What Happens When You Remove the Cats From a Rabbit-Laden Island?

Australians of European descent might be forgiven for thinking they could turn the continent into another Europe. Admittedly, there are regions that appear familiar to residents of the northern hemisphere. The rolling fields just west of the Blue Mountains, a bit more than an hour from Sydney, for instance, reminded me of rural Pennsylvania.

But that resemblance is only superficial, and Australia and Europe are really nothing alike. That didn't prevent a long list of Australians, though, from introducing various species to the country. And that didn't always work out well. (One exception is the dung beetle, which was imported from Africa and Europe from 1968 to 1984 to control the large quantities of cattle dung—which due to a dearth of fungi in Australia was not decomposing fast enough—and associated flies.)

Many of those introduced species became invasive, but subsequent efforts to control them sometimes create new problems. Take the example of Macquarie Island, a World Heritage site about 900 miles south of Tasmania. In the early 1800s, sailors accidentally introduced mice and rats to the island and then brought cats to control the rodents. A few decades later, they brought rabbits so that any shipwrecked colleagues could have something to eat; they were also an unintentional meal for the cats.

Since they breed like rabbits, the bunnies' numbers grew, despite the cat predation. They reached 130,000 by the 1970s when the Australians introduced the disease myxomatosis and the European rabbit flea that spreads the Myxoma virus. The rabbit population dropped to 20,000, and then the cats started feeding on burrowing birds.

In 1985, conservationists decided the cats had to go, and they started an eradication that was complete in 2000. But a new study, published in the Journal of Applied Ecology, finds that the loss of the cats led to even worse destruction as the kitties' former prey species have taken over in the absence of the hunter and stripped large portions of the island bare of vegetation. The rabbit population is back up to 130,000 (the Myxoma virus isn't enough to keep them under control), and there are now 36,000 rats and 103,000 mice. All this on an island just 50 square miles; it would fit into a tenth of Nashville.

The Tasmanian Parks and Wildlife Service, which oversees the island, intends to get rid of all the rabbits, rats and mice. Is that even possible? (New Zealand managed to remove all the rats from tiny Campbell Island, but they needed tons of poison.) And what unexpected results might their plans have?

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Sarah Zielinski is an award-winning science writer and editor. She is a contributing writer in science for Smithsonian.com and blogs at Wild Things, which appears on Science News.