Wallace's Line: A Result of Plate Tectonics

Abstract

Wallace's line is one of the sharpest and first detected zoogeographical boundaries on earth. This line, running north to south through the center of the Malay archipelago, separates predominantly Asian faunas to its west from predominantly Australasian faunas to its east. The sharpness of the boundary differs from group to group, dependent on the dispersibility of the group under consideration. For rain forest plants Wallace's line is a less marked boundary. To its west Australasian plants occur mainly on oligotrophic podzolized sands in heath forest. Asian plants massively penetrate to its east and the Australasian element is perhaps most predominant in certain mid-mountain forests rich in Nothofagus or the conifers Agathis and Araucaria. Malesia has an extremely rich flora, conservatively estimated to comprise 25,000 species of flowering plants (van Steenis, 1971) and this has contributed to the view that the 'cradle of the angiosperms' lies somewhere between Assam and Fiji (Takhtajan, 1969; Smith, 1970). Until recently different viewpoints on the palaeogeography and biogeography of the Malay archipelago had to stand or fall on their ability to fit all known facts into a plausible hypothesis. There was no external point of reference. Biologists tended to make geographical reconstructions and geographers to use biological evidence. The revolution in the earth sciences during the last two decades, which has resulted from the theory of plate tectonics, means that the palaeogeography of the globe during the Mesozoic and Cenozoic is now understood in general terms. Now biologists can for the first time work with a broadly painted background of established palaeogeographical facts, albeit still lacking much fine detail, rather than mere suppositions. The former dangers of circular argument, which have so often plagued discussions of biogeography, no longer befog the scene. The major events of global geology that affect the biogeography of the region of Wallace's line are the progressive break up of Gondwanaland from about 140 m.y.a. (Jurassic/Cretaceous boundary) and the drifting north of the Indian fragment to collide with Laurasia at about 55 m.y.a. and of the Australia/New Guinea fragment to collide with the southeastern extremity of Laurasia at only about 15 m.y.a., the mid-Miocene. Before that the Malay archipelago did not exist. Plants and animals could have reached modern Malesia from one of three sources, Laurasia, Gondwanaland via Australia, or Gondwanaland via India followed by southeastwards migration. Before we look at some present-day phytogeographic patterns and see how