Australia: The Land Where Time Began

A biography of the Australian continent 

Mitchell grass, mostly Astrabla species,  is the predominant vegetation of these grasslands of the arid zone, over an area of about 500,000 km², growing on the cracking clay soil plain formed by the accumulation of fine, fertile mud, derived from Great Artesian Basin (GAB) sediments deposited in the epicontinental sea of the Early Cretaceous, deposited over millions of years, that is swampy in the wet season but on drying in the winter dry season, it cracks and the columns of soil are separated by wide gaps that break any roots that grow horizontally. This is probably the reason the region is treeless. They are mostly in an arc along the eastern and northern margin of the desert areas. The Mitchell grass avoids the root problem by developing long vertical roots that are mostly unaffected by the cracking. There are up to about 50 native grass species amongst it. The Barkly Tableland in the Northern Territory is part of this grassland, the cracking clay soil of this region deriving from the sediments deposited in the Eromanga Sea, also of Cretaceous age, outside the GAB and its aquifers from the Jurassic and Cretaceous. The boundaries of the underlying Cretaceous rocks of the Rolling Downs Group coincides with the boundary of the tussock grasslands. In overgrazed tussock grassland in northeastern Queensland an introduced thorny bush (Acacia nilotica) has invaded the overgrazed areas converting them to open savanna.

In Victoria, prior to the introduction of agriculture, when the native vegetation was cleared and exotic pastures introduced, on the basalt plains to the west of Melbourne there were extensive areas of tussock grass savanna. Several genera of grasses predominated in this savanna, Themeda, Danthonia, Stipa and Poa. One of the consequences of the loss of this habitat and its reduction elsewhere, as well as introduction of foxes and cats, has led to reduction in numbers of the population of eastern barred bandicoots to the point at which they are endangered, only about 500 remaining. There are many other instances where the arrival of Europeans with their agricultural methods has decimated the native wildlife.

At the time of European settlement, tussock grasslands covered wide areas in semi-arid and some wetter areas of the continent, as far south as Tasmania. It has now been cleared and changed by the introduction of introduced pasture species, the areas of natural tussock grassland being changed by the selective grazing by sheep and cattle. An unforseen effect of the changes in pasture composition, noticed by farmers in the Riverina, was the spread of spear grass (Stipa) at the expense of the Dnathonia and  Tremada that were preferred by the sheep. The seed of spear grass has 3 curved prongs that act like the legs of a tripod, when the dry seed lands on the soil the pointed end of the seed is always pressed against the soil surface so that when it gets wet, as by the arrival of rain in its natural habitat, it twists, burrowing the seed into the ground. Unfortunately for the sheep, the seed can't distinguish between the source of moisture, burrowing into the nasal passages and even the skin of the sheep.

On the Southern Tablelands, winter frosts are the determining factor producing the open grasslands of the area. A similar situation is seen in the Highveld of South Africa.

Research has been carried out in the semi-arid zone near Blanchetown in the Murray Basin, on the grasses eaten by grey kangaroos, red kangaroos and hairy-nosed wombats. It has been found that with no introduced grazers, the native grasses do well in balanced communities. The balanced state of the area before the introduction of grazing species produced changing mosaics of habitats and populations. The composition of the vegetation is changed by heavy grazing by rabbits and sheep, the exotic species have their growth phases mainly in spring and early summer. This grazing pressure concentrated in part of the year has been found to be pushing ecosystems to a boom and bust type of economy, introduced animal populations expanding in good years with high rainfall and crashing in between these good years. One result is that the availability of grass species eaten by wombats has become out of phase with the breeding time of the wombats, resulting in suitable grasses being less available at the time the young wombats are being weaned. The relationships between native animals and the environment that is so erratic is being broken down by the grazing of introduced species that are not adapted to the variable local environmental conditions, making it unlikely the native animals will avoid extinction at some point in the future.

The fire-stick farmers disrupted the original balance of the environment, but the changed balance was maintained sustainably over thousands of years, but the changes wrought by modern agriculture is pushing semi-arid and arid zones to the type of boom and bust system that eventually leads to desertification. As the process continues, seedbanks in the soil are diminishing, until a point is reached when there can be no regeneration. The animals decline as the degradation of the vegetation they depend on continues. Over many millions of years the biota of the Australian arid and semi-arid lands have evolved into forms that can survive in the most erratic climate on the driest inhabited continent on Earth, often through specialising. These frail ecosystems, finely balanced to survive through the proverbial 'fire, drought and flooding rains', are unlikely to survive the continuous grazing by introduced species that evolved in places with much different climatic conditions, where the rain was more reliable and the soil was more fertile.

In places such as the northern Sahara, the process has been completed, a man-made desert.

Birds

In these grasslands there are many seed-eating birds such as budgerigars, cockatiels (quarians) and button-quails. The Australian bustard is also present, feeding on a variety of foods, insects, fruits and seeds. When it is approached it freezes to avoid being seen then slowly moves away after some time of remaining motionless. These grasslands are the habitat of the flock bronzewing and there are many venomous snakes.

Reptiles

Snakes are found in all of Australia's desert and arid regions, but outside the tussock grass areas they are mostly non-venomous, but here the venomous snakes are common, some, such as the speckled brown snake are endemic. The snakes can enter the cracks in the clay inhabited by the plague rats, planigales, and any other mammals that seek shelter in them, even bats. In this ecosystem snakes replace lizards as the most common reptiles, probably because of the high numbers of mammals for them to feed on. An exception is Spencer's monitor, a goanna. It specialises on eating the diurnal death adder, one of the most venomous snakes in a land of the most venomous snakes, avoiding the first strike of the snake, clamping its jaws on the snake before it can retract for a second strike.

Sources & Further reading

1. Penny Van Oosterzee, The Centre - The Natural history of Australia's Desert Regions, Reed Australia, 1993
2. Mary E White, After the Greening, The Browning of Australia, Kangaroo Press, 1994