Researchers from the University of Aberdeen, Waterford Institute of Technology in Ireland and the University of Massachusetts Amherst in the USA used DNA forensics to test the hypothesis that pine martens are suppressing grey squirrel populations in Scotland.
Over a three-year period they looked at three areas - the Scottish Borders, where pine martens have recently begun to recolonise; Central Scotland, which has a more established pine marten population; and the Highlands, where pine martens coexist with red squirrels but where greys remain absent.
In each, "multi-species feeders" with sticky patches to collect hair samples were deployed, supplemented by trail cameras.
DNA forensics were then used to identify individual pine martens and so identify their ranges, explained research co-leader Dr Emma Sheehy of the University of Aberdeen.
"From this we were able to quantify how much red and grey squirrels were exposed to these pine martens, and how this affected their distribution.
"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."
She suggested that reds have evolved to be cautious of pine martens whereas this trait is absent in the greys, adding: "Our evidence that pine martens suppress 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."
She said this role "will presumably spread as the pine marten’s range expands southwards through Scotland and into the North of England", but added this "is likely to be a slow and very gradual process".
Funded by the Irish Research Council, Forestry Commission Scotland and the EU, the research is published in Proceedings of the Royal Society B.
Sheehy earlier conducted research in central Ireland which appeared to show that an increase in the pine marten population was behind the "unusually low" grey squirrel numbers there.
Pine martens have been re-introduced in mid-Wales and there have recently been sporadic sightings of them in England following an absence of over a century.