Species diversity is the most popular index among the different indices used to characterize a community of organisms. It has two components- species richness and relative abundance of species.
The total number of all the species that are present in a community makes up the species richness. Relative abundance is the proportion of the different species present among the total individuals in a community.
The measurement approaches in biodiversity depend either on functional categories such as ecosystem, genetic diversity, species etc or on theoretical categories such as alpha, beta and gamma diversity.
Alpha diversity refers to the diversity of the species in a particular sample confined by either reproductive or geographical boundaries.
Beta diversity refers to changes in the sample composition with changes in environmental factors. For example, the species composition of forest lands across the slopes of mountain.
Gamma diversity is due to variations in the sample composition when different samples are combined and analyzed.
E.g. study of diversity of forest lands
Species richness is the simplest measure of diversity. But the species can be counted only in smaller and simpler ecosystems and the number of individuals in a species cannot be counted in all ecosystems. And, even if the counting is carried out, it consumes more time and therefore not feasible in all types of ecosystems.
Diversity indices provide measure of species richness and evenness of distribution of species in the total community. However, over 60 indices are in use in ecological studies which are used to indicate the relative abundance of species. There are two major forms of indices- dominance indices and information indices
Dominance indices give more weight to the abundance of the species in the community. Information indices are geared more towards evenness. Some of the most common indices used in ecological studies and their importance are presented.
1. Dominance index
It is referred as 1-J where J is the evenness of relative diversity (H'/ Hmax). Absolute evenness is given the value 1. It is used to compare the dominance of a single species under different conditions of environment or similar ecosystems.
e.g. Phytoplankton dominance across the lakes of a particular geographic region are calculated and compared through dominance index.
2. Simpson's Diversity Index
It is a dominance index which measure the probability of two organisms randomly picked up from a sample being of two different groups. Two trees randomly selected from a rain forest will have less probability for being in the same species than those selected from a boreal forest in a similar fashion.
It is calculated as
Ds = S(ni (ni -1)/N(N-1))
Where Ds represent the Simpson index after introducing bias correction, n1 is the number of individuals of species i and N is the total number of species in the community.
A lower diversity index represents higher diversity. The reciprocal form 1/ Ds or complimentary form 1-Ds is often used to represent the index. Here diversity increases as the index value increases. The value of D ranges from 0 to 1. 0 represents infinite diversity and 1 represents absence of diversity.
3. Shannon-Weiner index
It measures the uncertainties of species categories or evenness. It assumes that all the species are represented in a community and the sample is randomized. In short, it represents entropy in a community.
It is calculated as
H' = - S pi lnpi
Where pi is the proportion of the ith species and ln pi is the natural logarithm of pi.
The index is a representation of both species abundance and evenness. When either of these two factors increases, the diversity index also increases. Shannon's index has a maximum value when all the species are equally abundant in a community.
The index can be used to measure evenness of a particular species of a community.
Evenness = H'/ Hmax where H max is the maximum entropy of the ecosystem. In other words, H' = Hmax when all the species are equally abundant in an ecosystem i.e. there is maximum evenness. Evenness ranges from 0 to 1.
Communities with a single taxon have an index of 0 whereas those having more taxa have higher values. The index is also a measure of pollution of water bodies with values greater than 3 indicating clean water. Values less than 1 are typically highly polluted.
These indices are compared and analyzed across the different ecosystems for most studies. Modern ecologists use modified indices apart from these basic ones. They are discussed in the next part of the series.
Alpha diversity is the diversity of organisms within a selected habitat or sample, and is quantified by indices and by rank abundance models.
Beta diversity is an index of the rate of increase of alpha diversity as new habitats are sampled, so is a measure of the turnover of species along a spatial gradient.
Finally, gamma diversity is the full diversity (species richness) of an entire sampled landscape or gradient
Scalar/vector: Î± and Î³ diversity are scalar quantities, ie may be represented by a single number. ÃŸ diversity is a vector: it must have a directional component as well as a magnitude.
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