The vertebrate fauna of the Wallacean Island Interchange Zone: the basis of inbalance and impoverishment

Freshwater fishes, Amphibia, mammals and birds are used to produce an updated biogeography of Southeast Asia, Wallacea, and Wallace’s Line. The faunas are documented using newer data sets and interpreted relative to biological characteristics, and plate tectonics, geographical, and climatic, information. To give focus the material is arranged relative to two questions: (1) Why are the vertebrate faunas of Wallacea impoverished and unbalanced, that is, lacking in so many lineages dominant on the adjacent continents?; and (2) Why, given 10–15 million years of proximity has there not been a massive interchange of continental faunas through Wallacea. The survey confirms and amplifies earlier biogeographic knowledge. The extraordinarily rich the Oriental freshwater ichthyofauna is sharply delimited by Makassar Strait; it contrasts with the marine-derived ones of New Guinea - Australia, and Wallacea. In frogs contrasting Oriental and Australian lineages, only tenuously enter Wallacea from west and east. In mammals the exclusion of most lineages of the rich Bomeoan assemblage from Sulawesi is explained on historic and ecological grounds. Birds, the best group for such analysis because of their diversity and abundance are represented by a series of faunal “strata” with different histories, distributions, and ecologies. The Wallacean avifauna, far from being just a limited attenuation of the continental ones proves to be an integrated, island adapted unit, with a generally common structure. The geological data provides a background to the evolution of the contemporary biota and in addition to in toto generalized effects the differently adapted vertebrate lineages have obviously responded to its different components. Relative to the two basic questions posed it is noted that the New Guinea flora and insect and, to some extent, reptilian faunas, are Oriental in origin. Historically, then there must have been a considerable movement of biotas through Wallacea. The exclusion from Wallacea indicated by birds and mammals would, hence, be less general than appears. Relative to the “impoverishment” question it is pointed out that if the Wallacean avifauna is rich and diversified relative to ecological opportunities available the term does not really apply. It is depauperate only when measured against the continents, rather different environments. The status of Wallace’s Line is redefined relative to a cross-section of biological groups.