Insight into Habitat Diversity & Threatened Species
Environment Australia, 2001
Arid Habitats
Arid habitat are the most extensive in Australia. About ten million years ago the continent began drying out and the forests began to thin out to become woodlands. Pockets of grassland expanded. Low, heath-like vegetation dominated the dry stony ridges. To the north, trees became widely scattered with a grassy floor, a savanna. Southwards, stunted multi-stemmed eucalypt scrubland, the mallee areas, covered wide areas along with bluebush and in other places saltbush, the shrublands. Prevailing westerly winds in the south swept the beds of dry lakes forming eastern lunette dunes while from about 30,000 years ago the major dune systems of the interior were activated and expanded.
Acacia scrubland (mulga) and spinifex hummock grasslands became the most widespread arid vegetation. Stony ranges broke this mosaic creating havens for more humid vegetation types and the most productive of arid habitats, the outwash areas. As aridity increased, ancient river patterns were engulfed by sand to form sinuous lines of salt and in many places, lines of aquifers upon which depended lines of "Nature's Boarding Houses", the river red gums.
About 70% of the continent is covered by a diverse mosaic of arid habitats linked by dry (on the surface) watercourses and broken by ranges. Because change took place over ten or more million years a wide variety of wildlife became well adapted to parts of the mosaic. Some animals such as small dragons have ranges as limited as a dune. Others are restricted to a waterhole while kangaroos and birds, when stressed by water shortage, move over vast areas. Ability to move gives parrots, cockatoos and waterfowl use of the desert and other habitats once the prevailing drought breaks.
Invading rabbits and foxes
In the late 1800s and early 1900s the rabbit plague spread across the arid zone causing havoc in the dryland habitats by reducing much of the vegetation to a dustbowl condition. However the plague carried another unwanted guest, predatory foxes which, when drought had starved most of the rabbits, turned to hunting native fauna. Dorothy Tunbridge working with the Aboriginal people and from pastoralist records in the Flinders Ranges, has shown that some 60% of the arid animal species there had become extinct in the 25 years before 1900. Central Australia's decimation came 30-40 years later. The Mala, Wopilkara (greater stick-next rat) and golden bandicoot are just three of many species either extinct or on the brink of extinction.
These animals which through the adaptation of behaviour and physiology to survive the toughest conditions of the desert were, and in many cases still are, highly vulnerable to any new factor which reduces the quality of their habitat, for example food supply, water, shelter and special space needs.
The diagram shows three common arid habitats: arid dunes, mulga scrublands and mountainsides with terraces and cliffs.
Arid dunes
Dunes are of soft, deep sands with dense hummocks of spinifex species, acacia and numerous nectar-producing shrubs with clumps or lone bloodwood trees. Dune slopes and swales support desert oaks, a dense cover of spinifex and mallee or mulga scrub. Areas of softer perennial grasses succeed for several years after tough, smothering spinifex is burnt in bushfires. After rain numerous ephemeral herbs and grasses burst into vibrant life. Dune habitats favour the diggers with nocturnal roaming habits and mobile animals which can make use of flushes of growth and flowering - the herbivores and nectar or seed eaters. These flushes create a burst of food for the insects and later, their predators, the spiders, centipedes and small mammals. Then the inevitable drought returns and the dune system becomes a quieter place where short-term lives end and others slip into near sleeping modes of activity. Non-perennial plants bide their time as seed, so long as the seed-eaters don't find them first.
The dynamics of the other arid habitats are roughly similar. A flurry of life with the breaking of drought and a slow down during drought. In the mountains, the principal limiting factor is the location of permanent water. On the cliffed terraces of gorges, rock wallabies rest up in the caverns or under boulders, feeding on the grasses of the hillslope or at dusk moving into the gorge bottom. More mobile is the euro, or hill kangaroo which usually concentrates on the lower slopes. Both animals have a physiology which is very conservative of water. But key to ultimate survival is access to the higher humidity of caverns for daytime use, permanent water within range and a sustaining food supply. Aggressive feral goats successfully compete for shelter and the food.