From a traditional ecological viewpoint, diversity may be thought of as the number of groups (usually species) present in an assemblage, or how evenly those groups occur in the assemblage. These aspects of diversity are generally termed richness (species richness for example) and evenness. High richness equates to high diversity, and a highly dominated assemblage (i.e., one with low evenness) is considered to be less diverse than a more even one.

A large number of diversity measures have been formulated and these tend to be measures of richness (e.g., Margalefs Index), evenness (see Coastal and Estuarine Environments), or are constructed in such a way as to combine the two components in one measure (e.g., Shannon-Wiener Index, which is weighted toward the evenness component). Given that these measures are all based on a limited amount of information, namely the numbers of individuals in each group independent of their actual identities, it should not be surprising that many of them are very closely related to each other.

Many diversity measures calculated for samples suffer from severe sample-size or sampling-effort dependence. This is almost inevitable with richness measures, even those which are formulated to reduce the effects ofincreas-ing sample sizes capturing additional species. It is also true, to a lesser extent, of many indices weighted toward the evenness component of diversity. One measure which tends to be relatively sample-size independent is the Simpson index (Figure 1).

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