Species Diversity, Biogeography, and the Evolution of Biotas

SYNOPSIS. A central scientific problem for ecologists and systematists has been to explain spatiotemporal patterns of species diversity. One aspect of this question is how to understand the taxonomic assembly of biotas and their included ecosystems and communities. Four processes add or subtract species from a region: speciation, extinction, biotic dispersion, and long-distance dispersal. Speciation and biotic dispersion are postulated to result in historically structured (hierarchical) species assemblages, whereas long-distance dispersal results in assemblages that would be expected to be historically unstructured (nonhierarchical). Continental biotas, as exemplified by the Australian avifauna, are historically structured: they are segregated into areas of endemism having hierarchical relationships that presumably arose as a result of their history being dominated by cycles of biotic dispersion and vicariance. It is also proposed that these latter two processes are necessary, and in many cases probably sufficient, to explain the taxonomic composition of communities within these areas of endemism. Long-distance dispersal appears to play a much more minor role in the assembly of either continental biotas or their communities than current ecological theory would predict.