Modelling mammalian extinction and forecasting recovery: koalas at Iluka (NSW, Australia)
Lunney, D, O’Neill, L, Matthews, A & Sherwin, WB 2002, Biological Conservation, vol. 106, pp. 101-113.
The authors of this study modelled the effects of several population-boosting scenarios to evaluate the factors likely to be most important in the extinction and recovery of the koala population at Iluka, New South Wales. Improvements in neither mortality nor fertility rates reversed extinction risk. Only metapopulation planning to facilitate immigration into the koala population combined with increased fertility was found to restore the population to its initial size.
A population viability analysis (PVA) was employed to evaluate the potential effectiveness of different population management and recovery strategies to reverse koala population declines in Iluka, New South Wales. A model of the original population was pieced together using data from wildlife carers, radiotracking programs and the community. Scenarios including reduced mortality, increased fertility and increased immigration were then modelled to test their influence on mean population growth rate, median years to extinction and likelihood of survival. While a reduction in mortality delayed population decline to the point of extinction, this scenario alone did not prevent extinction. A 250% improvement in fertility would result in a positive growth rate for the population, but still indicated long term population decline. Similarly, increased immigration alone would improve the overall probability of survival but would not restore the population size to its initial numbers. A combination of increased immigration and improved fertility was projected to be the most effective contribution to population replenishment and sustainability long term. This would be possible as a result of the migration of koalas from other local populations; an action not previously considered.
The management of isolated koala populations in habitats subject to rapid urbanisation and land clearing presents challenges to their long-term sustainability. Koalas inhabiting the semirural landscape of Iluka, New South Wales have experienced rapid population decline since European settlement with some populations becoming locally extinct. Fragmentation in this area has limited the migration of koalas to potentially improve survival and fertility in the Iluka population.
Despite the sensitivity of the PVA test, the authors acknowledge the simulations do not account for other stochastic threats to the examined population but instead provide a guideline for the development of management protocols. Regardless, this study highlights the significance of metapopulations for contributing to immigration and reproductive activity in declining koala populations. Consequently, the authors emphasise the importance of managing koala populations at the metapopulation rather than only local population level. The lessons learnt from this case study can be applied in the management of other similar, small koala populations.
Summarised by Lauren Mousley
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