Australia-New Guinea
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Australia-New Guinea, also called Sahul or Meganesia, is made up of the continent of Australia and the islands of New Guinea and Tasmania. These land masses are separated by the Torres Strait (Australia and New Guinea) and the Bass Strait (Australia and Tasmania). However, from biological and geological points of view, they form a single unit. During the ice age, they formed a single continent.
Australia, New Guinea, and Tasmania all sit on top of a single tectonic plate, the Indo-Australian Plate, and are connected by a shallow continental shelf. All were joined onto Antarctica as part of the southern supercontinent Gondwana until the plate began to drift north about 96 million years ago (mya). For most of the time since then Australia-New Guinea has remained a single landmass.
As the continent drifted north, a unique flora and fauna developed. Marsupials and monotremes also existed on other continents, but only in Australia-New Guinea did they out-compete the placental mammals and come to dominate. Bird life also flourished, in particular the ancestors of the great passerine order that would eventually spread to all parts of the globe and account for more than half of all living avian species.
There were three main reasons for the enormous diversity that developed in both plant and animal life.
- While much of the rest of the world underwent significant cooling and thus loss of species diversity, Australia-New Guinea was drifting north at such a pace that the overall global cooling effect was roughly equalled by its gradual movement toward the equator. Temperatures in Australia-New Guinea, in other words, remained reasonably constant for a very long time, and a vast number of different plant and animal species were able to evolve to fit particular ecological niches.
- Because the continent was more isolated than any other, very few outside species arrived to colonise, and unique native forms developed unimpeded.
- Finally, the continent was already very old and thus relatively infertile. Where other continents had volcanic activity and/or massive glaciation events to turn over fresh, un-leached rocks rich in minerals, the rocks and soils of Australia-New Guinea were left largely untouched except by gradual erosion and deep weathering. In general, fertile soils produce a profusion of life, but only a relatively small number of species. This is because where nutrients are plentiful, competition is largely a matter of absorbing them as fast as possible. In contrast, infertile soils tend to produce a great variety of species, each one specialised for a particular niche: a single plant species, for example, can rapidly develop into several different but related species: one that specialises in slightly acid conditions, another that colonises dryer places, and so on.
For about 40 million years Australia-New Guinea was almost completely isolated. During this time, the continent experienced numerous changes in climate, but the overall trend was towards greater aridity. When South America eventually separated from Antarctica, the development of the cold Antarctic Circumpolar Current changed weather patterns across the world. For Australia-New Guinea, it brought a marked intensification of the drying trend. The great inland seas and lakes dried out. Much of the long-established broad-leaf deciduous forest began to give way to the distinctive hard-leaved sclerophyllous plants that characterise the modern Australian landscape.
For many species, the primary refuge was the relatively cool and well-watered Great Dividing Range. Even today, pockets of remnant vegetation remain in the cool uplands, some species not much changed from the Gondwanan forms of 60 or 90 mya.
Eventually, the Australia-New Guinea tectonic plate collided with the Eurasian plate to the north. The collision caused the northern part of the continent to buckle upwards, forming the high and rugged mountains of New Guinea and, by reverse (downwards) buckling, the Torres Strait that now separates the two main landmasses. The collision also pushed up the islands of Wallacea, which served as island 'stepping-stones' that allowed plants from Southeast Asia's rainforests to colonise New Guinea, and some plants from Australia-New Guinea to move into Southeast Asia. The ocean straits between the islands were narrow enough to allow plant dispersal, but served as an effective barrier to exchange of land mammals between Australia-New Guinea and Asia.
Although New Guinea is the most northerly part of the continent, and could be expected to be the most tropical in climate, the altitude of the New Guinea highlands is such that a great many animals and plants that were once common across Australia-New Guinea now survive only in the tropical highlands (where they are severely threatened by overpopulation pressures).