Grey squirrel invaders

The park’s grey squirrels are invasive aliens, brought here during the 19th Century, when the possession of rare and exotic species of plants and animals was the height of fashion. Grey squirrels, native to eastern North America, were first released into the wild in Britain, at Henbury Park, in Cheshire, in 1876.

They have thrived for a variety of reasons. Our winters are less extreme; grey squirrels rarely forage for food when the temperature is below freezing. Our native habitat of mixed woodland provides them with an ideal diet; they are opportunistic feeders whose basic diet is mast (the seeds of trees) but they need a range of other foods, from birds’ eggs and nestlings to fungi, to stay healthy.

There are fewer predators here. In their native America they are the prey of snakes, skunks, coyotes and wildcats; their most significant native predators, though, are birds of prey: owls and hawks. Here, foxes, domestic cats, and another invasive American alien, the mink, are their main predators. Our birds of prey take very few.

The grey squirrel not only out-competed our native red squirrel but brought with it  disease. Grey squirrels rarely die of the parapoxvirus; they evolved with it and over generations developed immunity but reds have no immunity at all and almost invariably die when they are infected.

Red squirrel habitat in England and Wales, was soon overtaken; red squirrels now only survive in places where the grey squirrel either has not reached or can be easily eliminated.

You would think that, by now, we would have learned of the dangers of transporting plant and animal species around the world, out of the environments in which they evolved and into environments where their success is momentarily applauded but soon becomes a problem.

Himalayan balsam, a real rarity fifty years ago, precious garden exotica fifty years before that, has invaded nearly all our waterways; in the park we set watch for it. Harlequin ladybirds, introduced into Europe in the 1980s as eco-friendly pest control, are out-competing our native species and were spotted in the park in their thousands last winter. Signal crayfish, an escape from commercial fisheries that introduced it in the 1970s to exploit a profitable European market, is a voracious predator, working its way through our native freshwater species.

Our squirrels are as cute as they come but every time we point and admire, we should remember the dangers of introducing alien species.

Red squirrel by Dave_S (CC2.0)

Header picture by DKG.

Other invasive species:

bluebells   Ladybirds    Celandine(Lesser)_2006_04_29_GlassonDock_030

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