Wallace Line - Biogeography

Biogeography

Understanding of the biogeography of the region centers on the relationship of ancient sea levels to the continental shelves. Wallace's Line is visible geographically when the continental shelf contours are examined; it can be seen as a deep-water channel that marks the southeastern edge of the Sunda Shelf linking Borneo, Bali, Java, and Sumatra underwater to the mainland of southeastern Asia. Australia is likewise connected via the shallow ocean over the Sahul Shelf to New Guinea; and the related biogeographic boundary known as Lydekker's Line, which separates the eastern edge of Wallacea and the Australian region, has a similar origin. During ice age glacial advances, when the ocean levels were up to 120 metres (390 ft) lower, both Asia and Australia were united with what are now islands on their respective continental shelves as continuous land masses, but the deep water between those two large continental shelf areas was, for over 50 million years, a barrier that kept the flora and fauna of Australia separated from those of Asia. Wallacea consists of islands that were never recently connected by dry land to either of the continental land masses, and thus were populated by organisms capable of crossing the straits between islands. "Weber's Line" runs through this transitional area (to the east of centre), at the tipping point between dominance by species of Asian against those of Australian origin.