Practice question

Shannon diversity is H′ = −Σpᵢ ln pᵢ, where pᵢ is the proportional abundance of each species. The index increases when more species are added and when individuals are distributed more evenly among the species already present. A rare species contributes relatively little because its pᵢ is small, while a strongly dominant species lowers overall uncertainty: a randomly chosen individual becomes easier to predict. For a fixed richness S, H′ reaches its maximum when every species has abundance 1/S, giving H′max = ln S. Evenness can therefore be expressed as J′ = H′/ln S. Dominance and unevenness move H′ downward, not upward, because most individuals become concentrated in a few taxa. “Gamma” describes regional diversity and is a spatial scale, not a direct driver within the formula. Shannon’s index is often interpreted as the uncertainty in predicting the species identity of a randomly sampled individual. High richness supplies more possible identities, and high evenness keeps those possibilities similarly likely. Its joint sensitivity to both components explains why two communities with equal species counts can have different H′ values.