Ecosystems
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
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
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 pyramid
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 rules:

• Energy is neither created nor destroyed, but merely changes form
• When energy changes form, some usable energy is lost as heat

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
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.