Sciurus carolinensis
(grey squirrel)

Preferred Common Name

• grey squirrel

International Common Names

• English: gray squirrel

Local Common Names

• Germany: Grauhoernchen
• Italy: scoiattolo grigio

• Domain: Eukaryota
•     Kingdom: Metazoa
•         Phylum: Chordata
•             Subphylum: Vertebrata
•                 Class: Mammalia
•                     Order: Rodentia
•                         Family: Sciuridae
•                             Subfamily: Sciurinae
•                                 Genus: Sciurus
•                                     Species: Sciurus carolinensis

The grey squirrel (Sciurus carolinensis) is a medium-sized tree squirrel with no sexual dimorphism in size or colouration. Ranges of external measurements (in mm) are: total length, 380-525; length of tail, 150-250; length of hind foot, 54-76; length of ear, 25-33. Adult body mass ranges from 300 to 710g. The back is grizzled dark to pale grey and may be washed with cinnamon on hips, feet, and head. The tail is white to pale grey. Underparts are white to grey to buff to cinnamon. In the native range of the species in North America, melanism is common in the north and albinism is rare (Koprowski, 1994).

Introduction pathways to new locations
Landscape/fauna "improvement":
Natural dispersal:

Local dispersal methods
Natural dispersal (local):

In overlap areas, the grey squirrel (Sciurus carolinensis) can cause the extinction of the red squirrel (see Sciurus vulgaris in IUCN Red List of Threatened Species) through competitive exclusion – the two species have similar diet and activity patterns; grey squirrels are known to digest acorns more efficiently (Bertolino, 2008). Grey squirrels also act as a reservoir for a poxvirus – the virus can be fatal to S. vulgaris, whereas it is less pathogenic to S. carolinensis. This has been postulated as another reason why red squirrels go extinct in the presence of grey squirrels; a phenomenon known as pathogen-mediated competition (Gurnell et al. 2006). In England and Wales, many grey squirrels and red squirrels are known to carry the virus, and recently, cases of the virus in both species have occurred in Scotland. In Ireland, some grey squirrels carry squirrel pox virus; until recently no infected red squirrels were reported but a few cases have now been reported (Invasive Species Ireland, 2012; Bertolino, 2008). In Italy, squirrel pox virus has not been recorded in either S. carolinensis or S. vulgaris (Bertolino, 2008).

The epidemiology of the poxvirus is poorly understood but a recent study (Bruemmer et al., 2010) indicated direct transmission (i.e. via contact or via common focal points where contamination could occur). If this is the case, caution is advisable when undertaking any management that brings red and grey squirrels into contact or attracts them to joint focal points, such as trapping or supplementary feeding.

Grey squirrels can cause damage to woodland through bark stripping activity; sycamore (Acer pseudoplantanus) and beech (Fagus sylvatica) are particularly badly affected (Bertolino and Genovesi, 2003). The main damage period is from April to July and trees aged between 10 and 40 years are mostly likely to have their bark stripped (Mayle et al., 2007). Bark stripping can reduce timber quality through stem deformation, or by allowing insects and diseases to degrade the timber. Damage tends to occur when grey squirrel density is greater than 5 per hectare and when there are large numbers of juveniles entering the population in summer (Forestry Commission, 2012). In 2003, the damage to beech, sycamore and oak woodlands alone in the UK was estimated to be £10 million (Lawton et al., 2010). Bark stripping of poplars cultivated for pulp and timber has been a problem in Northern Italy (UNEP-WCMC, 2010). Grey squirrels can also damage agricultural crops, notably, maize (Signorile and Evans, 2006).

Grey squirrels may negatively impact upon woodland birds. They are known to feed on bird eggs and may also complete with seed eating species such as finches (Lawton et al., 2010).

Grey squirrels can be a garden pest by digging up bulbs and eating the bark of ornamental plants.

• Threat to/ loss of native species

• Competition - monopolizing resources
• Pest and disease transmission
• Herbivory/grazing/browsing

Grey squirrels (Sciurus carolinensis) are harvested for food in Mississippi (USA). Squirrels are popular animals to watch throughout their native and introduced ranges.

Citations from GISD

Bomford, M., 2003. Risk Assessment for the Import and Keeping of Exotic Vertebrates in Australia. Bureau of Rural Sciences, Canberra. http://www.feral.org.au/feral_documents/PC12803.pdf

CONABIO. 2008. Sistema de información sobre especies invasoras en México. Especies invasoras - Mamíferos. Comisión Nacional para el Conocimiento y Uso de la Biodiversidad. Fecha de acceso. http://www.conabio.gob.mx/invasoras/index.php/Especies_invasoras_-_Mam%C3%ADferos

Forestry Commission, Great Britian., 2008. Management of grey sqirrels. http://www.forestresearch.gov.uk/forestry/kirn-5m5emv

Genovesi, P. and Bertolino, S., 2001. Human dimension aspects in invasive alien species issues: the case of the failure of the grey squirrel eradication project in Italy. In: McNeely, J.A. (Ed.), The Great Reshuffling: Human Dimensions of Invasive Alien Species. IUCN, Gland Switzerland and Cambridge, UK, pp. 113–119.

Genovesi, Piero (in press). Threats posed by the Grey squirrel in Europe and a strategy for the future: the Italian perspective. From a presentation at the 6th Meeting of the Group of Experts on Invasive Alien Species (Palma de Majorca, 9-11 June 2005) to be published by the Council of Europe.

IUCN/SSC Invasive Species Specialist Group (ISSG)., 2010. A Compilation of Information Sources for Conservation Managers Involved in the Prevention, Eradication, Management and Control of the Spread of Invasive Alien Species that are a Threat to Native biodiversity and Natural Ecosystems.

Koprowski, J.L. 1994. Sciurius carolinensis. Mammalian Species 480: 1-9.

Mayle, Brenda and Smith, Linda (in press). Non-Native Invasive Species - the Grey Squirrel Sciurus carolinensis. A particular example of the threat posed to European Biodiversity. From a presentation at the 6th Meeting of the Group of Experts on Invasive Alien Species (Palma de Majorca, 9-11 June 2005) to be published by the Council of Europe.

Murray, C. and C. Pinkham. 2002. Towards a Decision Support Tool to Address Invasive Species in Garry Oak & Associated Ecosystems in BC. Prepared by ESSA Technologies Ltd., Victoria, B.C. for the GOERT Invasive Species Steering Committee, Victoria, 96 pp. http://www.goert.ca/documents/GOEDSTreport.pdf

The Garry Oak Ecosystems Recovery Team (GOERT)., 2003. Annotated Bibliographies on the Ecology and Management of Sciurus carolinensis

The Garry Oak Ecosystems Recovery Team (GOERT)., 2003. Field manual of Sciurus carolinensis http://www.goert.ca/documents/InvFS_sciucaro.pdf