Macquarie Island in 2001 (top) and in 2007, after the rabbit boom. Credit: AAD

Australia's Macquarie Island, a severe-looking but biologically rich outpost of land between Australia and Antarctica, had too many cats. They were eating the island's bird life. So between 1985 and 2001, all the island's felines were shot.

But getting rid of the cats, researchers have discovered, has allowed the population of another invasive species — the rabbit — to boom. As a result, the island's soil and plant life are feeling the effects. Repairing the damage will cost Aus$24 million (US$16 million), say researchers.

The moral? Before conservationists remove an invasive species, they should look at the big picture, says ecologist Dana Bergstrom of the Australian Antarctic Division in Kingston, Tasmania.

"Try, if you can, to identify any unintentional consequences," she says. "How are invasive species connected to each other and how are they related to natives? These species become quite entwined in the hundreds of years they are together."

The plan backfires

Cats were introduced to the Macquarie Island in 1818; sealers introduced rabbits 60 years later.

The rabbits tore through the island's vegetation. In 1968, the rabbit flea was introduced. Once that had established the lethal myxomatosis virus — which the flea spreads — was introduced in 1978.

Rabbit numbers crashed, but then the cats, which had previously eaten rabbits, switched their attentions to the island's birds.

But once the cats were gone, the few hardy rabbits that had survived both the cats and the myxomatosis emerged and began doing what rabbits do best — breeding and eating.

Such consequences sound obvious in retrospect, but even simple systems can produce "surprise effects", says ecologist Franck Courchamp of the University of Paris-Sud.

"Removing the rabbit at the same time might have triggered a release of an invasive plant, barely noticeable when the rabbits were present" he notes.

The only possible solution he says, is to study the invaded ecosystem carefully before any control measures. Courchamp has done exactly that on Surprise Island near New Caledonia. There, they have tried to eradicate both the invasive mammals, mice and rats, simultaneously.

Been there before

Bergstrom thinks that no one realized how fast the rabbits could come back on Macquarie Island, and no one believed how few cats could keep them in check. She and her colleagues reports their findings in the Journal of Applied Ecology1.

The toll on the island could be severe. "Many of the plants that are being eaten are what are called regional endemics," says Bergstrom. "We only see them in this part of the world."

These plants include what Bergstrom calls "mega-herbs" including, waist-high "mega-celery". Erosion caused by grazing on slopes also leads to small landslides, one of which landed on a penguin colony, she says.

It has long been known that one of the chief perils of dealing with invasive species is that the cure is often worse than the disease.

For example, species introduced to prey on invasive organisms often become a problem themselves. The Indian mongoose (Herpestes javanicus), for example, was brought to Hawaii and other Pacific islands to kill rats. It did, but it also killed untold numbers of native birds, driving at least two to extinction2.

And on Little Barrier Island, near New Zealand, cat removal did not increase breeding success for Cook's petrels. The rats, freed from cat predation, multiplied and ate the petrels themselves3.

These stories show that removing a single invasive species can have many unintended consequences. On Macquarie Island, Australian federal and state governments will now have to spend millions on an "integrated rabbit, rat and mouse eradication programme".