Australasia is one of Earth’s eight ecosystems. This ecozone includes Australia, New Guinea (including Papua New Guinea and the Indonesian province of Papua), and the eastern region of the Indonesian archipelago (island of Sulawesi, Moluccan islands, and the islands of Lombok, Sumbawa, Sumba, Flores, and Timor). Several Pacific island groups are also part of the Australasia ecosystem including the Bismarck Archipelago, Vanuatu, Solomon Islands, and New Caledonia. The rest of Indonesia not stated here is part of the Indomalayan ecozone.
Australasia is a unique region with a common evolutionary history. There are a great number of unique plant and animal species that are either common within the entire region, or specific to only particular areas but still sharing a common ancestry. The isolation of Australasia from other continents allowed it to evolve mostly independently. This isolated evolution makes it home to many types of plant and animal species that are not found anywhere else in the world. Australia and New Guinea are both well-known for their marsupial mammals (kangaroos, possums, and wombats). The last remaining monotreme (having birdlike and reptile-like features) mammals (echidnas and platypus) are endemic to Australia. Before humans arrived 50,000 years ago, only 33% of the Australasian mammal species were placental.
The Wallace Line (named after naturalist Alfred Russel Wallace) is a geographical boundary between Australasia and Indomalaya. This line was established as Wallace recorded the differences in mammal and bird species between the islands on either side of the line. During the ice ages, water levels were lower and land bridges that connected certain islands with mainland made it possible for Asian land animals to inhabit these islands. The islands west of the Wallace Line, including Java, Bali, Borneo, and the Philippines all have mammal species (tigers, rhinos, apes, etc.) that are also found in East Asia. A number of islands east of the Wallace Line are separated by deep ocean waters and have relatively few animal species that are common in Asia and Australia. These islands are called Wallacea. While land mammals were not able to cross the Wallace Line, many species of reptiles, birds, and plants were able to make the crossover.
Australia, New Zealand, and New Caledonia are all former parts of the super continent Gondwanaland, which began to break up 130 – 65 million years ago. New Zealand separated first (around 80 million years ago), and then Australia broke free from Antarctica about 45 million years ago. All regions of the Australasian ecozone contain flora that was once part of the Antarctic region. As Australia moved north into desert latitudes, the continent became very hot and dry, and the soils were leached of nutrients. This caused the old Antarctic flora to retreat to the humid coastal regions of Australia.
While most of the land masses in the Australasian ecosystem are fragments of the ancient super continent Gondwanaland, some smaller islands in the zone were created by recent volcanic and tectonic origins. Australia, New Guinea, and Tasmania are connected by a shallow continental shelf that forms the largest fragment of Gondwanaland. The Zealandia micro-continent, which is made up of New Zealand and New Caledonia, is the other fragment of Gondwanaland in this region. Other smaller fragments exist as well, but are not as complex. The collision of the Australian plate and other ocean plates caused the formation of and rising of the islands of the Bismarck Archipelago, Admiralty Islands, Solomon Islands, and Vanuatu. These islands are collectively known as the East Melanesian Islands. The further north Pacific island groups of Micronesia, Fiji, and Polynesia, were formed from fairly recent volcanic activity. Although this region shares many ecological resemblances with Australasia, it is actually part of the Oceania ecozone.
Bats were the only mammals that were found in New Zealand until the arrival of humans. New Zealand lost many of its plant and animal families as they could not flourish in the cooler climate. Some animals that no longer are found in New Zealand are crocodiles and turtles. Some trees have been lost as well. Australasia has 13 endemic bird families including emus, cassowaries, kiwi, kagu, cockatoos, honeyeaters, and birds of paradise. Large reptiles, including monitor lizards, komodo dragons, and crocodiles are an ecologically important predators in Australia, new Guinea, and Wallacea.
When humans arrived in Australia and New Guinea (50,000 to 60,000 years ago, they brought dingoes to Australia, and dogs and pigs to New Guinea. Pigs and rats arrived with the first settlers of New Zealand about 800 years ago. The arrival of Europeans brought a wide range of animal and plant families including sheep, goats, rabbits and foxes. These animals have disrupted the ecologies of Australasia and now many native plants and animals are endangered.
Australia is currently the driest, flattest and the smallest continent. It also sits the lowest in elevation.