0.5 Species diversity

Strictly speaking, species diversity is the number of different species in a particular area ( species richness ) weighted by some measure of abundance such as number of individuals or biomass. However, it is common forconservation biologists to speak of species diversity even when they are actually referring to species richness.

Another measure of species diversity is the species evenness , which is the relative abundance with which each species is represented in anarea. An ecosystem    where all the species are represented by the same number of individuals has highspecies evenness. An ecosystem where some species are represented by many individuals, and other species arerepresented by very few individuals has a low species evenness. shows the abundance of species (number of individuals per hectare) in three ecosystems andgives the measures of species richness (S), evenness (E), and the Shannon diversity index (H).

Shannon's diversity index $H=-\sum {}_i\ln {}_i$

• ${}_i$ is the proportion of the total number of specimens $i$ expressed as a proportion of the total number of species for all species in theecosystem. The product of ${}_i\ln {}_i$ for each species in the ecosystem is summed, and multiplied by $-1$ to give $H$ . The species evenness index( $E$ ) is calculated as $E=\frac{H}{H_{\text{max}}}$ .
• $H_{\text{max}}$ is the maximum possible value of $H$ , and is equivalent to $\ln S$ . Thus $E=\frac{H}{\ln S}$

In , ecosystem A shows the greatest diversity in terms of species richness. However, ecosystem Bcould be described as being richer insofar as most species present are more evenly represented by numbersof individuals; thus the species evenness (E) value is larger. This example also illustrates a condition that is oftenseen in tropical ecosystems, where disturbance of the ecosystem causes uncommon species to become even less common, and commonspecies to become even more common. Disturbance of ecosystem B may produce ecosystem C, where the uncommon species 3 has becomeless common, and the relatively common species 1 has become more common. There may even be an increase in the number of speciesin some disturbed ecosystems but, as noted above, this may occur with a concomitant reduction in the abundance of individuals orlocal extinction of the rarer species.

Species richness and species evenness are probably the most frequently used measures of the total biodiversity of a region.Species diversity is also described in terms of the phylogenetic diversity , or evolutionary relatedness, of the species present in an area. Forexample, some areas may be rich in closely related taxa, having evolved from a common ancestor that was also found in that samearea, whereas other areas may have an array of less closely related species descended from different ancestors (see furthercomments in the section on Species diversity as a surrogate for global biodiversity ).