When you introduce a new species a frequent problem is that they leave behind in their native land all the predators and problems that would have kept them in balance with others in their new home.
Now that the pine martens are making a comeback after many years of being considered vermin by gamekeepers, it seels that grey squirrels are meeting their nemesis. It will be decades, though, before the pine martens spread into all areas, so we should not stop discouraging greys and helping reds all we can.
Greys are cute, but reds are native and cuter - and under threat from the more successful usurpers, who eat a wider selection of foods and can have several litters a year, with more kits in each litter. Greys also carry squirrel pox, which is fatal to reds.