Practice question

Shannon–Weaver diversity is calculated as H′ = −Σpᵢ ln pᵢ. The number of terms reflects species richness, while the relative abundances pᵢ determine evenness. Adding species generally raises H′, and distributing individuals more equally raises it further. For a fixed richness S, the maximum is ln S when every species has pᵢ = 1/S. This permits Pielou evenness to be calculated as J′ = H′/ln S. The index can be interpreted as uncertainty in predicting the species identity of a randomly selected individual: many equally likely species create high uncertainty, whereas dominance makes identity predictable and H′ low. Richness-only metrics ignore abundance, and dominance-only descriptions emphasize concentration without jointly representing both components in this form. Turnover is beta diversity and requires comparison among communities rather than calculation within one abundance distribution. Shannon values depend on logarithm base and sampling completeness, particularly detection of rare species, so valid comparisons need consistent methods. The index’s sensitivity to both how many species occur and how their individuals are apportioned explains its widespread use in community ecology.