Eradication, cost and confidence
By Duncan Sutherland (Phillip Island Nature Parks)
In 1907 a ‘pet’ fox jumped ship, abandoning the crew of a fishing vessel as it passed Phillip Island, and swam ashore. Lying at the entrance of Western Port, Phillip Island is home to extensive colonies of breeding seabirds including short-tailed shearwaters and little penguins. It’s also a refuge for threatened shorebirds such as hooded plovers. What a paradise this must have been for that enterprising fox! The following year there was evidence that the fox had caused the death of many short-tailed shearwaters.
Undoubtedly, other foxes soon joined it on Phillip Island and by 1918 the impact of fox predation on the local seabirds was considered a threat to their very existence. The local council decided to sponsor two residents to hunt foxes on the island and thus began a protracted campaign to control this damaging predator.
For the next 60 years, foxes were hunted opportunistically. Poison baits were laid on small scales, but seabirds continued to succumb to fox predation. In 1980, a formal fox control program was initiated to reduce predation on little penguins at the Penguin Parade, a tourism venue near the western end of Phillip Island. Initially hunters operated around the Parade and poison baits were laid out at high intensity in a couple of years. However, because of the perceived risks to domestic dogs, baiting was discontinued.
For the next 25 years efforts to reduce fox numbers on Phillip Island were continued. These efforts included day hunts with dogs, spotlighting at night, fox trapping and den fumigation. While many foxes were being removed, their population size was not being reduced.
In 2006, things changed. The aim was no longer to control foxes on Phillip Island, it was far more ambitious: complete eradication. There were three phases recognised in the eradication campaign: a ‘knock-down’ to substantially reduce the fox population, a ‘clean-up’ phase to remove the last remaining individuals, and a ‘post-eradication’ phase to monitor for, and prevent, future fox incursions.
To achieve ‘knock-down’, Phillip Island Nature Parks implemented an island-wide baiting program several times a year. This was on top of existing techniques. Staff were dedicated specifically to fox control and to coordinating a communication programme that involved local residents and stakeholders.
Now, in 2014, an effective ‘knock-down’ has been achieved with multiple signs of success. The number of little penguins known to be killed by foxes has dropped from about 200 per year to almost zero. Populations of several ground nesting birds including Cape Barren geese and masked lapwings have more than doubled since 2006. Importantly, the rate of fox detections has dramatically declined.
So we are now in the ‘clean-up’ phase, and we’ve been making preparations for the day when no more foxes are detected and we can declare that Phillip Island is fox free. But we know that we have to be careful about making such a declaration. Foxes are elusive creatures and we can go some time between fox detections, even while they persist on the island. So, the big question is: When can we safely declare that foxes have been eradicated?
In partnership with Dr Tracy Rout and Professor Michael McCarthy we have developed a Bayesian catch-effort model that estimates the probability of detecting foxes from the four main control techniques used between 1987 and 2012 as a function of the effort invested in each technique. From this model we can also estimate the number of foxes on Phillip Island given the effort expended, the number of foxes removed and the detection rate from each technique (Rout et al., 2014).
This modelling has revealed that fox numbers have dropped from about 150 in 2005 to less than 20 in 2012. From the estimated detection rates it can be discerned that poison baiting was the most effective control technique to remove foxes and that its implementation across Phillip Island in 2006 was critical for an effective ‘knock-down’.
So when can we declare eradication? Well, given the low detectability of foxes over the years, it will take at least four years without foxes being detected before we can be 80% sure that eradication has been achieved. And it’ll be longer if we want to be more certain of success. But with greater certainty comes increased management costs accrued during the campaign. The inverse is also true, shorter campaigns cost less, but success is less certain.
The net cost of a campaign incorporates two elements. Declaring too early allows the fox population to re-establish, incurring the costs of repeating the ‘knock-down’ and ‘clean-up’ phases. However, the probability of this being required declines over time. Declaring too late and we’re mounting a campaign against a fox population that’s not there, and the overall cost climbs. So, to optimise this trade-off we also modelled the net expected cost of the campaign as years progressed (Regan et al. 2006). Phillip Island Nature Parks spent about $160,000 per year during the ‘knock-down’ phase and this gives an optimised time to declare eradication to be at least three years after the last sign of foxes has been detected.
This has significant implications for the fox eradication campaign on Phillip Island and resourcing in general. Three years is a long time to maintain fox control across the island without seeing any foxes or their spoor. We haven’t even reached the start of the three years either; we still see some signs of fox presence on the island.
As a result of the environmental decision science that has been done on the topic of ‘when to declare eradication’, support has been given to continue fox management for an extended period and to increase our capacity to detect the last remaining foxes. Phillip Island Nature Parks recognises the need to increase our ability to detect foxes so that eradication is more certain and can be declared sooner. This will ultimately reduce the costs of the campaign while ensuring its effectiveness.
Publication of this work has been instrumental to attracting funding from the Ian Potter Foundation and the Penguin Foundation to support the fox eradication campaign into the future. So far, additional support has come in the form of a full time staff member for three years to handle two specially trained scent dogs that will join the hunt to sniff out the last remaining foxes from their refuges, and the implementation of extensive camera trapping across the island to improve detection rates.
Baiting, spotlighting and trapping will continue as before. The expectation is that these new techniques will increase our success at detecting the last remaining foxes and this data can be incorporated into a revised catch-effort model.
With more weapons in the arsenal, we hope the last remaining foxes will be located and removed, the revised model will indicate eradication can be declared sooner, and Phillip Island’s wildlife can again enjoy a fox-free environment.
More info: Duncan Sutherland firstname.lastname@example.org
Regan TJ, MA McCarthy, PWJ Baxter, F Dane Panetta & HP Possingham (2006). Optimal eradication: when to stop looking for an invasive plant. Ecology Letters 9:759–766.
Rout TM, R Kirkwood, DR Sutherland, S Murphy & MA McCarthy (2014). When to declare successful eradication of an invasive predator? Animal Conservation 17: 125–132.