Bird Species Diversity: Components of Shannon's Formula

Shannon's diversity index H' = Σpi log2 pi was calculated for 267 breeding bird censuses. The index was resolved into its components, species richness and relative abundance, to determine which components played a larger role in the determination of diversity patterns. Changes in diversity were correlated closely with species richness (r = 0.972), while the relative abundance component remained stable. Among the nine community types represented, diversity and species richness increased with foliage height diversity. The relatively abundance component was strikingly low in the marshes due to the presence of gregarious birds which nest in colonies and feed outside the community. Photoplankton differ from birds in that the relative abundance component is not stable from one collection to the next. This is attributed to differences in the environmental uncertainty encountered by the two groups, plankton inhabiting relatively uncertain environments and being "opportunistic," while birds occupy predictable environments and therefore "equilibrium" species. The distribution of relative abundances in birds is more even than in trees, herbaceous—shrub communities, or phytoplankton. Since this is in part the result of intraspecific territoriality, it is predicted that nonbreeding bird populations, species' relative abundances will be less even than they are during the breeding season. It is suggested that the regulation of diversity by either the species richness or relative abundance components represent alternative strategies which are suited to predictable/nonrigorous and unpredictable/rigorous environments, respectively. Therefore, differences similar to those observed between birds and phytoplankton might be expected in other groups of organisms.