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 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.
Carnivores or meat eaters feed only on other animals and are thus secondary or tertiary consumers such as lions, leopards, tigers, etc.
Omnivores feed on both plant and animal material. Human beings are good examples of omnivores.