Macquarie's feral cats: a delicate ecological balance

The extermination of Macquarie's feral felines was meant to safeguard the island's indigenous birds. But their demise has had an unexpected effect: rats and rabbits are flourishing - and pose a new threat to the native species.

Nestling in the icy waters of the sub-Antarctic is the World Heritage-listed Macquarie Island, home to more than a dozen species of threatened marine mammals and seabirds - and 100,000 rabbits.

The rabbits munching their way through Macquarie's vegetation are not native to the windswept Australian island in the Southern Ocean, nor are the rats that steal rare seabird chicks from their nests. They were introduced by whalers and sealers in the 19th century, along with the feral cats that proved the worst menace to the local wildlife.

A programme to eradicate the cats, funded by the Australian government, was heralded a great success. The last feline was humanely destroyed on the island - situated about 900 miles south-east of Tasmania - in mid 2000. In 2004 endangered grey petrels bred there for the first time in a century and there were hopes that the critically endangered blue petrel might return.

But attempts to restore the fragile ecological balance, upset by the influx of exotic predators, have had an unexpected effect. With the cats removed, the rabbit population on the 60 sq m island has exploded, increasing tenfold from 10,000 in the mid-1980s. The number of rats and mice has also soared.

Now, environmentalists are demanding urgent action to exterminate the rats and rabbits. A plan costing A$16.5m (£6.7m) has been formulated but has yet to be implemented because federal and state governments are arguing about who should pay.

While Macquarie, uninhabited by humans apart from visiting scientists, is part of the island state of Tasmania, the federal government has responsibility for it as a World Heritage site. As they wrangle about funding, the condition of the island has been condemned as an international embarrassment.

WWF Australia warned recently that the grey-headed albatross faces imminent extinction in Australia because its only known breeding site, located on a part of the island known as Petrel Peak, has been eaten away by rabbits. "There are only a few grains of sand left in the hourglass for this species in Australia," said Julie Kirkwood, the organisation's policy officer for invasive species.

Other species of albatross and six species of petrel are also under threat because of soil erosion and landslides caused by the rabbits' voracious nibbling. Breeding habitats and the seabirds' nests are being destroyed. At one breeding area for the light-mantled sooty albatross, almost half of the nesting birds failed to raise a chick at the site in one season.

Macquarie was discovered by accident in 1810 by the crew of a sealing vessel sailing from Australia to New Zealand. Within a decade one species of fur seal had been wiped out, and the killing of seals and penguins for oil continued until 1919. Located halfway between Tasmania and Antarctica, the island was also used as a stopping-point for expeditions to the frozen continent. In 1911 a scientific base was established by Sir Douglas Mawson, the Australian explorer, and replaced by a permanent research station in 1948.

Geologically unique, Macquarie is the only place in the world where the oceanic crust is exposed above the surface of the sea. It is a breeding ground for 100,000 seals and three to four million seabirds including four species of penguin.

Early visitors introduced foreign mammals including horses, donkeys, pigs, goats and sheep. The survivors were the rabbits, feral cats, black rats and house mice.

Efforts to eliminate cats began in the 1970s, culminating in a programme of intensive trapping in the late 1990s. Cats were located by spotlighting, with trained dogs brought in to make sure none were missed. Nearly 2,500 cats were caught. They were believed to have been killing up to 60,000 seabirds a year.

Rabbit numbers had been kept relatively low by destroying warrens and introducing the virus that causes myxomatosis. But since the cats disappeared, rabbits have bounced back, and time is running out to eradicate them.

An aerial poison-baiting programme has been mapped out but it has to take place in the southern hemisphere winter, when the birds are at sea. That leaves only a few weeks to reach agreement on funding. The last supply ship of the summer season calling at Macquarie Island leaves Hobart, the Tasmanian capital, in early April. If personnel and equipment are not on it, the project will have to wait another year. That could be too late for some of the 17 threatened species of marine mammals and seabirds that use the island.

Australia's Environment Minister, Malcolm Turnbull, offered Tasmania A$8.25m last week and urged the state government to find the balance. But Tasmania's Environment Minister, Paula Wriedt, said the protection of Macquarie - listed as a World Heritage Area since 1997 - was a national issue. "The federal government should stop penny-pinching and loosen its purse-strings," she was quoted as saying yesterday.

While politicians pass the buck, scientists are appalled by the delay. "It's urgent that they decide soon or the situation will be put back another year," Ms Kirkwood of WWF Australia said. "A lot of scientists are outraged by the damage being done. There's a possibility that it could be irreversible because the vegetation down there is very fragile and slow-growing."

Jenny Scott, from the University of Tasmania's school of geography and environmental studies, told ABC radio recently that the island's coastal slopes had been "trashed ... literally trashed, it's a horrible sight".

Dr Scott also said the small number of eco-tourists who visited Macquarie each year were horrified by what they saw. "It's a shocking situation, the tourists are really distressed," she said. If the baiting did not go ahead, "it's loss of biodiversity, it's loss of World Heritage values, it's loss of critical habitat, it's just an international disgrace," she added.

The island provides a critical habitat for the grey-headed albatross and wandering albatross. It is used by about 80 breeding pairs of grey-headed albatrosses. Blue petrels are now so severely threatened on Macquarie that they can breed only on offshore rock stacks.

The island, which has no trees or shrubs but is home to scores of mosses, lichens and algae, is a popular stop for Antarctic cruise operators, with its four species of seal and 850,000 pairs of royal penguins. Penguins make up 90 per cent of the island's total birdlife.

Apart from occasional tourists, the only humans on Macquarie are the 50 or so scientists and support personnel who staff the research base. Most of them are there only during the summer months.

Andreas Glanznig, WWF Australia's biodiversity programme leader, told local media that some species could be forced off the island if the culling did not happen soon. "This should have been sorted out a year ago," he said. "There's a significant risk to some species. Given that there's been an explosion in rabbit numbers since the 1990s, this could very much damage the long-term viability of the breeding population."