The Wallace Line: Division between Asia and Australasia

The Wallace Line (or Wallace’s Line) is a boundary that separates the zoogeographical regions of Asia and Australasia. 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 the apparent 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 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 voyage of Ferdinand Magellan 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, as many birds refuse to cross even the smallest stretches of open water. Many volant mammals (bats) have distributions that cross the Wallace Line, but non-volant species are usually limited to one side or the other, with a few exceptions (e.g., rodents [Hystrix]). Various taxa in other groups of plants and animals show differing patterns, but the overall pattern is striking and reasonably predictable.

An understanding of the biogeography of the region centers on ancient sea levels, and the continental shelves; Wallace’s Line is visible when one examines the sea contours, and can be seen as a deep-water channel which marks the southwestern edge of the Sunda Shelf linking Borneo, Bali, Java and Sumatra to the mainland of southeastern Asia. Australia, on the other hand, is united broadly with New Guinea, in the Sahul Shelf. The Lombok Strait is 1,300m deep and was still a barrier during the last ice age. At times when sea levels were lower, what are now islands were exposed and joined as continuous land masses, but the deep water between these two large shelf areas was — for a period in excess of 50 million years — a barrier that kept the flora and fauna of Australia largely separate from that of Asia.

A similar principle is behind 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).

Australasia does not conform to a single zoological area since New Zealand’s fauna are completely different to those on the Australian continent. Zoologists have suggested a term for the distinct area containing Australia, Tasmania and New Guinea that is dominated by marsupials. Suggestions are Meganesia, Sahul or Australinea.

Culturally Indonesia starts to divide after Lombok, with the islands to the west (Sumatra, Java, Bali) having strong high-culture (Buddhist, Hindu, Muslim) influences and the culture distinctly Asian. As one progresses eastward, through Sumbawa, Flores, Timor and the Mukuku’s etc. the culture becomes more similar to the western Pacific.