Wallace's line between Australian and Southeast Asian fauna. The deep water of the Lombok Strait between the islands of Bali and Lombok formed a water barrier even when lower sea levels linked the now-separated islands and landmasses on either side.
The Wallace Line (or Wallace's Line) is a boundary that separates the zoogeographical regions of Asia and Australia. West of the line are found organisms related to Asiatic species; to the east, mostly organisms related to Australian species. 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). Evidence of the line was also noted in Antonio Pigafetta's biological contrasts between the Philippines and the Spice Islands, recorded during the continuation of voyage of Ferdinand Magellan (after Magellan himself had been killed on Leyte) in 1521.
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.
An understanding of the biogeography of the region centers on ancient sea levels, and the continental shelves; Wallace's Line is visible geographically when you examine the ocean shelf contours, and 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. On the other hand, Australia is united underneath a shallow ocean with New Guinea, on the Sahul Shelf. At ancient times when the ocean levels were lower, what are now islands & Australia were united 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.
A similar principle is seen in the definitions of the related biogeographic boundaries known as "Weber's Line" and "Lydekker's Line", which also occur within this transitional area (known as "Wallacea").
Australia and New Zealand, etc., do not form a single zoological area - since New Zealand's fauna are really distinct from those on the Australian continent. The reason for this is clear because the Tasman Sea is a wide and deep part of the Pacific Ocean, and it does not even have any islands in it now. (It formerly gave a barrier to aviation, too.)
Zoologists have suggested a term for the distinct area containing Australia, Tasmania, and New Guinea which is dominated by marsupial animals in its land fauna. Suggestions for a name have been Meganesia, Sahul, or Australinea.
See also
References
Borneo
- Abdullah, M. T. (2003). Biogeography and variation of Cynopterus brachyotis in Southeast Asia. PhD thesis. The University of Queensland, St Lucia, Australia.
- Hall, L. S., Gordon G. Grigg, Craig Moritz, Besar Ketol, Isa Sait, Wahab Marni and M. T. Abdullah (2004). "Biogeography of fruit bats in Southeast Asia". Sarawak Museum Journal LX(81):191-284.
- Wilson D. E., D. M. Reeder (2005). Mammal species of the world. Washington DC: Smithsonian Institution Press.
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