Pine martens were reduced to fragments of their British and Irish range by persecution and habitat loss, then began recovering under legal protection. Surveys in Ireland, later repeated in Scotland and Wales, found a consistent pattern: as marten occupancy rose, grey squirrel occupancy fell and red squirrels returned to woods they had been absent from for decades.
The explanation on offer is behavioural, not dietary. Red squirrels evolved alongside pine martens in Europe; grey squirrels, introduced from North America, did not. Greys are heavier, spend more time foraging on the ground and appear less responsive to marten cues, which makes them easier to catch. Behavioural work has found reds showing stronger avoidance responses to marten scent than greys do.
The effect is real but should not be oversold. It has been recorded across several regions, yet its strength varies with woodland type and with how much of the landscape is broadleaf, where grey squirrels do best. Whether marten recovery alone can reverse grey expansion in the large broadleaf woodlands of southern England, rather than only in more coniferous northern and Irish landscapes, has not been shown.