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Managing over-abundance in koala populations in south-eastern Australia-future options

Roger Martin

Department of Ecology and Evolutionary Biology, Monash University, Clayton, Victoria, 3168

INTRODUCTION

The problem caused by an overabundance of koalas, recently in the news because of the situation On Kangaroo Island, South Australia, is not new for wildlife managers. Overabundant koala populations, and the resultant defoliation and death that they inflict on their food trees, have been a recurring management problem in Victoria for most of this century. Nor is it easily resolved. For a start such a situation defies the widely held perception that the koala is a rare and endangered species. Only last year it was nominated for listing under the Commonwealth Endangered Species Protection Act. This perception is not new either. The imminent extinction of the koala has been announced many times since it was first described in the early years of the 19th century, beginning with the prediction of John Gould that it was 'certain to become gradually more scarce and to be ultimately extirpated' (Gould 1863).

While the range of the koala has undoubtedly contracted since the European occupation of Australia, it is still widely distributed in eastern Australia (Phillips 1990). Perceptions of its conservation status are largely based on estimates of abundance, and low abundance populations are often incorrectly perceived to be declining ones. Koalas are not uniformly abundant across their range and their local abundance has much to do with geographical location and the carrying capacity of the habitat. For example, in the dry brigalow scrubs of Central Queensland viable koala populations with abundances as low as 1 animal per 200 ha have been reported (Melzer & Lamb 1994). This can be contrasted with the higher abundances commonly encountered in the far south of their range. Hindell (1984) reported abundances of between 0.7 and 1.6 animals/ha in the Brisbane Ranges, Victoria, and Mitchell & Martin (1991) between 6 and 8.9 animals/ha on French Island. Downes et al. (1997) recently reported average koala abundances in excess of eight animals/ha from a number of sites on the Strathbogie Plateau in north-eastern Victoria.

Why this thousand-fold difference in abundance between the northern and southern populations? The same trend is apparent with domestic stock and it most likely reflects the richness of the habitat and the reliability of the seasons. Droughts are few and population growth is unchecked in herbivores in the south. However, these higher abundances are not sustainable and over browsing and the death of food trees followed by the death of many animals is the usual consequence. This fact, combined with the remnant nature of much of the koala's remaining habitat (see, for example, the Victorian distribution of the koala superimposed over the distribution of forest cover in Menkhorst 1995, p. 85), constitutes the major koala management problem in Victoria.

In this paper I would like to review some of the history and biology of these erupting koala populations as well as consider the options we now have to deal with them.