A new paper in the Journal of Applied Ecology (Bergstrom et al (2009). Indirect effects of invasive species removal devastate World Heritage Island, doi: 10.1111/j.1365-2664.2008.01601.x) describes how, on Macquarie Island, an important breeding location for several species of penguin, the elimination of invasive cats in attempt to protect the penguins has triggered a trophic cascade as herbivore populations have boomed defoliating the island.

The study’s lead author Dana Bergstrom, of the Australian Antarctic Division, estimates that nearly 40% has been modified, with 20% having moderate to severe change, and that rabbits have convert vegetation from complex communities to short, grazed lawns or bare ground.

Rabbits caused plant cover to decline starkly on this royal penguin "run" between 2001 (top) and 2007 (bottom). BBC

Finch Creek on sub-Antartic Macquarie Island. Rabbits have stripped 40% of the island bare of vegetation, scientists say. Photograph: /Australian Antarctic Division

From the Guardian:

Things began to go wrong on Macquarie Island, halfway between Australia and Antarctica, 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.

Given easy prey, cats feasted on the hapless rabbits and feline numbers quickly grew. The island then lost two endemic flightless birds, a rail and a parakeet. Meanwhile, the rabbits bred rapidly and nibbled the island’s precious vegetation.

By the 1970s, some 130,000 rabbits were causing so much damage that the notorious disease myxomatosis was the latest foreign body introduced to Macquarie, which took the rabbit population down to under 20,000 within a decade.

“The island’s vegetation then began to recover,” Bergstrom says.

But what was good for the vegetation proved bad for the island’s wildlife. With fewer rabbits around, the established cats turned instead to local burrowing birds. By 1985, conservationists deemed it necessary to shoot the cats.

The last cat was killed in 2000, but the conservationists were horrified to see rabbit populations soar. Myxomatosis failed to keep numbers down, and the newly strong rabbit population quickly reversed decades of vegetation recovery. In 2006, the resurgent rabbits were even blamed for a massive landslip that wiped out much of an important penguin colony.

…
The Tasmanian Parks and Wildlife Service intends to fix the island once and for all, and has drawn up plans to eradicate all 130,000 rabbits, along with the estimated 36,000 rats and 103,000 mice that live there.

The move could yet provoke more unexpected side effects, Bergstrom says. “This is the largest island on which this type of eradication program will have been attempted.”