Cats, Rabbits, Myxoma Virus and Vegetation on Macquarie Island: A Comment on Bergstrom et al

Summary1.Eradication of a single pest species from a multiply invaded island system may have unpredictedand detrimental impacts. Bergstromet al.(2009) describe damage to vegetation following anincrease in the number of rabbits on Macquarie Island. They propose that the increase in rabbitnumbers was caused solely by eradication of cats.2.However, their modelling is flawed and their conclusion that cats were controlling rabbit num-bers is unsupported. We suggest the increase was because of some combination of four factors:reduced releases ofMyxomavirus, abundant food after 20 years of vegetation recovery, releasefrom cat predation and climate variability.3.Recent high numbers of rabbits on Macquarie Island are not unprecedented; vegetation has beendamaged in the past but has recovered. Rabbit numbers appear to be in decline again in the absenceof both cats andMyxomareleases, suggesting that other factors can contribute to regulation ofrabbit numbers in this system.4.We do not agree with the implication that pest management could have been better integrated.Eradication techniques for rodents and rabbits on an island the size of Macquarie were unavailablewhen cat eradication was deemed necessary. The benefits to seabirds of cat eradication have beenrapid. Our analysis further highlights the complexity of multiply invaded island ecosystems.