From an
ecological perspective the Australasian realm is a distinct region, parts of which have a common geologic and evolutionary history. The entire area has experienced a long period of biological isolation from other regions, and thus harbors a great many unique
plants and
animals. In this context, Australasia is limited to Australia, New Guinea, New Zealand,
New Caledonia, and neighbouring islands, including the
Indonesian islands from
Lombok and
Sulawesi eastward.
The
Wallace Line to the west divides areas in the Indomalayan realm of tropical
Asia which are or have at times been directly connected to the Asian mainland from islands that have never been so connected.
Borneo and
Bali lie on the western, Asian side. A second biological dividing line is
Lydekker's Line, which similarly separates islands isolated by surrounding deep water from those associated with the
Sahul Shelf of the Australian continent. Islands between the two lines (e.g. Sulawesi, the Moluccas and Lombok through Timor) form the biogeographical area of
Wallacea, a transition zone between the Indomalayan and Australasian realms populated entirely by aerial or
oceanic dispersal (although defined here as part of the Australasian realm).
Geology
Australia, New Zealand and New Caledonia are all fragments of the ancient supercontinent
Gondwana, the marks of which are still visible in the
Christmas Island Seamount Province and other geophysical entities. These three land masses have been separated from other continents, and from one another, for tens millions of years. All of Australasia shares the
Antarctic flora, although the northern, tropical islands also share many plants with Southeast Asia.
Australia, New Guinea, and Tasmania are separated from one another by shallow
continental shelves, and were linked together when the sea level was lower during
ice ages. They share a similar fauna which includes
marsupial and
monotreme mammals and
ratite birds.
Eucalypts are the predominant trees in much of Australia and New Guinea. New Zealand has no native land mammals, but also had ratite birds, including the
kiwi and the
moa. The Australasian realm includes some nearby island groups, like
Wallacea, the
Bismarck Archipelago,
Solomon Islands, and
Vanuatu, which were not formerly part of Gondwana, but which share many characteristic plants and animals with Australasia.
Ecology
Note that this zonation is based on flora;
animals do not necessarily follow the same
biogeographic boundaries. In the present case, many
birds occur in both "Indomalayan" and "Australasian" regions, but not across the whole of either. On the other hand, there are few faunistic commonalities shared only by Australia and New Zealand, except some birds. Meanwhile, Australia, Melanesia and the Wallacea are united by a large share of similar animals, but few of these occur farther into the Pacific. On the other hand, much of the
Polynesian fauna is related to that of Melanesia.