Practice question

Ten species represented by ten individuals each have identical relative abundance, pᵢ = 10/100 = 0.1. This is perfect evenness because no species is numerically dominant. For a fixed richness, equal abundance maximizes both Shannon diversity and the Gini–Simpson index. Shannon diversity would be H′ = −10(0.1 ln 0.1) = ln 10, and Pielou’s evenness H′/ln S would equal 1. Simpson dominance would be 10(0.1²) = 0.1, relatively low, while 1 − D would be 0.9. The community also has a richness of ten species; whether that richness is “high” depends on the taxon, area, and comparison community, but it is not low merely because each species has ten individuals. High dominance is excluded by the equal counts, and low diversity is inconsistent with the simultaneous richness and maximum evenness. This illustrates why abundance data improve on a species count alone. Two communities may each contain ten species, but one with 91 individuals of one species and one individual of each remaining species would have markedly lower evenness and composite diversity.