Posted on 09/10/2013 by Phillip Island Nature Parks
Introduced foxes established on Phillip Island in 1907 and quickly became a major threat to the island’s wildlife - inclusing the iconic little penguins. The fox population was held below carrying capacity for more than 20 years without preventing population-threatening predation on penguins and other seabirds. Successful control came following implementation of island-wide baiting four-times per year in 2005. First indications for success were a dramatic increase in the ratio of juvenile to adult foxes and strong recovery in little penguin numbers, coinciding with a decline to zero penguin deaths attributed to foxes.
When to declare fox eradication on Phillip Island?
After there are no more signs of foxes on Phillip Island, when should we declare that foxes have in fact been eradicated? We have a decision problem: fox management cannot continue indefinitely when individuals are no longer detected – at some point efforts must be reduced or ceased entirely. However, the risks of mistakenly declaring total eradication can be high: the species can bounce back, causing environmental and economic damage. Using different methods, we estimated that 11 foxes remained on Phillip Island as of June 2012 in a study published in Animal Conservation. Baiting was the most effective method for removing foxes while spotlighting was the most effective method for sighting foxes without removal.
If effort continues at current levels but no further foxes are removed or sighted in the next four years, the estimated number of foxes drops below a single fox with 69% probability of a successful eradication. This analysis allows decision-makers to assess the trade-offs involved in any decision to declare eradication.
Figure: The estimated fox population size (left axis, solid line = mean, dotted lines = 95% credible interval) and the probability foxes have been successfully eradicated (right axis, dashed line) under a future scenario where current management continues but no further foxes are detected from 2012.