Australasia ecozone

The Australasian zone is an ecological region that is coincident, but not synonymous (by some definitions), with the geographic region of Australasia. The ecozone includes Australia, the island of New Guinea (including Papua New Guinea and the Indonesian province of Papua), and the eastern part of the Indonesian archipelago, including the island of Sulawesi, the Moluccan islands (the Indonesian provinces of Maluku and North Maluku) and islands of Lombok, Sumbawa, Sumba, Flores, and Timor, often known as the Lesser Sundas. The Australasian ecozone also includes several Pacific island groups, including the Bismarck Archipelago, Vanuatu, the Solomon Islands, and New Caledonia. New Zealand and its surrounding islands are a distinctive sub-region of the Australasian ecozone. The rest of Indonesia is part of the Indomalayan ecozone.

From a biological point of view, Australasia is a distinct region with a common evolutionary history and a great many unique plants and animals, some of them common to the entire area, others specific to particular parts but sharing a common ancestry. The long isolation of Australasia from other continents allowed it to evolve relatively independently, and makes it home to many unique families of plants and animals.

Australia and New Guinea are distinguished by their marsupial mammals, including kangaroos, possums, and wombats. The last remaining monotreme mammals, the echidnas and the platypus, are endemic to Australasia. Prior to the arrival of humans about 50,000 years ago, only about one-third of Australasian mammal species were placental.

The boundary between Australasia and Indomalaya 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 to the west of the line, including Java, Bali, Borneo, and the Philippines share a similar fauna with East Asia, including tigers, rhinoceros, and apes. During the ice ages, sea levels were lower, exposing the continental shelf that links these islands to one another and to Asia, and allowed Asian land animals to inhabit these islands. Similarly, Australia and New Guinea are linked by a shallow continental shelf, and were linked by a land bridge during the ice ages. A group of Australasian islands east of the Wallace line, including Sulawesi, Halmahera, Lombok, Flores, Sumba, Sumbawa, and Timor, is separated by deep water from both the southeast Asian continental shelf and the Australia-New Guinea continental shelf. These islands are called Wallacea, and contain relatively few Australian or Asian mammals. While most land mammals found it difficult to cross the Wallace Line, many plant, bird, and reptile species were better able to make the crossing.

Australia, New Zealand, and New Caledonia are all portions of the ancient supercontinent of Gondwana, which started to break into smaller continents in the Cretaceous era, 130-65 million years ago. New Zealand broke away first, more than 80 million years ago, and Australia finally broke free from Antarctica about 45 million years ago. All the Australasian lands are home to the Antarctic flora, descended from the flora of southern Gondwana, including the coniferous podocarps and "Araucaria" pines, and the broadleafed southern beech ("Nothofagus"), and proteas ("Proteaceae").

As Australia moved north into the desert latitudes, the continent became hotter and drier, and the soils poorer and leached of nutrients, causing the old Antarctic flora to retreat to the humid corners of the continent in favor new drought and fire tolerant flora, dominated by the "Eucalyptus,Casuarina", and "Acacia" trees, and by grasses and scrub where the rainfall was too scarce to support trees. Presently Australia is the smallest continent, and also the driest continent and the flattest (lowest in elevation) continent.

Geology

The present distribution of Australasian plants and animals is partially a result of the geologic history of its land masses. Several of the land masses in the ecoregion are fragments of the ancient continent of Gondwana, while a number of smaller islands are of more recent volcanic or tectonic origin, and were never part of Gondwana.

New Guinea, Australia, and Tasmania, collectively known as Australia-New Guinea, Sahul, or Meganesia, are connected by a shallow continental shelf, and together form the largest fragment of Gondwana. The shallow continental shelf that presently separates the islands has served as a land bridge when sea levels were lower, most recently during the last ice age. New Guinea shares many families of birds and marsupial mammals with Australia. As the Indo-Australian Plate, which contains India, Australia, and the Indian Ocean floor in between, moved north, it collided with the Eurasian Plate, and the collision of the two plates pushed up the Himalayas, the Indonesian islands, and New Guinea's Central Range. The Central Range is much younger and higher than the mountains of Australia, so high that it is home to rare equatorial glaciers. New Guinea and Wallacea are part of the humid tropics, and many Indomalayan rainforest plants spread across the narrow straits from Asia, mixing together with the old Australian and Antarctic floras. Some botanists consider New Guinea and Wallacea to be part of the floristic province of Malesia, together with the other Indonesian islands and the Malay Peninsula, although Malesia is now mostly used to refer to only the Indomalayan side of the Wallace Line.

New Zealand and New Caledonia are the other former fragments of Gondwana in the region.

The island groups north and east of New Guinea and New Caledonia, including Bismarck Archipelago, Admiralty Islands, Solomon Islands and Vanuatu, were pushed up by the collision of the Australian plate with other oceanic plates. These islands, collectively known as the East Melanesian Islands, were colonized by plants and some animals from New Guinea and New Caledonia, and are considered part of the Australasian ecozone based on those affinities. Further north and east are the Pacific island groups of Micronesia, Fiji, and Polynesia, which are also of relatively recent volcanic origin, and constitute the separate Oceania ecozone, although they share many ecological affinities with Australasia.

Fauna

Bats were the only mammals of New Zealand until the arrival of humans. Birds adapted to ecological niches, such as grazers, insectivores, and large predators that have elsewhere been taken by mammals. New Zealand remained in the cool and humid latitudes, and lost many plant and animal families that were intolerant of its cool climate, including the araucarias and most proteas, as well as crocodiles and turtles.

Large reptiles, including crocodiles and huge monitor lizards (family Varanidae), like the Komodo Dragon ("Varanus komodoensis"), are ecologically important predators in Australia, New Guinea, and Wallacea.

There are 13 endemic bird families, including emus, cassowaries, kiwi, kagu, cockatoos, birds of paradise, and honeyeaters

Human impact

The arrival of humans to Australia and New Guinea 50-60,000 years ago brought dogs (dingos) to Australia, and dogs and pigs to New Guinea. Pigs and rats arrived on New Zealand with the first Polynesian settlers 800 years ago. The arrival of the first humans coincided with the extinction of much of the native megafauna (see Holocene extinction event). The arrival of Europeans brought a whole host of new animals and plants, including sheep, goats, rabbits and foxes, to Australasia, which have further disrupted the native ecologies; a great many Australasian plants and animals are presently endangered.

Australasia terrestrial ecoregions

ee also

* Australasia

External links

*commons-inline|Australasia
* [http://www.panda.org/about_wwf/where_we_work/ecoregions/maps/index.cfm Map of the ecozones]