Koalas on Kangaroo Island: from introduction to pest status in less than a century
Masters, P, Duka, T, Berris, S & Moss, G 2004, Wildlife Research, vol. 31, pp. 267-272.
The rapid increase in koala numbers after 18 individuals were introduced to Kangaroo Island, South Australia in the 1920s necessitated the development of a management plan to control the expanding population. Estimates of population size based on surveys carried out in 1994 placed the population at around 5000 individuals. At the time of implementation of a sterilisation and relocation program in 1997, however, it became apparent that initial surveys had greatly underestimated population size and an accurate measurement of the current population was needed.
In 2000 to 2001 a subsequent survey resulted in a much higher population estimate (of approximately 27,000) than the initial estimate of 3000 to 5000 in 1994. This later survey categorised koala habitat into low, medium and high quality based on the presence of preferred eucalypt species for koala consumption. A mark recapture method was utilised in different habitats to extrapolate data across the total area of suitable habitat for koalas in order to estimate a population size. Although this increase suggests rapid population growth from the time of introduction, the difference between the two survey counts may be due to underestimating the extent of occupancy of lower quality habitat which was excluded from the initial survey.
Availability of suitable habitat and low death rates, due to lack of disease and predators, are factors that may have led to the fast growth rate of koala populations on Kangaroo Island. The rapid increase in koala numbers since introduction and consequential overcrowding has resulted in increased herbivory of palatable plant species, leading to the removal of eucalypt species that provide high quality habitat for koalas. Although low quality habitat with fewer favourable plant species supports less koalas than high quality habitat, nearly half the population on Kangaroo Island inhabit these regions due to the extent of low quality habitat present.
Although initial introduction of koalas to Kangaroo Island was highly successful, overpopulation caused changes to plant species composition and has reduced the quality and amount of ideal habitat available for koalas. In order to manage a sustainable population after introduction, suitable ongoing management plans to control the number of individuals based on accurate population data must be considered.
Summarised by Meredith Kraina
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