Wallacea

Wallacea is islands within red area. Weber Line in blue.

Wallacea is a biogeographical designation for a group of Indonesian islands separated by deep water straits from the Asian and Australian continental shelves. The islands of Wallacea lie between Sundaland (the Malay Peninsula, Sumatra, Borneo, Java, and Bali) to the west, and Near Oceania including Australia and New Guinea to the south and east. The total land area of Wallacea is 347,000 km².

Geography

Map of Wallacea; upper right corner facing North. The red line denotes the western border of Wallacea. The eastern border corresponds to the light Australia-New Guinea shelf.

The boundary between Sundaland and Wallacea follows the Wallace Line, named after the naturalist Alfred Russel Wallace who noted the differences in mammal and bird fauna between the islands either side of the line. The Islands of Sundaland to the west of the line, including Sumatra, Java, Bali, and Borneo, share a similar mammal fauna with East Asia, including tigers, rhinoceros, and apes. During the ice ages, sea levels were lower, exposing the Sunda shelf that links these islands to one another and to Asia,^[1] and allowed Asian land animals to inhabit these islands. The islands of Wallacea have few land mammals, land birds, or freshwater fish of continental origin, who find it difficult to cross open ocean. Many bird, reptile, and insect species were better able to cross the straits, and many such species of Australian and Asian origin are found there. Wallacea's plants are predominantly of Asian origin, and botanists include Sundaland, Wallacea, and New Guinea as the Floristic province of Malesia.

Similarly, Australia and New Guinea to the east are linked by a shallow continental shelf, and were linked by a land bridge during the ice ages, forming a single continent that scientists variously call Australia-New Guinea, Meganesia, or Sahul. Consequently, Australia, New Guinea, and the Aru Islands share many marsupial mammals, land birds, and freshwater fish that are not found in Wallacea. The line dividing Wallacea from Australia–New Guinea is called Lydekker's Line. The Philippines (excluding Palawan which was part of Sundaland) are usually but not always considered a separate region from Wallacea.^[2] The Weber Line is the midpoint where Asian and Australian fauna and flora are approximately equally represented, and follows the deepest straits traversing the Indonesian Archipelago.

Biogeography

Wallacea is not merely a transitional region swamped by species from the adjoining areas, as it is frequently considered to be. In fact, Wallacea is a unique region with extensive autochthonous speciation and proportionately a large numbers of endemics; it is an important contributor to the overall mega-biodiversity of the Indonesian archipelago .^[3]

Although the distant ancestors of Wallacea's plants and animals may have been from Asia or Australia-New Guinea, Wallacea is presently home to many endemic species. Because many of the islands are separated from one another by deep water, there is tremendous species diversity among the islands as well. Conservation International has designated Wallacea as a biodiversity hotspot.

Wallacea was originally almost completely forested, mostly tropical moist broadleaf forests, with some areas of tropical dry broadleaf forest. The higher mountains are home to montane and subalpine forests, and Mangroves are common in coastal areas.

According to Conservation International, Wallacea is home to over 10,000 plant species, of which approximately 1500 (15%) are endemic. Endemism is higher among terrestrial vertebrate species; of 1142 species found there, almost half (529) are endemic. Most of Wallacea was originally forested; 45% retains some sort of forest cover, and only 52,017 km², or 15 percent, is in a more or less pristine state. Of Wallacea's total area of 347,000 km², only about 20,000 km² are protected. Wallacea is home to 82 threatened and six critically endangered species of terrestrial vertebrates.

Ecoregions of Wallacea

Tropical and subtropical moist broadleaf forests

• Banda Sea Islands moist broadleaf forests (Kai Islands, Tanimbar Islands)
• Buru rain forests (Buru)
• Halmahera rain forests (Halmahera, Morotai, Obi Islands, Bacan Island)
• Seram rain forests (Seram, Ambon Island, Saparua)
• Sulawesi lowland rain forests (Sulawesi, Banggai Islands, Sula Islands, Sangihe Islands, Talaud Islands)
• Sulawesi montane rain forests (Sulawesi)

Tropical and subtropical dry broadleaf forests

• Lesser Sundas deciduous forests (Lombok, Sumbawa, Komodo, Flores, Alor)
• Sumba deciduous forests (Sumba)
• Timor and Wetar deciduous forests (Timor, Barat Daya Islands, Banda Islands, Babar Island, Leti Islands)

Distribution between Asia and Australasia

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Australia may be isolated by sea, but technically through Wallacea it can be zoologically extended. An example of evidence for when Wallacea formed exists in a fossil site called Bluff Downs in northern Australia. Remains of basaltic lava show past oceanic subduction as Australia ploughed north through the Pacific. At the site, one of the earliest Australian rodent fossils have been found. Australia's rodents make up much of the continent's placental mammal fauna and include various species from stick-nest rats, hopping mice, and even giant beaver rats. Other mammals invaded west. Two species of cuscus from Sulawesi are among the most primitive possums in the world and the only marsupials in Asia.

Birds have expanded their range to and from Australia. Crows and shrikes invaded south into New Guinea and some into the Australian continent. Bustards and megapodes must have somehow colonized Australia. Cockateels similar to those from Australia inhabit Komodo Island in Wallacea.

Notes

References

• Abdullah MT. (2003). Biogeography and variation of Cynopterus brachyotis in Southeast Asia. PhD thesis. University of Queensland, St Lucia, Australia.
• Corbet, GB, Hill JE. (1992). The mammals of the Indomalayan region: a systematic review. Oxford University Press, Oxford.
• Hall LS, 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 DE, Reeder DM. (2005). Mammal species of the world. Smithsonian Institution Press, Washington DC.

External links

• Conservation International: Wallacea
• Too Many Lines; The Limits of the Oriental and Australian Zoogeographic Regions George Gaylord Simpson, Proceedings of the American Philosophical Society, Vol. 121, No. 2 (Apr. 29, 1977), pp. 107–120
• map of Wallace's, Weber's and Lydekker's lines

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