When I said last week that I was interested in squirrel conservation, I had imagined that I would find a lot of papers talking about the decline of European red squirrels (Sciurus vulgaris) in response to the presence of gray squirrels in the UK- and, I did. But I also found so much more: more species of squirrel than I knew existed, more conservation issues facing squirrels, more moments when I thought “What were they thinking?” So this week I’ll start with an overview of the red squirrel-gray (or grey) squirrel issue and then look at some of the other species around the world.
European red squirrels in the UK (a different species from the red squirrels in North America which are Tamiasciurus hudsonicus) are incredibly cute and also inhabit a range that is severely reduced– North American gray squirrels were introduced in 1876 in Cheshire and then in several other locations (why?) in the UK until 1937 when it became illegal to introduce or move around gray squirrel populations (Shuttleworth 2003). Research into interactions between red and gray squirrels indicates that red squirrels avoid areas with high gray squirrel densities and, while female red squirrels seem to be able to raise the same number of young whether or not gray squirrels are in the area, once those young red squirrels are on their own they seem to have reduced survival (Wauters et al. 2000). Disease is also a concern since gray squirrels can be asymptomatic carriers of parapox virus, a virus that causes lesions and death in the red squirrels (Rushton et al. 2000). And lest you think there is something about red squirrels in the UK that makes them uniquely vulnerable to competition and disease-transmission by gray squirrels, similar patterns of population decline after introduction of grays have been seen in Ireland and northern Italy (Teangana et al. 2000; Wauters and Gurnell 1999).
The threatened Northern Idaho ground squirrel has a USFWS recovery plan- photo by Diane Evans-Mack, Idaho DFG
Okay, European red squirrel populations are having trouble where they share space with introduced gray squirrels, but surely other squirrel species are doing fine, right? As it turns out, while squirrel species in Louisiana are doing well enough to have a hunting season, other species are facing considerably bigger challenges. Northern flying squirrels in North America, for example, are generally doing well across the northern part of their range (and a good thing, too, since they are an important food resource for predators like spotted owls), but two subspecies in the Appalachians were listed as endangered in 1985 (Loeb et al. 2000) and there are concerns about subspecies in the Rockies and Sierra Nevadas (Weigl 2007). The flying squirrel of Europe (once again, a different species from the one in North America- maybe we need to get a bit more creative with names?) is also declining in parts of its range- the culprits in general seem to be habitat loss and fragmentation (Reunanen et al. 2000). A subspecies of the fox squirrel, the big cypress fox squirrel is found only in Florida and is now considered a threatened subspecies due to loss of mature pine and cypress forests (Eisenberg et al. 2011).
Turning to species with more restricted distributions, the Franklin’s ground squirrel has been in decline in the eastern part of its range, for example in Indiana (Johnson and Choromanski-Norris 1992) and Illinois (Martin et al. 2003)- loss of habitat to agricultural land and the use of herbicides along railway tracks are believed to contribute to the population loss. Habitat loss and fragmentation seem to be the big issues overall, whether it’s loss of open prairie to agriculture as with the Perote ground squirrel in Mexico (Valdez and Ceballos 1997), loss of contiguous forest to roadways so that flying squirrels cannot reach new areas (Weigl 2007), or the introduction of alien plants to meadows for the Northern Idaho ground squirrel (which may not seem like a big deal for animals that eat plants, but, in this case, the new plants were considerably lower in polyunsaturated fatty acids and the squirrels could not get enough energy to last through hibernation- Sherman and Runge 2002). If you want to get a sense of how quickly these smaller populations can blink out, the third case is a pretty grim illustration- within 12 years the studied population had declined by more than 95%. And just to throw in the political side as well, the Mt. Graham red squirrel was listed as an endangered subspecies in Arizona in 1987 while living in a place that could also be used for large telescopes since the atmosphere is so clear- check out Warshall (1994) for the story of how the environmental impact review process doesn’t always follow proper procedure…
The list I just gave of species and subspecies in trouble is rather depressing- and I didn’t mention (until now) the prairie dogs I helped study in northern Mexico where colony sizes declined considerably over a ten-year period. So I want to close with a little look at the bright side– in 1994 in a remote location in northern Pakistan, a species not seen since 1924 was rediscovered (Zahler 1996). The woolly flying squirrel was known from specimens largely collected in the 19th century, but a female was caught and the remains of a second animal were found in the nest of an eagle owl in 1994, showing that this largely unknown species was still present in the mountains of central Asia. Squirrels can be resilient.
Now that you have a sense of the scale and general trends of squirrel conservation issues, for my next post I’m going to try to get more detail on recent developments and possible strategies for supporting those populations in need of help.
Works Cited:
Eisenberg DA, Noss RF, Waterman JM, Main MB. 2011. Distribution and habitat use of the big cypress fox squirrel (Sciurus niger avicennia). Southeast. Nat. 10:75–84.
Johnson SA, Choromanski-Norris J. 1992. Reduction in the eastern limit of the range of the Franklin’s ground squirrel (Spermophilus franklinii). Am. Midl. Nat. 128:325–331.
Loeb SC, Tainter FH, Cazares E. 2000. Habitat associations of hypogeous fungi in the Southern Appalachians: implications for the endangered northern flying squirrel (Glaucomys sabrinus coloratus). Am. Midl. Nat. 144:286–296.
Martin JM, Heske EJ, Hofmann JE. 2003. Franklin’s ground squirrel (Spermophilus franklinii) in Illinois: a declining prairie mammal? Am. Midl. Nat. 150:130–138.
Reunanen P, Monkkonen M, Nikula A. 2000. Managing boreal forest landscapes for flying squirrels. Conserv. Biol. 14:218–226.
Rushton SP, Lurz PWW, Gurnell J, Fuller R. 2000. Modelling the spatial dynamics of parapox disease in red and grey squirrels: a possible cause of the decline in the red squirrel in the UK? J. Appl. Ecol. 37:997–1012.
Sherman PW, Runge MC. 2002. Demography of a population collapse: the Northern Idaho ground squirrel (Spermophilus brunneus brunneus). Ecology 83:2816–2831.
Shuttleworth C. 2003. A tough nut to crack: red squirrel conservation in Wales. Biologist 50:231–235.
Teangana DO, Reilly S, Montgomery WI, Rochford J. 2000. Distribution and status of the red squirrel (Sciurus vulgaris) and grey squirrel (Sciurus carolinensis) in Ireland. Mammal Rev. 30:45–56.
Valdez M, Ceballos G. 1997. Conservation of endemic mammals of Mexico: the Perote ground squirrel (Spermophilus perotensis). J. Mammal. 78:74–82.
Warshall P. 1994. The biopolitics of the Mt. Graham red squirrel (Tamiasciurus hudsonicus grahamensis). Conserv. Biol. 8:977–988.
Wauters LA, Gurnell J. 1999. The mechanism of replacement of red squirrels by grey squirrels: a test of the interference competition hypothesis. Ethology 105:1053–1071.
Wauters LA, Lurz PWW, Gurnell J. 2000. Interspecific effects of grey squirrels (Sciurus carolinensis) on the space use and population demography of red squirrels (Sciurus vulgaris) in conifer plantations. Ecol. Res. 15:271–284.
Weigl PD. 2007. The northern flying squirrel (Glaucomys sabrinus): a conservation challenge. J. Mammal. 88:897–907.
Zahler P. 1996. Rediscovery of the woolly flying squirrel (Eupetaurus cinereus). J. Mammal. 77:54–57.