Sciurus carolinensis (grey squirrel)
- Summary of Invasiveness
- Taxonomic Tree
- Distribution Table
- Habitat List
- Biology and Ecology
- Means of Movement and Dispersal
- Impact Summary
- Threatened Species
- Risk and Impact Factors
- Uses List
- Prevention and Control
- Principal Source
- Distribution Maps
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PicturesTop of page
IdentityTop of page
Preferred Scientific Name
- Sciurus carolinensis Gmelin 1788
Preferred Common Name
- grey squirrel
International Common Names
- English: gray squirrel
Local Common Names
- Germany: Grauhoernchen
- Italy: scoiattolo grigio
Summary of InvasivenessTop of page
The grey squirrel (Sciurus carolinensis) is native to deciduous forests in the USA and has been introduced to the UK, Ireland, Italy and South Africa. In the introduced range grey squirrels can damage trees by stripping the bark. In Europe they cause the local extinction of red squirrel (Sciurus vulgaris) populations through competition and disease. This species has been nominated as among 100 of the "World's Worst" invaders.
Taxonomic TreeTop of page
- Domain: Eukaryota
- Kingdom: Metazoa
- Phylum: Chordata
- Subphylum: Vertebrata
- Class: Mammalia
- Order: Rodentia
- Family: Sciuridae
- Subfamily: Sciurinae
- Genus: Sciurus
- Species: Sciurus carolinensis
DescriptionTop of page
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).
DistributionTop of page
Native range: Grey squirrels (Sciurus carolinensis) are found in the eastern United States; the range extends west to the edge of the Eastern deciduous forest and north to southern Quebec and Ontario.
Known introduced range: Introductions occurred in California, Montana, Oregon, and Washington in the USA; and Quebec, New Brunswick, British Columbia, Manitoba, Nova Scotia, Ontario and Saskatchewan in Canada. Eastern grey squirrels were introduced to Italy and England from the USA, to Scotland from Canada, and to South Africa and Ireland from England. Grey squirrels were introduced from England to Australia in the 1880s but are now extinct there. (Koprowski, 1994). Grey squirrels seem able to establish viable populations in new habitats, with relatively few initial introduced individuals (Lawton et al., 2010).
Distribution TableTop of page
The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.
|Continent/Country/Region||Distribution||Last Reported||Origin||First Reported||Invasive||Reference||Notes|
|South Africa||Present||Introduced||01/Jan/1900||Not invasive||ISSG, 2011|
|Canada||Present||Present based on regional distribution.|
|-Alberta||Present||Introduced||later than 1914||Not invasive||ISSG, 2011|
|-British Columbia||Present||Introduced||01/Jan/1914||Invasive||ISSG, 2011|
|-Manitoba||Present||Not invasive||ISSG, 2011|
|-New Brunswick||Present||Not invasive||ISSG, 2011|
|-Nova Scotia||Present, few occurrences||2010||Huynh et al., 2010||First evidence of a breeding population in Nova Scotia reported in 2010 by Huynh et al.|
|-Ontario||Present||Not invasive||ISSG, 2011|
|-Quebec||Present||Not invasive||ISSG, 2011|
|-Saskatchewan||Present||Not invasive||ISSG, 2011|
|USA||Present||Native||Not invasive||ISSG, 2011|
|Pitcairn Island||Restricted distribution||Introduced||First reported in 1987||Invasive||ISSG, 2011|
HabitatTop of page
Grey squirrels (Sciurus carolinensis) survive best in mature broadleaf woodland where there is a mixture of nut producing species that produce food that can be stored over winter. Although they are most common in broadleaf woodland, they are also found in broadleaf/conifer mixes or conifer woodland nearby broadleaf woodland. In the UK they are common visitors to urban gardens where they frequently eat food left out for birds; they are also found in hedgerows and urban parks (Lawton et al., 2010).
Habitat ListTop of page
|Cultivated / agricultural land||Present, no further details|
|Disturbed areas||Present, no further details|
|Managed forests, plantations and orchards||Present, no further details|
|Urban / peri-urban areas||Present, no further details|
|Natural forests||Present, no further details|
|Scrub / shrublands||Present, no further details|
Biology and EcologyTop of page
Grey squirrels eat nuts (especially acorns, beechnuts, and chestnuts) and other large broadleaf seeds, and fluctuations of S. carolinensis populations are thought to be influenced by the availability of these key resources. Grey squirrels also eat buds, flowers, fruits, fungi, some insects and occasionally bird eggs. Grey squirrels will feed on maize if grown close to woodlands. During periods of food shortage, they strip bark from trees, although the reason for bark stripping remains unclear (Lawton et al., 2010). Bark stripping has negative impacts on the forestry industry (see ‘Impacts’ section).
