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Simpson’s index may be defined in different ways, but the original and simplest is that it is the probability that two individuals drawn at random from an assemblage will belong to the same species. As such it is a measure of dominance, and for a highly dominated (i.e., highly uneven) assemblage the probability of drawing two individuals from the same species will be high (approaching 1). For a completely even assemblage, in which all individuals belong to different species, the probability of drawing two individuals from the same species will be 0. Conventionally, more even assemblages are considered to be more diverse; therefore, this scaling appears counterintuitive as high values imply low diversity. The index is often, therefore, converted from a dominance measure into an evenness (or equitability) measure either by subtracting the dominance value from 1, or by taking its inverse. In comparison to other measures of richness and evenness, Simpson’s index can be shown to be relatively sample-size independent. Its simple definition also suggests methods for its estimation which do not require detailed taxonomic expertise.
|Publication Type:||Book Chapter|
|Copyright:||© 2008 Elsevier B.V.|
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