Practice question

A high value of Simpson’s dominance index D = Σpᵢ² indicates that abundance is concentrated in one or a few species. Since D is also the probability that two randomly chosen individuals belong to the same species, strong dominance makes this probability large and ecological diversity low. If all species are similarly abundant, each pᵢ is smaller, the squared terms shrink, and D declines. Species richness also tends to reduce D when additional species contribute appreciable abundance. Thus a high D does not imply high diversity or equal abundance, and it says nothing directly about turnover among sites, which is beta diversity. Numerical interpretation must follow the stated convention: some sources call 1 − D “Simpson’s diversity index,” in which case high values indicate high diversity, while 1/D is a reciprocal diversity measure with the same direction. Here the symbol D is being used consistently as the dominance form. For instance, a community with proportions 0.9, 0.05, and 0.05 has D = 0.815, reflecting far lower effective diversity than three equally abundant species, for which D is about 0.333.