Coexistence of congeneric species in the Avifaunas of Australian Islands

Abstract

Gause's model of competition among species in a confined space is applied to the breeding land and freshwater bird species on nine large (> 109 km2) and 77 small (< 30 km2) islands around Australia. If interspecific competition is the most important factor impoverishing island avifaunas, then islands should have fewer congeneric species than expected on the basis of a logarithmic association between numbers of species and genera, and of a Monte Carlo method. This was so for small islands, but large islands tend to have more congeneric bird species than expected. The latter results are contrary to the Gause model of ecological incompatibility and extinction. Their biological significance may be that congeneric species have similar ecological requirements and preferences, similar dispersal abilities, and are able to persist together as well as distantly related species can. The majority of small islands do not support any congeners, which is consistent with the Gause model.