The pine marten, a carnivorous relative of the weasel, was regarded as a pest for centuries and came close to extinction. But the population is growing, and researchers have noticed that when pine martens move into an area inhabited by grey squirrels, their numbers drop, The Scotsman reported.

The grey squirrels, a species introduced from North America, have replaced the native reds in much of Britain. The red squirrel is now an endangered species.

Rob Coope, a biodiversity manager for the Forestry Commission, said grey squirrels are ground feeders, while red squirrels spend much of their time high in the trees.

"Grey squirrels are a big threat to red squirrels, but what we have noticed is that in areas where pine martens are active, grey numbers go down and they almost disappear," he said. "Their march up the country has been pretty relentless, but in these areas it had been halted."

Naturalists in Ireland have observed the same relationship between pine martens and grey squirrels.