Practice question

Simpson’s D, in its dominance form, is calculated as the sum of squared relative abundances, Σpᵢ². Squaring gives disproportionate weight to common species, so D becomes large when one or a few taxa account for most individuals. It can also be viewed as the probability that two randomly sampled individuals are conspecific. Both interpretations make D a dominance measure: high D indicates concentrated abundance and therefore low diversity. Shannon’s index incorporates richness and evenness through −Σpᵢ ln pᵢ but is not conventionally labelled a dominance index. The Gini–Simpson index, 1 − D, reverses the scale and measures the probability that two individuals belong to different species. Alpha diversity is a spatial category—diversity within a local community—rather than a particular mathematical index. For example, two communities may each contain ten species, yet the one in which a single species forms 90% of all individuals will have much higher D. This sensitivity to abundant taxa makes Simpson’s D relatively robust to rare species missed by sampling, but also means it describes dominance more strongly than simple richness.