Transcending the Wallace line: do the western edges of the Australina region and the Australian plate coincide?

Abstract

The island of Sulawesi (Celebes), which lies at the heart of the Malay Archipelago, occurs in a region of exceptional tectonic complexity. Since Wallace first drew attention to the anomalous fauna of the island, debate has continued regarding the biogeography and geology of the area. Through an analysis of the distribution of the 183 genera and 470 species of butterflies known from Sulawesi (of which more than 200 species are regional endemics), two classes of biotic patterns linking the island to surrounding regions can be demonstrated. All, or virtually all of the genera on Sulawesi are Asian, but with no special link to Borneo. A set of younger patterns, derived from analysing species' distributions, links Sulawesi to the Moluccas, Philippines and the Lesser Sunda Islands, in addition to Asia. Of these younger patterns, the link between Sulawesi and the Moluccas is most pronounced . This is interpreted to suggest that current geological models, in which Sulawesi consists of at least two terranes, one Asian and one Australian in origin, are consistent with butterfly biogeography only if certain assumptions or constraints are imposed. Firstly, it must be assumed that Sulawesi has had a long independent history from Borneo; it seems most unlikely that Sulawesi and Borneo could have been contiguous 2 mya, as one geological theory has suggested. Secondly, before collision of the Asian and Australian plates about 15 mya, the advancing edge of the Australian plate must have been submerged during most if not all of the approach phase. If the collision has created new land by uplift in the eastern Sulawesi, Banggai and Sula region, then the strong species-level link between Sulawesi and the Moluccas is explicable by local dispersion over the last 15 mya. It is concluded that there is no sharp distinction, at least within Wallacea, between the Asian and Australian biota, as Wallace originally tried to demonstrate and as geological theories might predict: the western edges of the Australian biogeographic area and the Australian tectonic plate do not coincide.