Australia: The Land Where Time Began

A biography of the Australian continent 

 Yookamurra and Scotia - Earth Sanctuaries in the Malllee

The Yookamurra Sanctuary is about 100 km northeast of Adelaide, South Australia, in the mallee country between the Barossa Valley and the Murray River. Since 1988 captive breeding programs of some highly endangered animals have has been undertaken at this sanctuary. The release of animals into the sanctuary has allowed observation of the changes to the mallee environment that resulted from their loss, and the changes that took place when they were restores to the same environment in the protected sanctuary.

At another location, Scotia Sanctuary, on the western side of the Silver City Highway, about half way between Broken Hill and Wentworth, was established in 1997. In this sanctuary there are a number of vegetation types - saltbush plains, Belah woodland (Casuarina), arid grasslands and some mallee. Captive breeding programs have also been undertaken here. The fences around theses sanctuaries, as around all properties owned by Earth Sanctuaries, have been carefully designed to keep all feral animals out, and they have been eliminated from inside the encosures, but kangaroos and emus can pass through the enclosures. Alternating electrified and non-electrified wires above mesh rise to 1.8 m. It has been observed that emus part the wires and roll through on their chests and the kangaroos get though with a sideways dive. The electric charge is not enough to deter the emus and kangaroos, but doest deter the cats and foxes. A second fence of the same kind has been installed around the sanctuaries 40 m outside the inner fence. This was found to be necessary because of the practice of cats being thrown over the original fence.

In the western part of the Murray Basin, the western Division of New South Wales, as well as adjacent parts of Victoria and South Australia, places where the biota has had to evolve mechanisms to cope with arid conditions, on the sandplains and dunefields, semi-arid mallee vegetation is widespread. The dunefields are composed of short, low, linear dunes that align east-west. They rise between 2 and 6 m above the intervening swales, and are usually about 250-1000 m apart. In most cases, the south-facing slopes are steeper than the north-facing slopes, and contain about 7-20 % clay and 5-15 calcium carbonate formed as the alluvial B horizon.

Below the shallow A horizon, there are large number of places where a hard layer of groundwater calcrete is found below the shallow soil A horizon. Tree roots have to find cracks in this hard layer to reach the watertable in the Woorinen Formation sediments. The soils are mostly poor, after having weathered without renewal over long time periods, even on a geological scale. The biota recycles the small amount of nutrients that are mainly in the surface layer.

Seed germination, water balance and nutrient cycling rely heavily on the condition of the upper soil layer. The nutrient levels in this layer are dependant on the organic content. Water harvesting and moisture retention in the surface soil, essential for the establishment and functioning of microbial and plant communities in arid, nutrient-depleted environments, are affected strongly by the patchy soil disturbance caused by small mammals.

Mallee is the arid extreme of eucalypt woodland, the trees being multi-stemmed, growing from large underground lignotubers. Old growth mallee has been described as 'untidy' woodland, with old timber scattered among the stems, living and dead, that were often contorted, and with branches hollowed out by termites. On sandy areas the understorey is dominated by spinifex. In some places Grassy areas with herbs and areas where much of the surface is almost bare for much of the time, with a dry covering by biotic crusts.

The Murray Plains landscape before European colonisation

Prior to colonisation the soils were 'soft, spongy and very absorbent [and where] an inch of rain, then, in spring or autumn, produced a luxurious growth of fresh green grass, [there had been a] gradual deterioration of the country caused by stock, which had transformed the land from its original soft, spongy, absorbent nature to a hard, clayey smooth surface ... which instead of absorbing rain runs it off in a sheet as fast as it falls, carrying with it the surface mould, seeds of all kinds of plants, sheep manure, sand, etc., to enrich the lower lying country and plant it with pine, box and other noxious scrubs (James cotton, stock inspector)' (White, 2003).

According to the memories of another farmer, George Riddock, when he rode his horse on areas away from rocky ground the horse's hooves sank to the fetlocks in loose, friable soil. He compared it to a well-tilled filed in which rain penetrated the soil, fertilising the plants. Following the changes he said the ground was so hard the horse's hooves were almost ringing as he rode across it. According to diaries kept by early settlers, the rapid changes were actually welcomed, as it meant the runoff filled dams. They believed it was a positive change for their profitability. The reservoirs were all important, as there were no flowing rivers in the area. They were completely unaware of the costs to their future production, not realising the thin, surface layer of the soil, was being carried to the low lying areas, together with what little nutrients and soil microbes were present in it. In semi-arid areas the soil microorganisms are low in number, and function only when the soil is damp, are present only in the superficial layers, precisely the layers that were carried away in the runoff.

As stock grazing increased on the semi-arid land, overgrazing and drought, were widespread, and the small mammal fauna declined rapidly. There was a rapid decline of species such as the rat-kangaroos, potoroos, bandicoots, and bilbies, some to become rare and other to completely disappear. It is now known that these small mammals scratch and dig hopes in the soil as they forage for food. This has the effect on the soils of aerating, turning over the soil, improving infiltration of rainwater, spreading seeds and fungal spores. The result was a soil surface that rain could infiltrate as soon as it fell, reducing runoff, and keeping the soil damp longer to allow the microbes to do their job, recycling nutrients and minerals. The disturbed soil where the animals scratched or dug pits provided site for seed germination and establishment. 

According to White (2003) it has been estimated that a single brush-tailed bettong can churn up more than 6 tonnes of soil in a year. There is no doubt that this one species can be responsible for major perturbation. The activities of these and other species are important, especially in areas where the soils tend to be water-repellent, that are very common in many parts of southern Australia.

Effects of the reintroduction of small mammal burrowers and scratchers

The degradation that was previously believed to have been caused by the introduction of hoofed animals to the arid land in Australia, where their hoofs broke through the crust that exists on the sandy soil over much of the inland, was found to not recover when the hoofed animals were removed. In the enclosures of the captive breeding programs at places like Yookamurra it can be seen that what is required is the return of the small animals that have been exterminated over much of the continent by feral animals such as cats and foxes. In these enclosures it has been found that it is the burrowing and scratching of the small mammals that is returning the land to its state prior to the introduction of the livestock and feral animals. Grasses are once again growing on land that had become covered by nothing by biotic crusts. Highly degraded land is once again becoming functioning ecosystems. The archives have provided information on the ecological conditions of the area prior to the changes that resulted from European colonisation.

Sources & Further reading

1. Mary E. White, Earth Alive, From Microbes to a Living Planet, Rosenberg Publishing Pty. Ltd., 2003
2. Listen...Our Land is Crying