Australia: The Land Where Time Began

A biography of the Australian continent 

There are 6 species of large kangaroo, the red kangaroo being the most common, as well as being the only one that is restricted to the arid zone, inhabiting the better watered plains and low open woodlands, with its heartland in western New South Wales. It occurs in most parts of central Australia, with the exception of true desert areas, extending its range to the north as far as the northwestern coast. Its range covers areas where the unpredictable annual rainfall is less the 800 mm, and the mean annual temperature is above 15^oC. It is absent from areas with predictable rainfall, such as the southwest corner of the continent and southern and eastern areas and Tasmania, and from the tropical latitudes north of 14^oS. It occurred as far south as the southern coast 12,000 years ago, at time when the rainfall pattern or the mean temperature differed from the present (Flannery & Gott, 1984; Caughley et al., 1987b).

The euro occupies the hills and ranges in an overlapping distribution with that of the red kangaroo, which occupies the plains. Being adapted for hill-climbing, the euro is stockier and has shorter hind legs. The eastern wallaroo, a subspecies of euro, extends the distribution as far as the eastern coast of Australia, well beyond the rangelands. The distribution of the euro extends north into the tropics. In this part of its range there are 2 closely related species, the antilopine wallaroo and the black wallaroo. It is believed the populations of the euros and wallaroos are probably similar to that of red kangaroo, though they haven't been estimated (Edwards, 1989).

In the eastern and southern regions of the continent, 2 species of grey kangaroo dominate in areas with predictable rainfall that is seasonal. In eastern Australia and Tasmania, the eastern grey kangaroo has its heartland in southern Queensland and northern New South Wales, where the predominate season for rainfall is summer. In spite of a favourable climate in South Australia and Western Australia, it does not occur in these areas. The least common of the 4 species is the western grey kangaroo, occurring only where the main rainfall occurs in the winter, its heartland being in southern New South Wales, a gradient of lower density of population extending through South Australia to Western Australia. It is also found on Kangaroo Island, but not eastern Victoria or Tasmania. Western grey kangaroos have a high tolerance for sodium fluoroacetate, a poisonous substance that occurs in 33 plant species of the genera Gastrolobium and Oxylobium, that occur in Western Australia. No plants from eastern Australia are known to contain this poison. The western grey kangaroo has a high tolerance for this poison, as do other marsupials from Western Australia. The lethal dose is 20 mg/kg body weight (Oliver et al., 1979). Western grey kangaroos from Kangaroo Island and New South Wales share the tolerance exhibited by the Western Australian animals. It has been suggested that the tolerance for the poison in populations or western grey kangaroos from areas where the poison-containing plants are not found is evidence that the species originated in western Australia (Tyndale-Biscoe, 2005). If it was named from its distribution at the present it should be called the southern grey kangaroo, but this tolerance indicates that the name is more appropriate.

Though for different reasons, the red kangaroo and grey kangaroo species live south of latitude 14^oS, north of this latitude it is too hot for the eastern grey, too wet for the red kangaroo and too hot in summer and too dry in winter for the western grey kangaroo (Coughley et al., 1984, 1987b). The ranges of the 4 main species overlap considerably in the southern half of the continent, in parts of western New South Wales all 4 species are found together where all their ranges overlap.

The most important factor influencing the locality and extent of their range is the effective rainfall, the amount of rainfall in excess of evaporation, when there is enough soil moisture to induce seed germination leading to the growth of nutritious ephemeral plants that are their main food source and which are required for successful reproduction. Other factors kangaroos species of the inland arid areas must contend with are high temperatures, food of low nutritional value, and often with a high salt content and limited shade. The kangaroos of this area must have some physiological and behavioural mechanisms to overcome these problems in their environment, they must control heat load, and conserve water and  energy resources. The effectiveness of their responses to these adverse conditions determines their breeding success and hence their numbers. The red kangaroo and the euro have adapted most fully to these conditions, each in their own particular way.

Sources & Further reading

Tyndale-Biscoe, Hugh, 2005, Life of Marsupials, CSIRO Publishing.