Invasive Species Compendium

Detailed coverage of invasive species threatening livelihoods and the environment worldwide


Sciurus carolinensis
(grey squirrel)



Sciurus carolinensis (grey squirrel)


  • Last modified
  • 20 November 2019
  • Datasheet Type(s)
  • Invasive Species
  • Natural Enemy
  • Host Animal
  • Preferred Scientific Name
  • Sciurus carolinensis
  • Preferred Common Name
  • grey squirrel
  • Taxonomic Tree
  • Domain: Eukaryota
  •   Kingdom: Metazoa
  •     Phylum: Chordata
  •       Subphylum: Vertebrata
  •         Class: Mammalia
  • Summary of Invasiveness
  • 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 strip...

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Grey squirrel (Sciurus carolinensis); adult, feeding. Surrey, UK. January 2011.
CaptionGrey squirrel (Sciurus carolinensis); adult, feeding. Surrey, UK. January 2011.
Copyright©Peter Trimming=2011 - CC BY 2.0
Grey squirrel (Sciurus carolinensis); adult, feeding. Surrey, UK. January 2011.
AdultGrey squirrel (Sciurus carolinensis); adult, feeding. Surrey, UK. January 2011.©Peter Trimming=2011 - CC BY 2.0
Sciurus carolinensis (grey squirrel); adult. Cumnor, Oxford, Oxfordshire, UK. March 2016.
CaptionSciurus carolinensis (grey squirrel); adult. Cumnor, Oxford, Oxfordshire, UK. March 2016.
Copyright©Charles J. Sharp/via wikipedia - CC BY-SA 4.0
Sciurus carolinensis (grey squirrel); adult. Cumnor, Oxford, Oxfordshire, UK. March 2016.
AdultSciurus carolinensis (grey squirrel); adult. Cumnor, Oxford, Oxfordshire, UK. March 2016.©Charles J. Sharp/via wikipedia - CC BY-SA 4.0


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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 Invasiveness

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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 Tree

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


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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).


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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 Table

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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.

Last updated: 10 Jan 2020
Continent/Country/Region Distribution Last Reported Origin First Reported Invasive Reference Notes


South AfricaPresentIntroduced1900


United KingdomPresentIntroduced1876Invasive

North America

CanadaPresentPresent based on regional distribution.
-AlbertaPresentIntroducedFirst reported: later than 1914
-British ColumbiaPresentIntroduced1914Invasive
-New BrunswickPresent
-Nova ScotiaPresent, Few occurrences2010First evidence of a breeding population in Nova Scotia reported in 2010 by Huynh et al.
United StatesPresentNative


PitcairnPresent, LocalizedIntroduced1987InvasiveFirst reported: 1987


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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 List

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Terrestrial ManagedCultivated / agricultural land Present, no further details
Terrestrial ManagedManaged forests, plantations and orchards Present, no further details
Terrestrial ManagedDisturbed areas Present, no further details
Terrestrial ManagedUrban / peri-urban areas Present, no further details
Terrestrial Natural / Semi-naturalNatural forests Present, no further details
Terrestrial Natural / Semi-naturalScrub / shrublands Present, no further details

Biology and Ecology

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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 Dispersal

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

Local dispersal methods
Natural dispersal (local):

    Impact Summary

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    Native fauna Negative
    Native flora Negative


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    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 Species

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    Threatened SpeciesConservation StatusWhere ThreatenedMechanismReferencesNotes
    Sciurus vulgarisLC (IUCN red list: Least concern)Competition - monopolizing resources; Pest and disease transmissionISSG, 2011

    Risk and Impact Factors

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    Impact outcomes
    • Threat to/ loss of native species
    Impact mechanisms
    • Competition - monopolizing resources
    • Pest and disease transmission
    • Herbivory/grazing/browsing


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

    Uses List

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    Human food and beverage

    • Food

    Prevention and Control

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    Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

    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.


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    Citations from GISD

    Bomford, M., 2003. Risk Assessment for the Import and Keeping of Exotic Vertebrates in Australia. Bureau of Rural Sciences, Canberra.

    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.

    Forestry Commission, Great Britian., 2008. Management of grey sqirrels.

    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.

    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


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    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.

    Forestry Commission, 2012. Forest Research. Controlling grey squirrel damage to woodlands.

    Huynh HM; Williams GR; McAlpine DF; Thorington RW Jr, 2010. Establishment of the Eastern Gray Squirrel (Sciurus carolinensis) in Nova Scotia, Canada. Northeastern Naturalist, 17(4):673-677.

    Invasive Species Ireland, 2012. Species Alerts. Squirrel pox virus.

    ISSG, 2011. Global Invasive Species Database (GISD). Invasive Species Specialist Group of the IUCN Species Survival Commission.

    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.

    Links to Websites

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    GISD/IASPMR: Invasive Alien Species Pathway Management Resource and DAISIE European Invasive Alien Species Gateway source for updated system data added to species habitat list.
    Global register of Introduced and Invasive species (GRIIS) source for updated system data added to species habitat list.


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      Compiled by: IUCN/SSC Invasive Species Specialist Group (ISSG)

      Last Modified: Monday, October 17, 2005

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