Ecosystems are the basic functional unit of the enviroment. It is difficult to define what an ecosystems precisely is, but the following definition is acceptable:

An ecosystem is a more-or-less self contained ecological entity, consisting of both organisms and their complete biotic (living) and abiotic (non-living) environment found in a particular place at a particular time.

For convenience, fairly defined localities such as a forest ecosystem, pond ecosystem, a stream or a stretch of grassland are considered examples of ecosystems. A key feature is that members of a particular ecosystem are more likely to interact with members of the same ecosystem than they are with members of a different ecosystem.

An ecosystem possesses both biotic (living) and abiotic non-living components. The abiotic components include soil, water, light, inorganic nutrients and weather. The biotic components of the ecosystem can be categorized as either producers or consumers. Producers are autotrophic organisms with the capability of carrying out photosynthesis and making food themselves, and indirectly for the other organisms as well. In terrestrial ecosystems the producers are predominantly green plants, while in freshwater and marine ecosystems the dominant producers are various species of algae. Consumers are heterotrophic organisms that used food that has already been performed by other organisms. It is possible to distinguish four types of consumers, depending on their food source.

• Herbivores
  Herbivores or plant eaters feed on plants. They are termed primary consumers. These animals range from aphids which suck plant juices, to large browsing animals like springbok, giraffes and elephants.

  Elephants are good examples of herbivores

• Carnivores
  

  Carnivores or meat eaters feed only on other animals and are thus secondary or tertiary consumers such as lions, leopards, tigers, etc.

   

• Omnivores
  Omnivores feed on both plant and animal material. Human beings are good examples of omnivores.

  Monkeys are good examples of omnivores

• Decomposers
  

  Decomposers feed on food and decomposing matter or detritus. Detritus is the remains of plants and animals following their death and fragmentation by soil organisms, bacteria and fungi of decay are important decomposers, but so are other soil organisms, such as earthworms and small arthropods.

The word 'system' in ecosystem suggests an organized unit wherein all the parts, biotic and abiotic, are in constant interaction with one another. For example, consider a mosquito larva in a fresh-water ecosystem. It depends on the abiotic factor water to provide a medium in which to swim and it depends on the biotic microscopic algae for its food supply. In turn the predatory dragonfly nymph depends to some extent on the mosquito larva for food. The nymph is in turn eaten by fish. Other predators, such as the kingfisher bird, feed on the fish. The abiotic factor of light permits growth of the algae on which the larvae feeds.

In an ecosystem nothing is on its own. Ultimately everything is interdependent.

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