SPECIES-DIVERSITY, BIOGEOGRAPHY, AND THE EVOLUTION OF BIOTAS

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 a nd their included ecosystems and communities. Four processes add or su btract species from a region: speciation, extinction, biotic dispersio n, and long-distance dispersal. Speciation and biotic dispersion are p ostulated to result in historically structured (hierarchical) species assemblages, whereas long-distance dispersal results in assemblages th at would be expected to be historically unstructured (nonhierarchical) . Continental biotas, as exemplified by the Australian avifauna, are h istorically structured: they are segregated into areas of endemism hav ing hierarchical relationships that presumably arose as a result of th eir history being dominated by cycles of biotic dispersion and vicaria nce. It is also proposed that these latter two processes are necessary , and in many cases probably sufficient, to explain the taxonomic comp osition of communities within these areas of endemism. Long-distance d ispersal appears to play a much more minor role in the assembly of eit her continental biotas or their communities than current ecological th eory would predict.