Scottish island wins battle against predatory rats

Severin Carrell, The Guardian 7 Jun 08;

It has taken almost half a million pounds, more than 4,200 hand-laid traps and the expertise of rodent catchers flown in from New Zealand, but today the small Hebridean island of Canna will be officially declared rat-free.

The low-lying, crescent-shaped island lying just to the south of Skye had previously been famous for its more welcome settlers: Iron Age farmers, early Christian monks and the influential Gaelic folklorists John Lorne Campbell and Margaret Fay Shaw.

But when the voracious brown rat, Rattus norvegicus, took hold, it threatened to extinguish the island's precious sea bird colonies and kill off one of its smallest and rarest inhabitants – the Canna mouse. Experts estimated that up to 10,000 rats had spread across the 4.5-mile-long island.

In 2005 the National Trust for Scotland (NTS), the island's owners, set up the most ambitious rat eradication programme attempted on an island in the UK to date, after it emerged that the rats were decimating Canna's Manx shearwater population. After 3,000 birds were counted in 1972, it was virtually extinct in 2004. Other ground-nesting birds – shags and razorbills – were also being taken in large numbers.

With the Royal Zoological Society of Scotland, the trust also evacuated 150 Canna mice – a distinct sub-species of mouse – to prevent them being eaten by rats or dying from the poison, rehousing them as an insurance policy in Edinburgh zoo and a wildlife park in the Highlands.

The last rat was seen in early 2006, and the official end of the eradication programme will be marked by the Scottish environment minister Michael Russell in a small ceremony on Canna today. "Rats, while being fairly innocuous creatures in their natural environment, can have a devastating impact in a fragile ecosystem such as that of Canna," he said yesterday.

Despite its small size, specially designed baits had to be laid every 50 to 90m across a carefully calculated grid across the entire island's steeply slopped and craggy interior. Unlike on uninhabited islands where baits can be dropped by air, each was laid by hand; Canna now has a population over 20 people, after the trust introduced two new families to the island after a worldwide advertising campaign for new settlers.

The poison was fixed in wax blocks, carefully placed in plastic tunnels, and searches for dead rats made every week to ensure the corpses and poisons were not eaten by Canna's sea eagle population. The project was immediately effective: every rat was killed in the first year of the bait-laying.

Richard Luxmore, the trust's senior nature conservation adviser, said they needed two more years to guarantee that no rats survived. It was essential, he said, for the eradication to be swift and ruthless. "It you tackle rats in a half-hearted way or without adequate resources, you're not getting 100% eradication and 99% eradication is 100% failure, if you leave on only pregnant rat behind. That's absolutely clear," he said.

Several Scottish islands and isolated rural areas are threatened by invasive predators, including mink. A similar rat eradication project is being considered on the neighbouring island of Rum, a nature reserve six times the size of Canna. This summer, its owners, Scottish Natural Heritage, will begin rat control trials around its Manx shearwater colonies and rat surveys to assess the scale of the problem.

Islanders on the Orkney island of Egilsay have approached Wildlife Management International, the New Zealand-based firm used by the NTS on Canna, to advise them on tackling their rats damaging their corncrake nests. Elizabeth Bell, the project leader on Canna, said the problem was growing worldwide.

Climate change meant rats and mice – an invasive species on Gough island now eating its way through the southern Atlantic island's albatross, petrel and shearwater chicks - are surviving winters in much larger numbers and colonising higher ground than before, she said.

"They can be a serious problem for an island's endemic species because these ecosystems have often developed without mammals at all. So none of these animals are used to dealing with predators, and being so delicate, the balance can be unsettled very easily" she said.

There is one sad footnote for Canna: the 150 mice which were taken off the island to ensure their survival were allowed to die in captivity. A small residual population had survived the rats on Canna, and the zoo feared their captive mice could take home disease – a risk too great to take.