Areas with growing pine marten populations have seen grey squirrel numbers fall as they provide easy prey for the predators – unlike native reds, a new study shows

The invasion of grey squirrels that has decimated native reds across the UK is reversed when pine martens prowl the woods, new research has shown. Unlike reds, grey squirrels appear to be easy meat for the predator.

Pine marten populations have also been drastically reduced in the past. But where they are recovering, they send grey squirrel numbers plummeting while reds thrive, according to scientists.

The spread of pine martens from their Scottish stronghold into England is the best long-term solution to controlling grey squirrels, said Emma Sheehy at the University of Aberdeen, who led the work: “It should be all that you need. But it may be some time getting to that place.” As a result, she said it would be foolish to stop current control programmes now.

There were once 3.5 million red squirrels across the UK, but the introduction of “ornamental” grey squirrels from the US in the 1870s has left just 250,000 reds, mostly in Scotland. The larger greys carry a pox disease which is deadly to reds, and also outcompete their smaller cousins.

Pine martens have also suffered, having been killed for their fur and as predators of poultry and game birds, and are essentially extinct in England. They only gained legal protection in the 1980s but there are now a few thousand in Scotland.

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The new research, published in the Proceedings of the Royal Society B, examined in detail the relationships between red squirrels, greys and pine martens in the Borders, central belt and Highlands of Scotland. The team placed more than 200 feeders in woods containing hazelnuts – a squirrel favourite – and peanuts, which pine martens enjoy. A sticky patch on the trap collected animal hairs, and the DNA was analysed to identify individual animals.

Earlier work in Ireland had suggested pine martens might suppress grey squirrels, but the new results from Scotland were “unequivocal”, according to the researchers. The more pine martens there were, the lower the number of greys and the higher the number of reds.

The scientists found that in places where pine martens were more common, red squirrels were much less likely to visit the nut feeders, but greys were not. “It suggests greys are totally naive to the risks of pine martens as a predator,” said Sheehy. “In their native range, they don’t have similar predators and that leaves them much more susceptible here.”

“We are very excited to finally have some research which clearly shows that the recovery of the pine marten is having clear benefits for native red squirrels in Scotland,” said Mel Tonkin, at the Saving Scotland’s Red Squirrel project.

Pine martens have recently been detected crossing the border into England in the Kielder forest. “However, it is important that people understand that it may take several decades for pine martens to recover to densities that will allow us to put away our traps altogether. If we stop the work we are doing now, there won’t be red squirrels left for the pine martens to save,” said Tonkin.

Some Scottish pine martens are being moved to Wales and a project in the Forest of Dean is being considered, but Sheehy said a large scale reintroduction was not feasible, due to cost and scarce suitable habitat.

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Adrian Vass, at the Red Squirrel Survival Trust, welcomed the new pine marten evidence and said it coincided with the development of an oral contraceptive that prevents grey squirrels from breeding: “This innovation offers the potential for grey squirrel management in areas without predatory species.”

Grey squirrels also damage trees such as oak, beech and sycamore by stripping their bark. Jonathan Spencer, at Forest Enterprise and Forestry Commission England said: “The insights emerging from this elegant study are very welcome indeed. The damage inflicted by grey squirrels on native trees runs into millions of pounds each year.”

Sheehy said the new research also has wider implications for protecting biodiversity: “It is really encouraging news in a global context, as it supports the concept that ecosystems with native predators can offer greater resistance to invasive species.”