Australia Geography: Ecosystems

The expanse of the island continent embraces a variety of ecosystems: dynamic interactions of the physical environment and living organisms. Tropical rainforests and wetlands are found in the north of Australia, as are offshore coral reefs; eucalypt forests dominate in the south-east and south west; and deserts and arid lands feature in the vast centre.

The typical image of Australia, of a mostly dry and harsh landscape, is largely true; but it wasn't always this way. The familiar arid-land flora and fauna had rainforest-dwelling ancestors - their fossil remains are imprinted in the rocks of the outback. Living and recognizable descendants of this wetter past flourish in the wet tropics of the north and the temperate rainforests of the south-east; a few reside in the deep gorges of central Australia.

Contemporary ecosystems reflect this Gondwanan ancestry, the effects of continental drift and a gradually drying climate, the influence of Ice-age events, and, more recently, migrations of other animals (including humans) and plants from the Eurasian continent.

Tropical Wetland

The tropical wetlands of northern Australia witness remarkable seasonal change. Growing, flowering, feeding and breeding are all governed by the annual cycle of Wet and Dry. Towards the end of the Dry, the receding swamps and lagoons attract thousands of water birds; noisy geese are joined by fish-hunting jabiru and pied heron. A series of spectacular storms precedes the Wet.

By the end of December heavy rains have arrived, signaling the estuarine crocodile to nest and forcing the antelope wallaroos to higher ground. In the warm, wet environment, plant growth is rapid. Many water birds now nest among the rushes and wild rice. Further storms flatten the tall grasses, and by about May, the Wet is over. As the land dries, natural fires and aboriginal hunting fires continue to shape this environment.

Tropical Rainforest

There is scarcely a niche in the warm, damp environment that is not occupied by lush, green vegetation. The tree canopy filters sunlight and resounds to the call of birds and fruit bats. The trunks of trees support lichens, ferns and orchids, and the understorey and forest floor are a profusion of palms, more ferns and the essential fungi and microorganisms that decompose the rich forest litter.

Pythons, bandicoots, cassowaries and native mice are the largest inhabitants of the forest floor, while among the tree branches clamber possums, clumsy tree kangaroos and the monkey-like cuscus. Interrupting the almost uniform green are brightly colored birds and butterflies, often seen near water or displaying themselves in patches of sunlight.

Most of Australia's tropical rainforest is restricted to a few mountain ranges and along river courses on the northeast coast. The wet tropic region supports many endemic species of plants and animals, some of which closely resemble the inhabitants of the cool rainforest that once covered much of Australia. These remnant rainforests are highly regarded for their beauty and biodiversity.

Coral Reef

Unlike ecosystems of great diversity, coral reefs do not have a conspicuous flora. There are, however, symbiotic algae living within coral, encrusting coralline algae which help to hold the reefs together, and a thin algal turf covering most of the reef. Close to the mainland, in shallows protected from ocean swells by the reef, are sea grass meadows, home to dugongs and a nursery for many fish.

Six of the world's seven species of sea turtle feed and breed in these waters. Among the coral swim and crawl are not yet fully recorded variety of fish, crustaceans, echinoderms and mollusks. Above and along the reef edge prowl barracuda, sharks and sailfish. Overhead, gannets, terns shearwaters; gulls and frigate birds search for a feed of fish; the isolated coral cays provide a relatively safe nesting ground for thousands of seabirds.

Eucalypt Forest

The eucalypt tree is typically Australian but there is no typical eucalypt forest. Depending on climate and soil, you may find mountain ash (the world's tallest flowering plant), stunted alpine gum, hardy arid land ironbark or desert gum. Understorey ranges from moist ferns to dry acacias, sedges and grasses.

Eucalypt-associated faunas reveal similar variety. Although there are over 600 species of eucalypt, only a handful of species in the south-east of the continent are eaten by the koala. Less particular are the common possums, the brushtail and the ringtail, who both supplement their diets of eucalypt leaves with fruits and insects. Scratching around on the forest floor, the superb lyrebird is an excellent mimic - various bush sounds, including other bird calls, accompany male's impressive courtship display.

Announcing its presence with a familiar laugh, the kookaburra is a daytime hunter of lizards, snakes, frogs and small mammals. Grey kangaroos and wallabies may be seen moving into open forest in the evenings to browse shrubs and graze native grasses. Also out for a nightly forage, the common wombat, a relative of the koala, grazes its home range before returning to one of its large and conspicuous burrows.

Central Desert

In the arid centre of Australia life is most conspicuous in shaded gorges and along dry river courses where river red gums, home to colorful and noisy parrots, are able to tap deep reserves of water. On this ancient, eroded landscape, sparse vegetation and red sandy soils are interquently and temporarily transformed by rain into a carpet of wildflowers.

Tell-tail tracks in the sand lead to clumps of spinifex grass and burrows. Small marsupials and mice are mostly nocturnal; the rare and endangered bilby was once common to much of Australia but is now only found in the deserts of central Australia.

A few of the lizards, such as the thorny devil, will venture out into the heat of the day for a feed of ants. Among the scattered mulga and desert oak, mobs of kangaroos, the males brick-red and over two meters tall, seek shelter from the sun; but seemingly impervious to the heat, emus, with an insulating double layer of feathers, continue to search for seeds and fruit In the evenings rock-wallabies emerge from rocky outcrops to browse on nearby vegetation. Most animals breed in the cooler winter-their eggs and young attracting the attention of dingoes, eagles and perenties.