Biogeography of mammals in SE Asia: estimates of rates of colonization, extinction and speciation

Abstract

Four categories of islands in SE Asia may be identified on the basis of their histories of landbridge connections. Those islands on the shallow, continental Sunda Shelf were joined to the Asian mainland by a broad landbridge during the late Pleistocene; other islands were connected to the Sunda Shelf by a middle Pleistocene landbridge; some were parts of larger oceanic islands; and others remained as isolated oceanic islands. The limits of late Pleistocene islands, defined by the 120 m bathymetric line, are highly concordant with the limits of faunal regions. Faunal variation among non-volant mammals is high between faunal regions and low within the faunal regions; endemism of faunal regions characteristically exceeds 70%. Small and geologically young oceanic islands are depauperate; larger and older islands are more species-rich. The number of endemic species is correlated with island area; however, continental shelf islands less than 125 000 km2 do not have endemic species, whereas isolated oceanic islands as small as 47 km2 often have endemic species. Geologically old oceanic islands have many endemic species, whereas young oceanic islands have few endemic species. Colonization across sea channels that were 5–25 km wide during the Pleistocene has been low, with a rate of about 1–2/500000 years. Comparison of species-area curves for mainland areas, late Pleistocene islands, and middle Pleistocene islands indicates that extinction occurs rapidly when landbridge islands are first isolated, with the extent of extinction dependent upon island size; extinction then slows to an average rate of 1–2%/10 000 years. The great majority of the non-volant Philippine mammals arrived from the Sunda Shelf, the geographically closest of the possible source areas. Speciation within the Philippines has contributed substantially to species richness, perhaps exceeding colonization by a factor of two or more as a contributor to species number. Colonization, extinction and speciation rates differ among taxonomic groups, with murid rodents being most successful and carnivores least successful. In order for any model of island biogeography to be widely applicable to insular faunas, the model must include speciation as a major variable. It is suggested that insular mammalian faunas typically are not in equilibrium, because geological and climatic changes can occur as rapidly as colonization and speciation.