Practice question

In the dominance convention, Simpson’s D = Σpᵢ² is the probability that two randomly selected individuals belong to the same species. A value close to 1 occurs when one species contains nearly all individuals. The community is then highly dominated, the identity of a sampled organism is predictable, and diversity is low. Maximum evenness would spread abundance among species, reduce each squared proportion, and drive D downward. Infinite or maximum diversity is therefore incompatible with D near 1. The notation requires care because some texts label 1 − D as “Simpson’s index”; that complementary form has the opposite direction, approaching 1 as diversity increases. Others use 1/D, whose effective-number interpretation also rises with diversity. Here the answer set and surrounding items clearly employ D as dominance. For illustration, proportions 0.95, 0.03, and 0.02 give D = 0.904, whereas three equal species give D ≈ 0.333. Richness alone cannot determine D because the distribution of individuals matters strongly, and squaring makes abundant species much more influential than rare ones.