Measurement of Bio-Diversity

Rajkumari Ashalata / Laishangbam Sanjit *

Biodiversity is a science that is still in its infancy or in other words, at the "what" stage. Inventorying and monitoring works of biological diversity has just started in a few taxonomic groups and has yet to be started in most of the important taxa. As biodiversity equals variety at the species level of biological organization, the terms species richness and species diversity have become key topics in conservation biology. Both are important characteristics of community structure.

Therefore, much has been studied and reported on the measurement of the species richness and species diversity of communities. However, a certain looseness in the use of these terms is still found in literatures and school/college text books, e.g., by using them interchangeably. This led to a confusion of the concepts. In this article, an attempt has been made to discuss the concepts of species richness and species diversity in the light of recent developments in this area and suggest that a clear distinction should be made between the two to avoid the confusion surrounding these terms.

Species diversity is a better tool for ecologists who are interested only in understanding the mechanisms of certain ecological phenomena, such as pollution, environmental disturbances, etc. It is a function of the number of species present (species richness or species number) and the evenness with which the individuals are distributed among these species (species evenness or species equitability or abundance of each species).

This definition could be the best one we have at this moment; and its meaning probably should be restricted to at least this extent. Otherwise, ambiguity of the term may arise because species richness is neither a synonym nor a measure of species diversity. Species richness is easy to measure and understand, whereas measurement of species diversity is much more complicated because of different priorities in the reconciliation of its two factors: richness and evenness, which led to the invention of many indices, each of which has own limitations.

Moreover, species diversity, as it is usually measured, is an aspect of community structure, and structurally, many rare species are minor components of their community (e.g., Shannon's index is insensitive to rare species, which are very important in the studies of biodiversity). Since diversity has never had a single, unequivocal definition, we can muddle along with a plethora of indices, each supported by at least one person's intuition and a few recommended by fashion. One among these is Shannon-Wiener index, which is notoriously sample size dependent and weighted towards richness. Another measurement is Simpson's index which is weighted towards evenness.

McIntosh first coined the term species richness in 1967, but the concept itself is the oldest and most fundamental meaning of biological diversity. Species richness can refer to the number of species present (in a given area or in a given sample) without any particular regard for the number of individuals examined. It can be numerical species richness (or simply "species richness") or areal species richness (or simply "species density"- the number of species present in a given area).

Species-Area (SA) relationship of Rosenzweig (1995) expressed that 'the number of species that can exist in a given land area is a function of the area: smaller areas have fewer species and larger areas have more species, all other things being equal'. Finally, we have also the direct measurement of species number (NO), which seems to be one of the simplest and best methods of expressing species number relation.

Since species richness simply denotes the number of species, it is an unweighted measure of species number relations. In species diversity, species are weighted by some measure of their importance, viz. abundance, productivity, or size. As conservation of biological diversity is the most meaningful reason for the study of biodiversity, this particular branch of science is mostly dominated by conservation biologists, who are mainly interested in the few rare species that characterize most biota.

Therefore, we opine that species richness is the best tool for conservation biologists because it gives little emphasis to many common dominant species in a community. Another useful measurement of pattern of species distribution is spatial and temporal a, ß and ?- diversities. Since most of these measurements use the species presence-absence data, it is also suggested that it will be much clearer if we use the terms a, ß and ?- richness.

In fact, species richness (number of species) gives as much information related to biodiversity as species diversity. This leads to using both terms interchangeably, which in turn leads to a confusion of the concepts. Therefore, it is suggested that ecologists take approaches that are more direct to the study of species-numbers relations, by relying on direct measurement of number of species (species richness) rather than going through with many diversity indices which are simply derived variables.

The species abundance curve (of Dr Sanjit Laishangbam and Dr Dinesh Bhatt, 2005) is a simple approach to depict the species number, species abundance and species diversity together. During three years' sampling of birds in the Loktak Lake, the curve shows the presence of many rare species (having less than 10 individuals) and the dominance by a few abundant species which may act as an ecological indicator of the lake's changing ecosystem.