The aggressive North American gray (or grey, when it gets to England) squirrel is threatening to displace the English red squirrel. Immortalized in Beatrix Potter’s The Tale of Squirrel Nutkin and much beloved by English nature lovers, the red is the island nation’s only native species of squirrel. But it is being driven to extinction by the invading gray’s combativeness and a squirrel pox it carries, against which the reds have no resistance. And so, with characteristic national pluck, a cross-section of English men and women — from lords to priests to artists to farmers — has risen up to turn back “the grey menace” and save the reds. Nutkin’s Last Stand is touching and often humorous. But species loss is no laughing matter, and the film rises to a haunting evocation of the stakes in the survival of a little red squirrel.