The total number of species and their varieties present in a system is referred to as the biodiversity. In absence of any stress on the system, under clean unpolluted conditions, the total number of species present in a system depends largely on the climatic conditions. As conditions deteriorate and the stress induced by pollution rises the weak and the susceptible forms are first to be affected.
They disappear. This results in a general reduction of biodiversity. Those forms which are capable of surviving in stressful environment grow and multiply rapidly as resources are now in plenty due to disappearance of weaker forms. Finally under extreme conditions only one or two species are able to survive in the system.
However, due to their capability to grow and multiply under polluted conditions coupled with lack of much competition these species built up huge populations. It is this phenomenon which has been made use of in a number of systems designed for biomonitoring of environmental quality. Using the total number of different species and the number of individuals of each species present, arbitrary system of numbers are devised for the assessment of degree of degradation or the extent of pollution in the habitat concerned by many workers.
Changes in population densities, frequencies, age-structure etc. often provide an early information system about the fate of species concerned and likely composition of biotic community in near future. Population of species composed primarily of young individual’s shows that it is reproducing rapidly and is likely to attain an important position in the community in near future.
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The species represented by old scenescent individuals suggests that it is going down and is about to disappear. A species losing its dominance to some other form for no obvious reason indicates that some subtle change is taking place in the environment. Shift in pattern of distribution, changes in frequency etc. in some forms can be co-related with changes in environmental conditions. In a large and complicated ecosystem it is not possible to study and document all species in this way and these observations are usually restricted to a few selected species or group of species which are more conspicuous in the system. Such species are usually referred to index species.