Australasian Science: Australia's authority on science since 1938

Killing Koalas with Cars, Dogs and Disease

By David Salt

Managing threatened species requires management of multiple threats. Conservation of koalas is a point in case.

It’s rare that saving a threatened species involves doing just one thing. Take the koala, for example.

Across much of eastern Australia the koala is declining due to habitat loss, disease, vehicle collisions, dog attacks and climate change. Many of these threats result in higher death rates and often occur together. Therefore recovery strategies for koala populations need to employ strategies that address multiple threats.

That’s a complex challenge, and until now there has been little attempt to quantify the relative impact of each threat on rates of mortality. A new analysis by researchers at the University of Queensland has sought to remedy this. They hope to enhance efforts to protect koalas while providing a more robust approach to drawing up recovery plans for species facing multiple threats.

“Understanding the combined effects of multiple threats for threatened species is a key priority in the design of species recovery plans,” says Dr Jonathan Rhodes, the lead researcher of the study and a Chief Investigator with the ARC Centre of Excellence for Environmental Decisions. “Without quantitative information, identifying appropriate recovery actions is difficult, and yet it’s an issue that has received limited attention to date. We addressed this problem for koalas by quantifying the consequences of multiple threats on a declining koala population in the Koala Coast region of south-east Queensland.”

The team’s analysis appears in the journal Biological Conservation. The researchers believe that the decline in the Koala Coast koala population appears to be largely driven by high mortality rates.

Estimated annual birth rates of 0.7 are reasonably high and consistent with birth rates observed even in overabundant koala populations in southern Australia. Consequently, birth rates do not appear to be the primary limiting factor for the growth rate of this population. On the other hand, death rates are relatively high, especially for 2–3-year-old males.

The proportion of total mortality arising from disease and vehicle collisions was high, but dog attack mortalities were also a major component of male koala mortality. They found evidence that, as forest cover is lost, total mortality and the proportion of that mortality arising from vehicle collisions increases. This emphasises the importance of management strategies to reduce wildlife mortality in urban areas.

Importantly, the results of the analysis suggest that the problem of declining koala populations cannot be solved by simply addressing any one of the threats in isolation of the other threats. “We found that a 39% reduction in the total mortality arising from the key threats would be required to stabilise the Koala Coast koala population,” Rhodes explains. “However, any recovery strategy focused solely on reducing a single cause of mortality, except possibly for disease, would be inadequate in stopping the population decline.

“Disease mortality on its own would even require a substantial reduction of 59% to prevent further declines, but we currently don’t have a way of reducing disease in koala populations. Similarly, if habitat restoration were the sole recovery strategy, even restoring 100% of currently non-forested land in the Koala Coast would be insufficient to prevent further declines.

“Such large reductions in individual threats or the restoration of such large areas is unlikely to be achievable in any case in such a highly urbanised environment. In addition, large investment in a single recovery action, such as reducing disease mortality, may be highly cost-inefficient due to diminished marginal returns.

“Consequently, recovery strategies must consider combinations of threat mitigation measures if they are to be successful in reversing the current decline. Although habitat must be conserved and restored to ensure the long-term survival of koalas, this must also be combined with recovery strategies that reduce mortality rates.”

David Salt is Knowledge Broker for the Applied Environmental Decision Analysis centre at the Australian National University.