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.