The pine marten was once the second most common carnivore in Britain but woodland clearance coupled with a drive to eradicate the predators by Victorian gamekeepers pushed the little mammal to just a few remote areas of Scotland.
But a new study suggests that reintroducing the creatures could provide a natural way of eradicating grey squirrels, and allow red squirrels to re-establish their populations.
Red squirrels are small and light enough to scamper to the ends of branches out of reach of hungry pine martens, but the heavier greys are slower and spend more time foraging on the woodland floor, where they are easy prey.
A new study by the University of Aberdeen found that in areas of Scotland where pine marten are present, red squirrels were found in far greater numbers.
"Our study has confirmed that exposure to pine martens has a strong negative effect on grey squirrel populations, whereas the opposite effect was observed in red squirrel populations who actually benefitted from exposure to martens,” said Emma Sheehy, of Aberdeen.
"Our evidence that, in addition to their intrinsic value, pine martens provide an ecosystem service by suppressing invasive grey squirrel populations is good news for both red squirrel conservation efforts and the timber growing industry, due to the detrimental impact of the invasive grey squirrel on both."
Red squirrels have been in serious decline since the North American native grey squirrel was introduced as an ornamental species in the 1870s. Not only does the grey outcompete the red, it carries a deadly pox.
The UK population of reds has dropped from around 3.5 million to between 120,000 to 160,000 individuals and is thought to be as low as 15,000 in England.
For the new three study, the team deployed feeders at sites across Scotland which had sticky tabs attached to capture hair from the mammals which visited. They then used DNA analysis to work out which animals were in the areas.
They found that when pine martens were present, grey squirrel numbers fell and red squirrel numbers rose suggesting the larger mammals were keeping the non-native species in check.
Kenny Kortland, a species ecologist for Forest Enterprise Scotland, said: "The findings of this research are extremely encouraging.
“It seems we have a very welcome ally in our efforts to protect red squirrel populations on the national forest estate.
“The research demonstrates that the return of native predators can have beneficial impacts for other native species."
The Forestry Commission is currently deciding whether to allow the reintroduction of pine martens into the Forest of Dean in Gloucestershire. So far some 40 pine martens have been released from Scotland into Wales, and are being monitored with tracking devices.
“The new insights emerging from this elegant study are very exciting indeed," said a spokesman for the Forestry Commission.
"Landscapes supporting pine martens could reduce the numbers of grey squirrels to the point where the damage they cause is significantly reduced or even stops."
“We are exploring the feasibility of reintroducing pine martens to the Forest of Dean working with the Gloucestershire Wildlife Trust, Vincent Wildlife Trust and others partners."
Christopher Sutherland of the University of Massachusetts, who carried out his doctoral work at Aberdeen added: “Our analysis suggests that we can achieve conservation objectives twice over by allowing a native species, the pine marten, to spread naturally while conserving our precious red squirrel.
"We've potentially found an answer that doesn't require the high cost of eradication."
The research was published in the Proceedings of the Royal Society B.