In reply to Timmd:
I read the whole paper which Monbiot bases his premise re pine martins. I also own and read the Duchas (Irish Heritage Service) book on Irish mammals.
The Irish mammal book states that both grey and red have regular population and distribution range changes which cannot be linked to the pine martin as it does not have as wide a distribution as either the grey or red squirrel.
However, in Ireland the pine martin hunts mainly on the ground. Amongst the diet it states that the pine martin 'occasionally eats squirrels', although which of the two species isn't mentioned. Although it is widely believed that the pine martin find it easier to catch grey squirrels I've not found evidence based studies to back this up. The fact that they eat greys (16% of their diet in the USA) does not mean they eat the same percentage here or in Ireland.
The author of the quoted report seems to be basing their conclusions on what they've red elsewhere, and some of this goes back to the 1980s. There is little if any direct evidence that pine martins have any direct impact on grey squirrels. - not that I have any problems with the spreading of pine martins mind you!!