Earlier this year the UK government finally gave up on trying to control the American grey squirrel in the UK. Officially gave up, that is; they had given up in practice decades earlier. The sign that the towel had been well and truly thrown in was the removal from the statute book of the Grey Squirrels (Prohibition of Importation and Keeping) Order, which, since 1937, had made it an offense not to report the presence of grey squirrels on your land. With grey squirrels now numbered in millions (they're eating the ripening plums on the tree in my garden as I write this) and the native red confined to a few northern and island outposts, you can see why the government admitted that they hadn't taken many calls under the 1937 act in recent years.
Which prompts the question: Why has the grey squirrel been such a success in Britain? Some of the reasons are well known: It's bigger than the red, lives at higher densities, is better adapted to deciduous woodland, is less wary of humans, and carries a virus fatal to the red squirrel. For those who see alien species as problems in themselves, quite independent of any human alteration of the environment, that's enough. But new research from Ireland, from researchers at the National University of Ireland in Galway, and reported in the journal Biodiversity and Conservation, tells a different story.
One of the most striking -- but also easy to overlook -- things about grey squirrels is that they have no predators. The pine marten, easily the most arboreal of the native members of the weasel family, used to occur throughout Britain, but a combination of persecution and habitat destruction almost led to its extinction, and, although its decline has been halted, it's still more or less confined to northern Scotland, a region from which the grey squirrel is still largely absent.
In Ireland not only did the pine marten never decline as severely, but its recovery is more advanced. The new research shows that in the Irish midlands, where the pine marten is strongly recovering, the grey squirrel population has dramatically collapsed, to the extent that they are now rarely seen. Furthermore, the red squirrel has shown a strong recovery in exactly the same region. Pine martens will eat both squirrels, if they can catch them, but they only rarely succeed in catching the small, agile red. The larger, slower grey, however, makes up a significant part of the pine marten's diet where their ranges overlap (as it does for the American marten in North America).
A final piece of evidence is that the grey squirrel in Ireland has never penetrated west of the river Shannon, into a region where pine martens are common and always have been, which makes the authors of the report (and me) wonder if grey squirrels would ever have made much progress in England if they had been faced from the start by a healthy pine marten population.
It's traditional among a certain type of ecologist/conservationist (the majority, in fact), when faced with an expanding population of an alien species, to look no further than the alien itself. If X is causing a problem, then the solution -- obviously -- must be to trap, shoot or poison X. But it pays to remember that in the modern world, most alien species are spreading into ecosystems that have been radically transformed by human activity, and it's that transformation that often provides a golden opportunity for alien species. In other words, Cassius had it right in Julius Caesar: "The fault, dear Brutus, is not in our stars, but in ourselves."