Skuas point to heritage island's recovery

·2-min read

Seven years ago, world heritage-listed Macquarie Island in the sub-Antarctic was declared free of invasive species.

And recent analysis of a top-order predator paints a promising picture of the rugged outcrop's recovery.

Nest numbers of brown skua, a large seabird, fell by almost half when their adopted prey of feral rodents and rabbits disappeared during eradication efforts.

Nests have since recovered to about 75 per cent of pre-eradication levels, according to a study led by Institute for Marine and Antarctic Studies PhD candidate Toby Travers.

"Skuas are at the top of the food chain, so their recovery depends on the recovery of all the other components of the island's ecosystem," he told AAP.

"Because of this, top-order predators are a great indicator of overall ecosystem health.

"The skua recovery we observed in the last few years of our study is a promising sign that the seabird prey of skuas, and vegetation that supports them, is recovering well from the impacts of invasive species."

Macquarie Island, roughly halfway between Tasmania and Antarctica, is the only island in the world composed entirely of oceanic crust.

Rabbit numbers reached an estimated 130,000 in the 1970s after they were brought there a century earlier as food for sealing and whaling crews.

Their population dropped to 10,000 in the early 1980s when the disease myxomatosis was introduced but had again increased to more than 100,000 by 2006.

A $24 million extensive aerial baiting and trapping plan targeting mice, ship rats and rabbits was launched in 2007, culminating in success in 2014.

The skua research, published in the Journal of Applied Ecology, looked at the flow-on effects of feral eradication programs.

In some areas, secondary poisoning of skuas that scavenged dead rabbit carcasses after baiting had a significant impact on their numbers.

"Macquarie Island at the time was the largest multi-species eradication conducted anywhere," Mr Travers said.

"It became a real key example of what can be achieved. Because it was at the forefront there's a lot of lessons to be learned."

Supplementary feeding and breeding programs could be used in future eradications if severe prey deficits or slow recovery of native prey are predicted, study authors say.

About 3.5 million seabirds arrive on Macquarie Island yearly to breed and moult.

Mr Travers says parts of the island recovered quite quickly after feral animals were wiped out.

"Now you've got fully vegetated slopes where once you had heavily grazed slopes," he said.

"Where you were getting land slips and erosion from so much rabbit grazing and burrows, these slopes have completely stabilised.

"That provides habitat for burrowing seabirds to return."

The island, which was awarded world heritage status in 1997, is also home to an Australian Antarctic Division research station.

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