I stirred up some controversy by posting a photograph of a grey squirrel on a nature forum. Suppose we lay aside the emotion and look at the facts.
Greys do not directly harm red squirrels and are thus not in the same category as feral mink, unwisely released by people who mistakenly thought they were doing good. Among its several advantages, the grey is hardier and more efficient at storing fat through hard winters than the red, and is immune to the squirrel pox virus, which it may however carry.
But, like American Indians, the red squirrel can, over time, develop immunity to initially fatal imported diseases which decimated their numbers. (The date is not still 1870). The red will nevertheless not out-compete the grey, because it is simply less efficient in most habitats. It is however possible to take action to preserve reds in the areas where they remain dominant.
I have been advised that grey squirrels, being less agile than red, are more vulnerable to pine martens. They are therefore less dominant where martens are prevalent.
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