It is the variety and variation of living forms that exists in an ecological complex. The living form may vary from plants, animal life and micro-organisms. The plants may include species of herbs to large trees and animal life may vary from tiny insects to huge mammals. There are three levels at which biodiversity can be classified.

1.       Ecosystem biodiversity: It consists of various habitats and the organisms living under the different ecosystems. Hence it is broader level of biodiversity and determines the structure and function of the particular ecosystem through keystone species i.e. those species which determine the ability of other communities to exist in the ecosystem.

2.       Species biodiversity: It takes into account different number of species of organisms that exists in an ecosystem. It is measured by two parameters.

(a). Species richness: It is a number of species per unit area. Larger is the area, higher will be the species richness and greater will be the species biodiversity.

(b). Evenness or Equitability: It refers to the distribution of organisms of various species at a particular area. Let us take an example of sample area 1 and sample area 2 having three species A, B & C each. Sample area 1 has four organisms of species A and 1 each of B & C. Similarly sample area 2 has 2 organisms of A, B & C. The evenness of sample area 2 is greater as there is uniform distribution of various organisms of any particular species. More is the evenness, higher is the species richness.

3.    Genetic biodiversity: It is the variation among the genes of organisms of any species. It deals with speciation i.e. evolution of new species.  It is a part of species biodiversity but is considered to be a complex level of biological diversity.