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Wallace line |
The Wallace Line (or Wallace's Line) is a boundary that separates the zoogeographical regions of Asia and Wallacea (which is a transitional zone between Asia and Australia). West of the line are found organisms related to Asiatic species; to the east, a mixture of species of Asian and Australian origin are present. The line is named after Alfred Russel Wallace, who noticed this clear dividing line during his travels through the East Indies in the 19th century. The line runs through the Malay Archipelago, between Borneo and Sulawesi (Celebes); and through the Lombok Strait between Bali (in the west) and Lombok (in the east). Antonio Pigafetta had also recorded the biological contrasts between the Philippines and the Spice Islands (situated on opposite sides of the line) in 1521 during the continuation of the voyage of Ferdinand Magellan (after Magellan himself had been killed on Mactan).
The distance between Bali and Lombok is small, a matter of only about 35 kilometers. The distributions of many bird species observe the line, since many birds refuse to cross even the smallest stretches of open ocean water. Some volant (flying) mammals (i.e., bats) have distributions that cross the Wallace Line, but non-volant species are almost always limited to one side or the other of the line, with a few exceptions (e.g., very mobile rodents such as the Hystrix genus.) Various taxa in other groups of plants and animals show differing patterns, but the overall pattern is striking and reasonably consistent.
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 m 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 a period in excess of 50 million years — a barrier that kept the flora and fauna of Australia separated from that of Asia. Wallacea consists of islands that were never recently connected by dry land to either of the continental land masses, and thus was populated by organisms capable of crossing the straits between islands. "Weber's Line" runs through this transitional area (a bit to the east of center), at the tipping point between dominance by species of Asian vs. Australian origin.
Australia and New Zealand do not form a single zoological area, since New Zealand's fauna are quite distinct from those of Australia. The reason for this is clear: the Tasman Sea is a wide and deep part of the Pacific Ocean, which doesn't even have islands now. (It was formerly a barrier to aviation, too.)
Zoologists have tried to come up with a term for the distinct biogeographical area comprising Australia, Tasmania, and New Guinea, which is dominated by marsupials. Among the suggested names are Meganesia, Sahul, and Australinea.
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