Wallacea/wɒˈleɪsiə/ is abiogeographical designation for a group of mainlyIndonesian islands separated by deep-waterstraits from theAsian andAustraliancontinental shelves. Wallacea includesSulawesi, the largest island in the group, as well asLombok,Sumbawa,Flores,Sumba,Timor,Halmahera,Buru,Seram, and many smaller islands. The islands of Wallacea lie between theSunda Shelf (theMalay Peninsula,Sumatra,Borneo,Java, andBali) to the west, and theSahul Shelf includingAustralia andNew Guinea to the south and east. The total land area of Wallacea is 347,000 km2 (134,000 sq mi).[1]
Wallacea is defined as the series of islands stretching between the two continental shelves ofSunda andSahul, but excluding thePhilippines. Its eastern border (separating Wallacea from Sahul) is represented by azoogeographical boundary known asLydekker's Line, while theWallace Line (separating Wallacea from Sunda) defines its western border.[2][3]
TheWeber Line is the midpoint, at which Asian and Australian fauna and flora are approximately equally represented. It follows the deepeststraits traversing theIndonesian Archipelago.
The Wallace Line is named after theWelsh naturalistAlfred Russel Wallace, who recorded the differences between mammal and bird fauna between the islands on either side of the line. The islands ofSundaland to the west of the line, including Sumatra, Java, Bali, and Borneo, share a mammal fauna similar to that of East Asia, which includes tigers, rhinoceros, and apes; whereas the mammal fauna of Lombok and areas extending eastwards are mostly populated by marsupials and birds similar to those in Australasia. Sulawesi shows signs of both.[4]
During the ice ages, sea levels were lower, exposing the Sunda shelf that links the islands of Sundaland to one another and to Asia[5] and allowing Asian land animals to inhabit these islands.
The islands of Wallacea have few land mammals, land birds, or freshwater fish of continental origin, which find it difficult tocross open ocean. Many species of birds, reptiles, and insects 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 thefloristic province ofMalesia.[citation needed]
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 callAustralia-New Guinea, Meganesia, Papualand, or Sahul. Consequently, Australia, New Guinea, and theAru Islands share manymarsupial mammals, land birds, and freshwater fish that are not found in Wallacea.[3]
Although the distant ancestors of Wallacea's flora and fauna may have been from Asia or Australia-New Guinea, Wallacea is home to manyendemic species. There is extensiveautochthonousspeciation and proportionately large numbers of endemics; the area is an important contributor to the overall mega-biodiversity of the Indonesian Archipelago.[6]
Fauna includes thelowland andmountain anoa, or dwarf buffalo (Bubalus sp.), and the babirusa, or "deer-pig" (Babyrousa sp.), both found on Sulawesi, among other islands. Maluku shares a number of similar species with Sulawesi, albeit with fewer total, given the differences in size between the two islands—Sulawesi has at least 4,000 recorded terrestrial plant and animal species,[7] while Maluku has just over 1,000, by comparison. Sulawesi is home to over 2,000invertebrate species (with over 1,000 known species ofarthropod, not including nearly 900lepidopterans), 100 species ofreptiles andamphibians, and 288 bird species. Maluku has around 70 reptile and amphibian, 250 avian, and over 550 invertebrate species.Seram Island is particularly noted for itsbutterflies and birds, including theMoluccan king parrot. Smaller mammals, including somecarnivorans (such ascivets),marsupials (such as thecuscus),primates androdents are common throughout the region.
A large portion of the waters surrounding Wallacea are part of theCoral Triangle, considered to be the richestcoral reef and marine ecosystems on earth, with the highest number of species, adding to the totalbiodiversity of the region.
Wallacea was originally almost completely forested, mostlytropical moist broadleaf forests, with some areas oftropical dry broadleaf forest. The higher mountains are home tomontane andsubalpine forests, andmangroves are common in coastal areas. According toConservation International, Wallacea is home to over 10,000 plant species, of which approximately 1,500 (15%) are endemic.[1]
Endemism is higher among terrestrial vertebrate species; out of 1,142 species described there, almost half (529) were endemic. 45% of the region retains some sort of forest cover, though only 52,017 km2 (15%) is in a pristine state. Of Wallacea's total 347,000 km2-area, about 20,000 km2 are protected.[1]
Tropical and subtropical moist broadleaf forests:
Tropical and subtropical dry broadleaf forests:
Australia may be isolated by sea, but technically through Wallacea, it can be zoologically extended. Australian Early-Middle Pliocene rodent fossils have been found in Chinchilla Sands andBluffs Down in Queensland, but a mix of ancestral and derived traits suggestmurid rodents made it to Australia earlier, maybe in the Miocene, over a forested archipelago, i.e. Wallacea, and evolved in Australia in isolation.[8]
Australia's rodents make up much of the continent'splacental mammal fauna and include various species fromstick-nest rats tohopping mice. Other mammals invaded from the east. Two species ofcuscus, theSulawesi bear cuscus and theSulawesi dwarf cuscus, are the westernmost representatives of the Australasian marsupials.[9]
Thetectonic uplift of Wallacea during the collision between Australia and Asiac. 23 million years ago allowed the global dispersal ofpasserine birds from Australia across the Indonesian islands.[10]Bustards andmegapodes must have somehow colonized Australia.Cockatiels similar to those from Australia inhabit Komodo Island in Wallacea.[citation needed]
A few species ofEucalyptus, a predominant genus of trees in Australia, are found in Wallacea:Eucalyptus deglupta on Sulawesi, andE. urophylla andE. alba in East Nusa Tenggara.[11] For land snails Wallacea and Wallace's Line do not form a barrier for dispersal.[12]
