AS ISLANDS go, you would be hard pressed to find one more remote. Deep in the southern Atlantic Ocean sits South Georgia, a haven for wildlife in the midst of ferocious seas. Over 30 million birds of 31 species breed here and a further 50 species have been spotted. It is home to grey-headed albatrosses, northern giant petrels, white chinned petrels, Antarctic prions, half of the entire population of macaroni penguins and most of the planet鈥檚 population of the South Georgia blue-eyed shag.
But it is not as idyllic as it sounds. Under the surface lurks a menace that is slowly ripping the ecosystem apart: rats.
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The rodents were stowaways on sealing and whaling ships that visited the island until the mid-20th century. When the hunters stopped coming, the rats were left to their own devices along with a small population of reindeer that had been brought for food and now roam wild. Without natural predators, the rat population has swollen to many million, eating their way through tens of millions of ground-nesting birds鈥� eggs and chicks in the process. As a result, the island鈥檚 endemic wildlife is under threat, and its only songbird, the South Georgia pipit, is on the brink of extinction.
Now the is going to fight back. In what will be the largest mass extermination ever attempted, the SGHT plans to poison every rat on the island. 鈥淭he difference between success and failure is the survival of two rats on the entire island,鈥� says Tony Martin, project manager of the SGHT Habitat Restoration Programme. 鈥淲e don鈥檛 have to get rid of most or even 99.9 per cent of the rats 鈥� we have to eradicate 100 per cent.鈥�
Absolute eradication is the only option because rats breed rapidly. They can live for around two years, achieve sexual maturity at two months old and are able to produce seven litters of 8 to 10 offspring a year. Female rats reach menopause at around 18 months. Even in the harsh climate of South Georgia, a sexually mature female is likely to have around four litters a year. If just one couple survive, it will only take a few years before the island is overrun again (see diagram).
SGHT鈥檚 eradication plan, costing 拢7 million, has been put before the UK government, which administers South Georgia as an overseas territory. If it gets the stamp of approval, the first wave of attack should begin in February 2011. But what chance does the scheme have of working? And what might go wrong?
More than 300 islands worldwide have successfully removed their rodent pests. While that is an encouraging sign, it is worth bearing in mind that South Georgia is a very different proposition. Stretching to a length of 120 kilometres, it is about eight times as big as the largest island that has been tackled so far. Its isolated location means the operation will be a logistical nightmare. 鈥淕etting a ship, helicopter, pilots, staff, bait and equipment there is not straightforward or cheap,鈥� says Keith Springer, head of the Macquarie Island Pest Eradication Project in Tasmania, Australia, who has been advising the SGHT on its rat removal plan. 鈥淭he weather will also place constraints on spreading the bait by helicopter.鈥� On top of this, South Georgia is uninhabited, covered in glaciers (see map), and heavy snowfall in the winter would conceal the poison.
The presence of glaciers is a double-edged sword for the project. They act as natural barriers that confine rats to specific areas, which 鈥渕eans the job can be tackled in bite-sized chunks, each of which has a higher chance of being successful, with minimal risk of re-invasion between operational seasons鈥�, says Springer. 鈥淏ut the clock is ticking as the glaciers are retreating.鈥� As they melt, rats will be able to invade the entire island, making their eradication impossible 鈥� it is unfeasible to treat such a large area in one hit. The sooner the rat catchers get to work, the better.
So how will the rats be killed? The idea is to liberally sprinkle toxic cereal pellets over the rat-infested regions from a helicopter. The poison of choice is brodifacoum, an anticoagulant that finds its way to the liver where it interferes with the synthesis of clotting factors that help stop bleeding. Symptoms kick in a few days after consumption, and victims die slowly of internal haemorrhaging.
Bad death
鈥淚t is a nasty way for the rats to die,鈥� admits Martin. 鈥淏ut these rats are eating their way through the young of millions of birds, who also die in a nasty way 鈥� the rats eat them alive. I have no doubt it鈥檚 justifiable to kill invasive predators that are wiping out native wildlife.鈥�
Brodifacoum has been chosen as it is extremely toxic 鈥� a few micrograms is all it takes to kill a rat. Yet its use is controversial. If brodifacoum accumulates in the food chain, it could , including the very birds the programme is trying to save.
The main concern is that some birds, particularly the pintail duck, may eat the baited cereal, says Martin. This fear is justified: when rats were eradicated from Motuihe Island in New Zealand, 60 per cent of the waterfowl also died.
What could make it worse this time round is that the proposed time of baiting 鈥� February 鈥� is when the ducks are at maximum stress because they are moulting and cannot fly, so may be more likely to eat the cereal bait. SGHT is hopeful this won鈥檛 happen. In tests, they found that captive ducks avoided eating the pellets, perhaps because of their size and blue colour. To be on the safe side, they plan to monitor duck populations by attaching radio transmitters to a random sample of 15 birds from across the island. 鈥淚f we find the transmitters attached to bones then things will be worse than anticipated and we would need to rethink,鈥� says Martin.
A secondary way of ingesting the poison would be to scavenge the carcasses of poisoned rats, says Martin. Fortunately for the birds, most rats are likely to die in their burrows, out of harm鈥檚 way, because the poison makes them sensitive to light.
Pete McClelland of New Zealand鈥檚 Department of Conservation is optimistic that the impact will be low. 鈥淕iven that not all the rats on South Georgia will be eradicated at once, the birds will not all be put at risk at the same time. I would be confident that any species in any 鈥榖lock鈥� will soon recover as birds from the neighbouring blocks move in. This is one major advantage South Georgia has over smaller islands where whole populations and species are put at risk at once.鈥� This is what went wrong in the rat eradication programme on Fr茅gate Island in the Seychelles in 1996, which was halted on the first attempt when the endangered magpie robin suffered secondary poisoning (see 鈥淥ther eradication projects鈥�).
Despite McClelland鈥檚 optimism, Martin concedes there is a chance the programme on South Georgia could fail. 鈥淲e could fail for several reasons: by using inadequate bait, or because our landscape is more complex than any island treated up until now, or simply because South Georgia is so large,鈥� he says. Should any individual areas be left with any rats after baiting, they would have to be baited again, and more thoroughly, before the glaciers melt. 鈥淚f we make a mistake in one area, then by definition we will have failed.鈥�
No one knows how long South Georgia has got until its glaciers disappear. But what is clear is that if they do melt before the rats have been eliminated, the entire island will be a rat free-for-all, and the days of native birdlife will be over. Crucially, for the mission to succeed, the South Georgia government will have to agree to kill, or remove, all of the 2000 reindeer living on the Island. This is because they could eat the bait, killing them and reducing the amount of poison available for the rats. 鈥淚 believe it will be impossible to get rid of the rats if the reindeer were to remain,鈥� says Martin. To get round this problem, the reindeer will have to either be shipped out to the Falkland Islands, or shot.
If the programme wipes out all the rats, the population of pipits should be the first to recover, says Martin. 鈥淭hey are the equivalent of a canary in a coal mine. It would be astonishing if they didn鈥檛 come back to many times their existing numbers,鈥� he says. Not all species will recover so quickly: longer-lived bird species could take decades. Even so, the consensus is that once the rats are removed, millions of birds will eventually return to South Georgia, making it a bird鈥檚 paradise once more.
鈥淚f the programme wipes out all the rats, the population of pipits should be the first to recover鈥�
