Practice question

Simpson’s dominance index D = Σpᵢ² measures the probability that two randomly selected individuals belong to the same species. Taking its complement gives 1 − D, the probability that the two individuals belong to different species. This complementary measure is called the Gini–Simpson index and increases with both richness and evenness. A community dominated by one species has D near 1 and 1 − D near 0; an increasingly even, species-rich community has smaller D and a Gini–Simpson value approaching 1. The alternatives 1/D and D represent the reciprocal Simpson index and dominance index, respectively, while D − 1 would be non-positive over the usual range and is not the standard diversity transformation. For a finite sample, an unbiased form may use counts as 1 − Σnᵢ(nᵢ − 1)/[N(N − 1)], but its probabilistic interpretation remains the same. Naming the formula matters because “Simpson’s index” is used inconsistently across texts. Stating 1 − D removes that ambiguity and makes larger numerical values correspond intuitively to greater diversity.