Rampant rabbits trash World Heritage island
14 January 2009 by Rachel Nowak
Good intentions have rarely gone so awry. When conservationists tried to save an island's birds by culling its feral cat population, they overlooked one critical consequence: rabbits. A new study reveals that removing just 160 feral cats triggered a boom in Macquarie Island's rabbit population from about 4000 in the year 2000 to 130,000 in 2006.
The cats were shot as they had been preying on the island's burrowing birds. But the newly rampant rabbits have devastated vegetation over 40% of the island. Clearing up the mess is expected to cost at least $16 million, and it remains unclear whether the island will ever fully recover. (See a live webcam of Macquarie Island.) A landslip in 2006 that badly damaged a penguin colony has been blamed on rabbit destruction of the vegetation.
A World Heritage Site halfway between Australia and the Antarctic, the Macquarie Island case is a tragic demonstration of why agencies in charge of conservation need to analyse all aspects of an eradication programme. "We need a culture change," says Hugh Possingham of the University of Queensland in Brisbane, Australia. "It's a generalisation, but people who do environmental work are often adverse to mathematics, and so avoid quantitative risk assessments."
The Tasmanian Parks and Wildlife Service (TPWS) eradicated the cats to protect native birds, such as the sooty albatross, believing that the continued release of myxomatosis virus would limit the expected increase in rabbits.
However, according to study leader Dana Bergstrom of the Australian Antarctic Division in Tasmania, had they done their calculations they would have spotted the risk of a population explosion and may have been able to avoid it. Several species of albatross nest on coastal slopes that are being denuded by rabbits.
Mathematics-based risk-assessment tools for environmental interventions have been available for decades, but they are not universally used outside of academia. On Macquarie, the consequences of failing to conduct a risk assessment have been particularly obvious because its simple, cold-climate island ecosystem meant that the ecological damage was not camouflaged by myriad more subtle changes, says Mark Burgman of the University of Melbourne, Australia.
Despite the devastation, a TPWS told New Scientist that eradicating the feral cats was a "major conservation achievement". They are now planning a rabbit and rodent eradication programme to start in 2010, and say this time they will conduct a quantitative risk assessment first.