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Italy Faces Invasion of American Killer Squirrels

The views expressed are those of the author and are not necessarily those of Scientific American.


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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

John R. Platt About the Author: Twice a week, John Platt shines a light on endangered species from all over the globe, exploring not just why they are dying out but also what's being done to rescue them from oblivion. Follow on Twitter @johnrplatt.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. bigtruck797 8:00 pm 10/5/2012

    I am puzzled, the gray squirrels we have in Central Illinois are smaller than our reds, are our greys or our reds a different species or race than those discussed in the article? When I was a kid we had no greys,now greys outnumber the reds.

    Link to this
  2. 2. Bird/tree/dinosaur/etc. geek 10:34 pm 10/5/2012

    Yes, our red squirrels are of a different species (*Tamiascurius hudsonicus*) than European reds.

    Link to this
  3. 3. John R. Platt in reply to John R. Platt 8:52 am 10/6/2012

    Amazingly, there are nearly 300 squirrel species, including tree squirrels, ground squirrels and chipmunks. (I didn’t realize that chipmunks were squirrels until I researched this article.)

    Bigtruck notes that grays in their area are now more numerous — eastern gray squirrels have been moving further to the west in the U.S. and invading new territories in our own country, too.

    Link to this
  4. 4. Skeptuckian 5:39 pm 10/7/2012

    Are you sure you are talking about red squirrels? Fox squirrels are reddish (although more orange) and are bigger.

    Link to this
  5. 5. John R. Platt in reply to John R. Platt 10:00 am 10/8/2012

    I’m not sure who you’re responding to, Skeptuckian, but there are multiple species in the U.S., some of whose habitats naturally overlap.

    Link to this
  6. 6. evodevo 2:10 pm 10/8/2012

    @ bigtruck – American red squirrels – Tamiasciurus hudsonicus – are smaller than greys and ours in Ky are mostly found in pine forests in Eastern Ky, or the extensive National Forests. The bigger eastern American squirrel, grey with orange overtones, is a fox squirrel. We have both in central Ky., but the one found mostly in subdivisions and the city is the grey squirrel.

    Link to this
  7. 7. IslandGardener 6:48 pm 10/8/2012

    Yes, here in Britain our native squirrels were reds, Sciurus vulgaris, and as North American squirrels have spread the red squirrels have disappeared, and in only a century.

    We still have red squirrels on the Isle of Wight (part of southern England) but most British squirrels are indeed in Scotland. Greys were first released in London (in England), and as Scotland is further away from London than Wales and the non-London bits of England, Scotland’s reds have lasted longer. See
    http://www.naturenet.net/blogs/2011/10/book-review-dormice-by-pat-morris-2/
    and
    http://www.wightsquirrels.co.uk/.
    The reds are declining more slowly in Ireland than in Britain, apparently – see for example http://www.dardni.gov.uk/forestservice/index/environment/wildlife/red-squirrels/causes-of-red-squirrel-decline.htm

    On a pedantic note:
    I assume when John Platt talked about ‘England and Ireland’, he meant ‘the British Isles’, which means the many islands of Britain and Ireland (politically, this means ‘United Kingdom + Republic of Ireland + Isle of Man + Channel Islands’).

    The United Kingdom gets name-checked later, but this means Britain + Northern Ireland.

    ‘Britain’ means ‘England + Wales + Scotland’.

    ‘England’ is not really acceptable shorthand for ‘Britain’.

    If you don’t believe me, try Wikipedia:
    http://en.wikipedia.org/wiki/British_Isles

    Thank you!

    Link to this
  8. 8. Scoiattolo Grigio 9:00 am 10/10/2012

    Actually the virus has never been recorded in Italy, and it is now more than 60 years that gray squirrels live here.
    In the meantime they have indeed occupied some 1,000+ square kilometers, which means that they should take some 18,000 years to complete their “invasion” of Italy, and it seems a bit fanciful worrying about who will inhabit Italy in the year 20,000 or so, and even more because the population of red squirrels is declining everywhere (i.e. where there are no gray squirrels as well) because of the loss of habitat, that is because of the invasiveness of the human species.
    A website (www.scoiattologrigio.org) has been recently published, although unfortunately in Italian language only, in order to underline why the idea of culling gray squirrels should be rejected both for ethical and for practical considerations.

    Link to this
  9. 9. David Kelly 2:37 am 10/11/2012

    IslandGardener, as a Scot I have to agree with your comments on the proper nomenclature for our island. There are Grey Squirrels in Scotland, mainly in the central belt. Where I live they are descended from escapes from Edinburgh Zoo in the 1930s. Red Squirrels hang on in pine plantations along the east coast. It’s certainly only Greys I see stealing my bird food!

    Link to this
  10. 10. Rosso Scoiattolo 5:39 am 09/13/2013

    Answering “Scoiattolo Grigio”‘s propaganda, I’m linking a site (in English too) made by SCIENTISTS about grey-squirrels-menacing-red-squirrels problem in Italy:
    http://www.rossoscoiattolo.eu

    Link to this

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