Grey squirrels cache food in small pits to see them through the winter. They have excellent spatial memory allowing these caches to be relocated. They sometimes pilfer the caches of red squirrels.
Placental, sexual, polygamous. 2-3 young per litter, 1-2 litters per year.
Means of Movement and DispersalTop of page
Introduction pathways to new locations
Local dispersal methods
Natural dispersal (local):
Impact SummaryTop of page
ImpactTop of page
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.
Threatened SpeciesTop of page
Risk and Impact FactorsTop of page Impact outcomes
- Threat to/ loss of native species
- Competition - monopolizing resources
- Pest and disease transmission
UsesTop of page
Grey squirrels (Sciurus carolinensis) are harvested for food in Mississippi (USA). Squirrels are popular animals to watch throughout their native and introduced ranges.
Uses ListTop of page
Human food and beverage
Prevention and ControlTop of page
The UK Forestry Commission have a research programme that includes investigating the impact of grey squirrels on woodland biodiversity, managing impacts on timber production and identifying efficient control strategies. Please follow this link for a summary of their research . The Forestry Commission have also published guidance documents on grey squirrel management including detailed information about control methods: ‘Controlling Grey Squirrel Damage to Woodlands’ and ‘Red Squirrel Conservation’.
Physical control: Physical management of grey squirrels includes live-trapping using baited cages, tunnel trapping, and shooting.
Chemical control: Warfarin (anti-coagulant) is the only cost-effective method of control currently available; however the use of an oral immunocontraceptive is under investigation. Warfarin is permitted for use in Britain but not in Italy (UNEP-WCMC, 2010).
The aim of control varies according to whether the main objective is protecting timber yields or protecting red squirrels. If the former is the case, the aim of control is to reduce the grey squirrel population below the level at which significant damage occurs (around five grey squirrels per hectare). On the other hand, to protect red squirrels, local eradication of the grey squirrel population should be the aim. Likewise, the types of control methods used is determined by whether red squirrels are present in the area where the control operation is taking place. If red squirrels are present, then cage trapping and shooting can be used, whereas warfarin can be used only if red squirrels are absent (Lawton et al., 2010).
Grey squirrels can recolonize the area after eradication and populations can recover after eradication efforts (Lawton and Rochford, 2007). In addition, eradication attempts have faced protests from animal rights groups, which can prevent the control operation from proceeding, which may have severe consequences (see Lawton et al., 2010).
Please read the document by UNEP-WCMC (2010) for management information in Europe, according to country.
BibliographyTop of page
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
ReferencesTop of page
Bertolino S, 2008. Introduction of the American grey squirrel (Sciurus carolinensis) in Europe: a case study in biological invasion. Current Science:903-906.
Bruemmer CM; Rushton SP; Gurnell J; Lurz PWW; Nettleton P; Sainsbury AW; Duff JP; Gilray J; McInnes CJ, 2010. Epidemiology of squirrelpox virus in grey squirrels in the UK. Epidemiology and Infection, 138(7):941-950. http://journals.cambridge.org/action/displayFulltext?type=6&fid=7785766&jid=&volumeId=&issueId=07&aid=7785765&bodyId=&membershipNumber=&societyETOCSession=&fulltextType=RA&fileId=S0950268810000816
Forestry Commission, 2012. Forest Research. Controlling grey squirrel damage to woodlands. http://www.forestry.uk/website/forestresearch.
Invasive Species Ireland, 2012. Species Alerts. Squirrel pox virus. http://invasivespeciesireland.com/species-alerts/squirrel-pox-virus/.
Lawton C; Cowan P; Bertolino S; Lurz PWW; Peters AR, 2010. The consequences of introducing non-indigenous species: two case studies, the grey squirrel in Europe and the brushtail possum in New Zealand. Revue Scientifique et Technique - Office International des Épizooties, 29(2):287-298.
Lawton C; Rochford J, 2007. The recovery of grey squirrel (Sciurus carolinensis) populations after intensive control programmes. Biology and Environment: Proceedings of the Royal Irish Academy, Section B, 107B(1):19-29.
Mayle B; Ferryman M; Pepper H, 2007. Controlling Grey Squirrel Damage to Woodlands. Forestry Commission Practice Note 4 (Revised). Forestry Commission, Edinburgh.
Signorile AL; Evans J, 2010. Damage caused by the American grey squirrel (Sciurus carolinensis) to agricultural crops, poplar plantations and semi-natural woodland in Piedmont, Italy. Forestry:89-98.
United Nations Environment Programme; World Conservation Monitoring Centre (UNEP-WCMC), 2010. Review of the Grey Squirrel (Sciurus carolinensis). UNEP-WCMC, Cambridge.
ContributorsTop of page
- Last Modified: Monday, October 17, 2005
Distribution MapsTop of page
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