Practice question

Species richness supplies the number of different taxa, while evenness describes how uniformly individuals are distributed among them. A community high in both components has many possible species identities and little numerical dominance, producing high values of composite diversity measures. Shannon H′ rises with richness and reaches its maximum for a fixed richness when all species are equally abundant. Simpson dominance D becomes low under even abundance, while its complement 1 − D becomes high. Thus “low Simpson index” could be misleading: it is true only for the dominance form D, not for the Gini–Simpson diversity form. “High biodiversity” is the convention-independent description. High dominance directly contradicts high evenness, and low diversity contradicts the combination of many species and equitable abundance. Ecologically, such communities may use resources complementarily and can show insurance against environmental fluctuation, although diversity does not guarantee stability in every food web. Sampling effort and taxonomic resolution must be comparable when communities are contrasted. This example shows why richness alone is incomplete: a species-rich assemblage dominated by one taxon can be less diverse by abundance-sensitive indices than a somewhat poorer but highly even assemblage.