Ecology

Floraedit

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.

Typical Southern Hemisphere flora include the conifers Podocarpus (eastern Australia and New Guinea), the rainforest emergents Araucaria (eastern Australia and New Guinea), Nothofagus (New Guinea and Tasmania) and Agathis (northern Queensland and New Guinea), as well as tree ferns and several species of Eucalyptus. Prominent features of the Australian flora are adaptations to aridity and fire which include scleromorphy and serotiny. These adaptations are common in species from the large and well-known families Proteaceae (Banksia), Myrtaceae (Eucalyptus or gum trees, Melaleucas and Callistemons), Fabaceae (Acacias or wattles and Casuarina or she-oaks) and Grevilleas, which are typically found in the Australian mainland. The flora of New Guinea is a mixture of many tropical rainforest species with origins in Asia, such as Castanopsis acuminatissima, Lithocarpus spp., elaeocarps, and laurels, together with typically Australasian flora. In the New Guinean highlands, conifers such as Dacrycarpus, Dacrydium, Papuacedrus and Libocedrus are present.

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 million years ago. 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.

Among the fungi, the remarkable association between Cyttaria gunnii (one of the "golf-ball" fungi) and its associated trees in the genus Nothofagus is evidence of that drift: the only other places where this association is known are New Zealand and southern Argentina and Chile.

Faunaedit

Due to the spread of animals, fungi and plants across the single Pleistocene landmass the separate lands have a related biota. There are over 300 bird species in West Papua, of which at least 20 are unique to the ecoregion, and some live only in very restricted areas. These include the grey-banded munia, Vogelkop bowerbird, and the king bird-of-paradise.

Australia has a huge variety of animals; some 83% of mammals, 89% of reptiles, 24% of fish and insects and 93% of amphibians that inhabit the continent are endemic to Australia. This high level of endemism can be attributed to the continent's long geographic isolation, tectonic stability, and the effects of an unusual pattern of climate change on the soil and flora over geological time. Australia and its territories are home to around 800 species of bird; 45% of these are endemic to Australia. Predominant bird species in Australia include the Australian magpie, Australian raven, the pied currawong, crested pigeons and the laughing kookaburra. The koala, emu, platypus and kangaroo are national animals of Australia, and the Tasmanian devil is also one of the well-known animals in the country. The goanna is a predatory lizard native to the Australian mainland.

As the continent drifted north from Antarctica, a unique fauna, flora and mycobiota 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. New Guinea has 284 species and six orders of mammals: monotremes, three orders of marsupials, rodents and bats; 195 of the mammal species (69%) are endemic. New Guinea has a rich diversity of coral life and 1,200 species of fish have been found. Also about 600 species of reef-building coral—the latter equal to 75 percent of the world's known total. New Guinea has 578 species of breeding birds, of which 324 species are endemic. Bird life also flourished – in particular, the songbirds (order Passeriformes, suborder Passeri) are thought to have evolved 50 million years ago in the part of Gondwana that later became Australia, New Zealand, New Guinea, and Antarctica, before radiating into a great number of different forms and then spreading around the globe.

Animal groups such as macropods, monotremes, and cassowaries are endemic to Australia. There were three main reasons for the enormous diversity that developed in animal, fungal and plant 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 animal, fungal and plant 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, despite the fact that the continent was already very old and thus relatively infertile, there are dispersed areas of high fertility. Where other continents had volcanic activity and/or massive glaciation events to turn over fresh, unleached 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, and a relatively large number of species/level of biodiversity. This is because where nutrients are plentiful, competition is largely a matter of outcompeting rival species, leaving great scope for innovative co-evolution as is witnessed in tropical, fertile ecosystems. In contrast, infertile soils tend to induce competition on an abiotic basis meaning individuals all face constant environmental pressures, leaving less scope for divergent evolution, a process instrumental in creating new species.

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.