The world's biggest rat-hunt is being mounted to rid a South Atlantic island of the rodents eating their way through millions of endangered seabirds.
The first phase of the eradication programme will start next February on South Georgia in the hope of returning the island to its previous state as a globally important breeding site for seabirds. Over the centuriers, the rats arrived on South Georgia off whaling ships and sealers. Since then, their population has grown to several million, feeding on the eggs and live chicks of the ground-nesting birds that breed on the island, nearly 950 miles east of the Falklands.
Invasive rodents have been successfully removed from more than 300 islands worldwide but the South Georgia operation is by far the biggest and most ambitious, involving two helicopters spreading poisoned bait over every square metre of ice-free land on the 170km-long island, with the precision of GPS navigation. But the hunters have to make sure every rat is killed. They reproduce so swiftly that leaving even one pair alive means 15,500 more within a year.
"It's easily the largest rat-eradication effort in the world, at least seven times bigger than anything anyone has tackled before," said Tony Martin, professor of animal conservation at the University of Dundee who is in charge of the multi-million pound project. "My job is to nail every last rat and to make sure we don't poison too many non-target species. It's no good if we end up missing the last couple of rats because they reproduce so quickly and within a few years their numbers will be back up to present levels."
South Georgia, a British-dependent territory, is home to about 30 million birds. Some 31 species breed on the island, including grey-headed albatrosses, northern giant petrels, white-chinned petrels, Antarctic prions, half of the world's entire population of macaroni penguins and the South Georgia blue-eyed shag. A further 50 species are known to visit the island.
The South Georgia pipit is close to extinction because of rats. Ground-nesting birds have no natural mammalian predators and so their chicks and eggs are defenceless against the brown rat, Rattus norvegicus, which has now spread across most ice-free areas of the part-glaciated island.
The only barriers to the rat on South Georgia are the ice tongues of the glaciers which protrude out to sea and isolate some bird colonies from the rats. But these glaciers are retreating rapidly inland and as soon as they shrink away from a beach rats are able to find new bird colonies to attack, Professor Martin said.
"The glaciers are retreating at a hell of a pace, sometimes visibly changing year on year. When the snout of a glacier melts away from a beach the rats just walk around it. It's happening very quickly which is why we need to work fast."
Pellets of a cereal bait loaded with an anti-coagulant poison will be spread across all rat-infested parts of the island by helicopter because it is crucial for every rat to be exposed to at least two pellets. A key advantage of the bait is that it can be detected by rats up to 300m away because they are so attracted by its smell.
The poison, which quickly degrades and does not dissolve in water, works by getting into the liver and causing internal bleeding. It is does not kill immediately and makes the rats sensitive to light so it is hoped they will retreat to their burrows to die underground rather than dying out in the open so that their poisoned corpses are eaten by the seabirds. The island's 2,000 reindeer will probably be moved to the Falklands while the rat-hunt continues.
"I'm not going to pretend that it's a nice way to die but it's the way rats are killed worldwide and if you leave them the seabird chicks will suffer an even nastier death of being eaten alive," Professor Martin said. "I have no qualms about killing invasive predators that wipe out native wildlife."
The first phase of the project will begin at the start of the southern hemisphere autumn when many migratory birds have left the island and the rats have stop breeding for the winter. After four seasons, the scientists hope to have completed the eradication. They will then comb the island with two specially trained dogs that can sniff out the odour of any living rats still left on the island.
Once the rats have gone, it is hoped that seabird numbers will increase to their previous levels, and threatened species such as the pipits should be the first to recover. "This is perhaps the most exciting project of my career because the legacy of this will last for millennia," Professor Martin added.