The cats ate the birds until the humans killed the cats, but now the rabbits are out of control. That's the sad state of affairs on Macquarie Island, an island near Antarctica that was declared a world heritage site in 1997 due to its status as the sole breeding ground for the royal penguin. For decades researchers have attempted to get rid of the invasive species that have altered the island's ecological balance, but a new study notes that the latest effort, an all-out push to eradicate feral cats, has had the unintended consequence of allowing a boom in the rabbit population. Those rabbits have quickly denuded the landscape of its vegetation, researchers say.

Things began to go wrong on Macquarie Island ... soon after it was discovered in 1810. The island's fur seals, elephant seals and penguins were killed for fur and blubber, but it was the rats and mice that jumped from the sealing ships that started the problem. Cats were quickly introduced to keep the rodents from precious food stores. Rabbits followed some 60 years later, as part of a tradition to leave the animals on islands to give shipwrecked sailors something to eat [The Guardian].

The invasive species all thrived to the detriment of local species, and by the 1970s biologists were concerned enough to introduce a rabbit-killing disease called myxomatosis, which thinned the rabbit herds considerably. However, that left the cats with less available prey and caused them to begin hunting the island's native burrowing birds. By 1985 conservationists decided that all of the cats would have to be shot, and the job was done by 2000. But that move allowed the rabbit populations to flourish once more.

Since the eradication of cats eight years ago there are now an estimated 100,000 rabbits munching the foliage of Macquarie Island.... Removal of plant cover is thought to make penguins more vulnerable to predation [BBC News].

Now scientists say they must wipe out the rabbits completely, and say that it will cost about $17 million to eradicate the invaders and restore native plant life. They expect to begin dropping poisoned bait around the island next year. In the study that sums up this history, which will be published in the Journal of Applied Ecology, study coauthor Dana Bergstrom says that Macquarie Island should serve as an example of what not to do.

"Our study shows that between 2000 and 2007 there has been widespread ecosystem devastation and decades of conservation effort compromised. The lessons for conservation agencies globally is that interventions should be comprehensive, and include risk assessments to explicitly consider and plan for indirect effects, or face substantial subsequent costs" [Telegraph].

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