Each and every living organism has specific surroundings or medium with which is continuously interacts, and to which it is fully adapted. This surrounding is the ‘natural environment’. The word ‘natural environment’ brings to mind broad aspects of landscape, such as soil, water, desert or mountains which can be more exactly described in terms of physical influences such as differences in moisture, temperature, texture of soil, and biological influences. Thus, environment is the sum total of living and non-living components; influences and events surrounding and organism. Living components are called biotic components while non-living is called abiotic components.

No organism can live alone without interacting with other organisms so each has other organisms as a necessary part of its environment. We know that all animals are directly or indirectly dependent upon green plants. However, plants also depend on animals for a few things such as pollination of flowers and dispersal of seeds or fruits.

What would be the components of its environment? The abiotic factors in the pond’s environment are light, temperature and water in which nutrients; oxygen, other gases and organic matter are dissolved. The biotic components include microscopic as well as larger plants and animals. The plants are of different kinds such as phytoplankton, partly submerged plants and plants growth around the edge or the pond. The animals consist of zooplankton, bacteria, insects, worms, mollusks, tadpoles, frogs, ducks and several kinds of fishes. The characteristic features of the pond will also depend upon such abiotic features as the intensity and amount of sunlight, altitude, temperature, depth of water and nature of the underlying rocks etc.

Now we consider another example, of fish in this pond. The living and non-living constituents in the pond would make the environment of the fish. We can call it the external environment. There is another environment within the body of fish, its internal environment. The body surface acts as an exchange barrier between the external and the internal environment. The internal environment is relatively stable as compared to the external environment. However, it is not absolutely constant. Illness, injury or environmental stress can upset it. But when the cause for the upset is removed, internal environment comes back to its natural condition.