| Fig. 6. As a measure of species diversity in a community, a species richness index is extremely influenced by the presence of rare species, as shown here in species-abundance diagrams for four hypothetical communities, two in which species richness (the actual number of species in a community) is 10 (a,b) and two in which species richness is five (c,d). Each community set has one community in which the relative abundances of species (or the evenness of the distribution) are equal (b,d) and one in which they are not (a,c). By the criterion of species richness, a) and b) have equal diversity and c) and d) have equal diversity, although b) and d) have much higher diversity by any index that takes into account the relative abundances of the species. A harvest that alters the abundance of a species must therefore alter diversity measured by any of those indices, but a similar harvest would not produce detectable change in diversity if diversity is measured simply as species richness. | |
