Stoats are a much greater threat to island birdlife than previously thought.
Steve Everett; CC
Introduced predators – stoats, feral cats and rats – are second only to habitat destruction as the scourge of New Zealand’s native bird species. Thanks to the efforts of generations of wildlife conservators, predator-free islands such as Kapiti, Codfish and Little Barrier have allowed species like the kakapo, saddleback and stitchbird to recover and pull back from the brink of extinction
But how safe are our “predator-free” islands from recolonisation? Not very, according to new research by University of Auckland PhD student Andrew Veale, who has been studying stoat genetics while based at Landcare Research.
Previous wisdom was that stoats could swim up to 1200m. Veale blew this figure out of the water last year after Department of Conservation workers found a stoat in a trap on Rangitoto Island, 4km from Takapuna Beach, almost a year after a mammal eradication campaign.
Using archived hair samples from stoats from the island, along with DNA samples from mainland stoats, Veale determined the trapped stoat was definitely from the mainland. Without his genetic analysis, scientists would have assumed the stoat was an island stoat that had evaded the trapping programme.
Then late last year, a Department of Conservation contractor saw a stoat on Kapiti Island, a bird sanctuary 5.2km from the mainland. An initial search collected scat and a dead kakariki that were sent to Veale for analysis. “We got stoat DNA off both the scat and the kakariki, showing the stoat was definitely there and was definitely killing native birds.”
A $30,000 trapping and monitoring campaign finally worked and the stoat was trapped in February this year, although it had been there for months and had killed an unknown number of birds.
“Why would they want to eat a mouldy old egg in a trap when they’ve got a saddleback to eat?” says Veale.
Now Veale is using genetics to try to tie the stoat caught on Kapiti to the scat samples and the dead kakariki. “Hopefully it was just the one male stoat that swam out there.”
For some islands, such as Maud Island in the Marlborough Sounds, recognition of the risk of reinvasion has led to mainland trapping programmes on the coast near the island. The focus of Veale’s work is now on Secretary and Resolution islands in Fiordland, about 1000m and 560m from the mainland respectively.
After several years of trapping on the islands, stoats are still being caught. Veale is using genetics to help the Department of Conservation determine where these stoats are from.
Steve Everett; CC
“It’s similar to paternity testing or forensics,” says Veale, who is gathering enough genetic data about the island and mainland populations to be able to determine whether trapped stoats are mainland-born or island-born, and so determine the invasion rate. If most of the trapped stoats are mainland ones that have swum to the islands, then mainland trapping needs to increase, but if most of the trapped stoats turn out to be island ones that have escaped previous trapping campaigns, trapping rates on the island need to be increased.
Stoats were intentionally introduced by Europeans in the 1880s to prey on the already out-of-control introduced rabbit population. With such great food sources on the mainland – not just rabbits and mice, but native birds and their eggs – what would inspire a stoat to jump in the sea and swim several kilometres?
“Maybe they’re just dispersing,” says Mick Clout, Veale’s PhD supervisor and director of the University of Auckland’s Centre for Biodiversity and Biosecurity. “Dispersing is just what animals do – in particular males. They are looking for a new place and a mate.
“Whether it’s a deliberate dispersal to an island they can see on the horizon, or they just get caught in the tide, we really don’t know.”
Whichever way, new information from Veale’s genetic studies means as long as stoats live on the New Zealand mainland, our islands are vulnerable to stoat colonisation.