Squirrels: So bushy-tailed, so ubiquitous—so deadly.

We don't normally think of squirrels as killers, but North America's eastern gray squirrels (Sciurus carolinensis) have actually been called one of the worst invasive species on the planet. When introduced into other ecosystems, the larger and more aggressive grays tend to out-compete native squirrels for food and intimidate them enough that they stop breeding. Even worse, the invaders also carry (and are immune to) a virus called squirrel parapoxvirus, which can be deadly to other squirrel species. The pox causes lesions and bleeding from the eyes and mouth; infected squirrels die in as few as four days.

Gray squirrels have already put quite the hurt on European red squirrels (S. vulgaris) in England and Ireland, where five million grays now dominate the landscape. (They were first imported to the British Isles in the late 19th century as a garden novelty.) Only 120,000 to 140,000 reds remain in the U.K., three quarters of which are in Scotland, where pox did not show up until 2005. Some scientists estimate that red squirrels could be extinct in the U.K. in as few as 20 years.

I've been following the gray invasion in the U.K. for several years, but now we have word that they are also wiping out red squirrels in Italy, where a 1,150-square-kilometer region in the northern part of the country is now devoid of native squirrels. According to scientists from the universities of Turin, Genoa and Insubria, the gray squirrel is now poised to spread farther through Italy and possibly all the way to France.

The Italian invasion dates to 1948, when a U.S. ambassador presented four of the cuddly critters as a gift. They escaped and have been expanding their territory exponentially ever since.

Invading grays pose a broader threat beyond the extinction of native reds. Gray squirrels are generalist eaters and consume nine times more food than the much smaller reds, which rely on a more specialized diet. Natasha Collings, project coordinator for the Cornwall Red Squirrel Project, told the Guardian that gray squirrels are doing millions of dollars of damage to commercial forests annually. They also hurt other woodland areas, and even eat frogs and bird eggs. This doesn't happen in North America, where the squirrels have coevolved with a broader selection of edible nuts and other vegetation, although grays here will eat through just about anything, including wood walls and electrical wires. (Just ask my neighbor about his attic.)

Can the invasion be stopped? There are efforts to cull gray squirrels in both the U.K. and Italy, but the methods are controversial and not all that effective to date. Unless red squirrels suddenly develop immunity to the pox (or someone comes up with a vaccine), their days will grow shorter and shorter.

Photos: Red squirrel photographed in Italy by Walter Kun via Flickr. Gray squirrel photographed at the National Botanical Gardens in Ireland by William Murphy via Flickr. Both used under Creative Commons license