Ecological studies are made at different levels. The main aim of study at these different levels is based on different ecological units like individual, groups of individuals etc., The ecological units are as follows. Species: This is the lowest stable unit in taxonomic hierarchy. This forms the main aim of study when an individual species has to be studied in relation with others. Ecological study where an individual species is the centre of attention is known as autecology.
A population may be defined as a group or collection of individuals belonging to a single species. The units of study are populations. Study of interaction between individuals in terms of growth, reproduction, dispersal etc., constitutes population ecology.
A community may be defined as populations of different species bound together in a common habitat. The members of a community may be plants as well as animals. Only the living components of a community however are studied here mainly to analyse the nature of interdependence.
An ecosystem may be defined as the total environment including both biotic and abiotic components. In any environment a mere study of interactions between living organisms will not give a total picture of the complexities because in the growth, development and other parameters of living beings the abiotic components play great role.
The role of the abiotic components such as climate, soil, temperature etc., have to be analysed before one can understand the total parameters of living beings. It is this realisation of ecologists that has lead to the concept of ecosystem. The concept of ecosystem was first proposed by Tansley (1935) who defined it as “the system resulting from the integration of all living and non living factors of environment”.
Thus according to Tansley not only the living communities but also the non living components constitute an integral part of ecosystem. Odum (1971) defines ecosystem as the basic functional unit of organisms and their environment interacting with each other and within their own components.