritus).
Energy flow within an ecosystem is often described by an energy
pyramid. Approximately 90% of energy available within a tropic
level is used to maintain organisms at that level. This energy
is ultimately converted to heat. Only 10% of the energy
within a tropic level is available to consumers at the next
tropic level. Energy flow within ecosystems follow two
- Energy is neither created nor destroyed, but merely changes
- When energy changes form, some usable energy is lost as
These rules are also known as the first and second laws of
thermodynamics, respectively. These laws help to explain why
energy is lost when moving up the food web.
Energy available to organisms
In a short food web, more energy is available to organisms.
For example, a larger population of humans can be sustained
by eating grain than by eating animals. To put this in numbers,
100 kg of grain fed directly to people becomes roughly 10
human kg. If the 100 kg of grain is fed to cattle, and the
resulting 10 kg of beef is fed to people, this generates only
1 human kg!
Photosynthesis is the process that the Producers
undergo to produce food for themselves and feed the Consumers.
Learn about the structures and processes involved in photosynthesis.