The biotic environment concerns all kinds of living things which an organism comes into contact with. The place where an organism lives is its habitat which may be a pond, a stream, a river, a forest or a desert. An organism in any habitat is never an independent unit. Its daily existence depends on and is influenced by other organisms around it. The interdependence of the various organisms is illustrated by the nitrogen cycle, the carbon cycle and the food chains. Every organism in the habitat has an important part to play. The function of an organism or the role it plays in the habitat is known as the ecological niche. The biotic environment consist of different kinds (or species) of plants and animals. A population is a group of individuals of the same species occupying a given area. It is expressed in terms of density, i.e. The number of individuals per unit area. The rate of change of a population depends on the birth rate (or the number of new individuals added to it), the death rate and migration. When populations of plants and animals live together under the same environmental conditions they constitute an ecological community. Ponds, open plains, forests and oceans are some examples of communities.