Invasive animals know no boundaries; they don't get stopped for a Passport check at the border. Therefore, they are a global problem. Not only do numerous species from countries across the oceans end up here in Pennsylvania, but species native to our country end up causing trouble overseas. We hear a lot about the foreign species that make life difficult for us here, but what is it like for those living with our “exotic” species?

It may come as a big surprise to hear that the same grey squirrels that inhabit Pennsylvania parks, casually munching on acorns, are wreaking havoc in Great Britain. Grey squirrels (Sciurus carolinensis) were deliberately imported into Great Britain in 1876 and there are now an estimated 2 million of them living in the British Isles. By some accounts they are one of the most commonly seen mammals in Great Britain. The Isles are not devoid of their own native squirrels; the red squirrel (Sciurus vulgaris), a smaller species roughly half as large as the greys, makes its home there. Once the larger grey squirrels established themselves, however, the red squirrels started to decline. One estimate puts the red squirrel’s population at just under 120,000 in some isolated pockets to the north in the U.K.

Red squirrels, native to Great Britain, are being negatively impacted by invasive grey squirrels from the U.S. (Photo: Wikimedia)

Grey squirrels out compete their red cousins for food and habitat and transmit the “Squirrel Pox” disease (a parapoxvirus). Grey squirrels are not killed by the pox, but for some reason the reds are. Already losing their homes and food to the grey squirrels, perhaps the pox is what puts them over the edge? All the negative impacts of grey squirrels combined have placed them on the International Union for Conservation of Nature's (IUCN) list of International Union for Conservation of Nature's 100 of the world's worst invasive alien species.

Another mammal from the U.S. that is considered invasive in the U.K. (as well as other European countries, Russia, parts of South America and Japan) is the American mink (Mustela vison). These animals were introduced into Great Britain in 1929 for fur farms but many either escaped on their own or were deliberately released after the decline of the fur industry. They have been able to colonize most of the U.K. waterways, threatening native species like the endangered water vole. Mink are predators that eat
the eggs and chicks of native waterfowl, fish and small mammals. Some of these animals had no natural predators before the arrival of mink, so they have not adapted to dealing with such attacks. The American mink is also in direct competition for food and territory with the European mink (Mustela lutreola), which is now limited to a few fragmented populations.
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Water voles, an endangered speices in the U.K., is being threatened by non-native american mink. (Photo: Wikimedia)