Here in the United States, we feel beleaguered by invasive species that have been introduced from elsewhere, but species introduction is not a one-way street.
Consider the eastern gray squirrel (Sciurus carolinensi), in the tree-squirrel genus. Americans have long been conflicted about our relationship with this rodent. While we tend to enjoy its antics and admire its ingenuity and pluck, we also can lose patience with its propensity for raiding bird feeders, digging up gardens, nesting in attics and feeding on crops.
Partly because it has easily adapted to living among us humans, the gray squirrel has been able to expand its range. And conservationists are concerned that this poses a threat to other, less-adaptive squirrel species, including the smaller, shyer American red squirrel (Tamiasciurus hudsonicus), in the pine-squirrel genus. In the US, the red’s range extends from the Northwest, where it is more common, to the Appalachians, although populations are patchy in some areas.
Across the pond, the issue of red versus gray is much more heated, according to a June 9 article by Erik Stokstad in Science magazine online. Since the gray was introduced in the UK and Ireland, it has “muscled out” the native Eurasian red squirrel (Sciurus vulgaris), which is in the same genus as the gray and is now considered endangered in the UK, Stokstad writes.
The exact reasons why the nonnative gray overmatches the Eurasian red is a point of controversy, but the gray’s larger size, greater strength, and greater ability to store fat for winter are thought to be factors, according the Encyclopedia of Life, or EOL. A study published in the Journal of Animal Ecology has also shown that grays compete for seeds cached by the reds in the spring, resulting in fewer and smaller young red squirrels being produced.
The gray squirrel also faces few predators in the UK, other than the pine marten (a native weasel), which seems to tamp down gray-squirrel populations while not greatly affecting the reds, as is usually the case when predators and prey evolve together. Almost extinct 50 years ago, the marten is rebounding in Scotland and Ireland, thanks to conservation efforts, according to an article in British newspaper The Guardian, but still remains “rare” in England and Wales.
Perhaps the most devastating threat the gray poses to the Eurasian red is squirrelpox (Parapoxvirus), a virus that the gray carries but is generally immune to. Stokstad describes the virus as causing a “gruesome” death in red squirrels, enabling the grays to expand their range “by as much as 34 square kilometers per year” where the virus is present — 25 times faster than where the red squirrels are healthy.
Some people in the squirrel war zone are literally beating back the gray intruder. Under the UK’s Wildlife and Countryside Act 1981, if an eastern gray squirrel is trapped, it is illegal to release it or to allow it to escape into the wild, and should instead be “humanely dispatched,” according to Rural Development Service Technical Advice Note 09. The technique for doing so is also pretty gruesome, although quicker than squirrelpox: trapping the squirrel, putting it in a bag and then bludgeoning it to death with a large stick. “I don’t like doing this, but they don’t belong here.” Stokstad quotes one biologist saying.
As with the introduction of many “exotic invasives,” the gray squirrel did not travel outside of its native habitat under its own steam. According to Stokstad, in 1890 the 11th Duke of Bedford, Herbrand Russell — the species’ “most ardent enthusiast by far” — set 10 free on his estate, near London. He then helped the species’ expansion by giving away offspring pairs as gifts, including six pairs to someone in Ireland. DNA studies show that all of Ireland’s grays descended from those six pairs.
The gray was subsequently also introduced into northern Italy and now threatens Eurasian red squirrel populations there as well, according to an article in the journal Biological Invasions. While in 1970 “red squirrels were still widespread and greys were restricted to forests near the introduction site,” the article says, the gray’s range “doubled in the successive two decades.” A project is now underway to control the gray in northern Italy, with recent trade restrictions and trade bans as a “first step” in trying to keep the squirrel from invading other countries.
Along with the native-versus-nonnative issue, culture is on the side of the red squirrel in the UK, Stokstad writes: it’s featured in children’s books, such as Beatrix Potter’s “The Tale of Squirrel Nutkin,” and in a well-known cartoon. But some conservation groups oppose the killing of gray squirrels to save the red squirrels, which themselves were considered a pest and were killed by foresters in “untold numbers” in the 1900s, according to Stokstad.
The war on the grays in the UK so far has only been successful in one area: Anglesey, a 714-square-kilometer island. While grays had been introduced there, they were eradicated in by 2014 through a campaign that lasted more than 10 years. The island is now “fairly secure, because squirrels can reach it only by scampering across bridges,” Stokstad writes, but elsewhere in the UK, gray squirrels “likely outnumber reds 200 to one.
While some conservationists are worried about the gray’s displacing our native red squirrel, pine squirrels such as the red prefer coniferous forest (hence the genus name), while the grays prefer deciduous forest, so are not often in competition. Places where the species and forest communities overlap, particularly in the Pacific Northwest, can be battlegrounds, but at least squirrelpox does not seem to be an issue in the US. Gray squirrels here carry a different strain of the virus than the grays in the UK, which doesn’t seem to have a big effect on our native reds.
Even if the UK succeeds in eradicating gray squirrels, it would be a drop in the bucket in stopping the threat posed by invasive nonnative species globally. And should we choose winners and losers, even if we can?
While many factors are driving species to extinction and thus threatening biodiversity, introduction of invasive species and habitat destruction typically top the list. And we humans are not likely to escape the effects of this sixth global extinction, which was primarily caused by our species.
© 2016 Pam Owen