GWCT National Gamebag Census & Tracking Mammals Partnership

Grey squirrel Sciurus carolinensis

Taxonomy: Class: Mammalia; Order: Rodentia; Family: Sciuridae

Description

Grey squirrel photo
Grey squirrel © Peter Thompson

The grey squirrel is a rodent native to the eastern half of North America. This species lacks the prominent ear tufts of the indigenous red squirrel. It was introduced to many sites across Britain between 1876 and 1929, and spread rapidly. It has displaced the red squirrel through better exploitation of deciduous woodland (Gurnell 1987) and infection by squirrelpox, a disease fatal to reds but not to greys. At high densities, the grey squirrel can cause considerable damage to forestry, crops and gardens (Sheail 1999). It may be culled all year round.

Further information:
Mammal Society website grey squirrel page.

Conservation status and legislation

Status:
UK: Non-native
World: Least Concern (IUCN Red List)

Legislation:
 
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Distribution and abundance

The grey squirrel is widely distributed across England and Wales, including the Isle of Wight. It has colonised the southern half of Scotland (including,Arran and Bute), and has been recorded in scattered locations further north. It occupies most of the eastern half of Ireland, although a scarcity of records means that this is not apparent from the map.

Estimates of grey squirrel abundance (numbers of individuals in the spring) across the UK, from Harris et al. (1995):

United Kingdom 2,520,000
England 2,000,000
Scotland 200,000
Wales 320,000
N Ireland no estimate

Recent trends from the National Gamebag Census

United Kingdom

Index of bag density from 1961 to 2009 (see statistical methods and interpretational considerations).
Error bars represent 95% confidence intervals.

Grey squirrel trend United Kingdom

The bag index remained relatively stable until the mid-1990s, then increased steadily thereafter. This resulted in an overall significant doubling of the index between 1961 and 2009. This reflects the ongoing range expansion and increasing abundance of this introduced species.

Change in grey squirrel bags over time, with 95% confidence limits (see statistical methods):

Country Sites Start
year
End
year
Change (%)
1961-2009
Change (%)
1984-2009
Change (%)
1995-2009
United Kingdom 983 1961 2009 97*
44 to 160
113*
73 to 161
59*
44 to 79

* significant at P < 0.05

England

Index of bag density from 1961 to 2009 (see statistical methods and interpretational considerations).
Error bars represent 95% confidence intervals.

Grey squirrel trend England

The bag index remained relatively stable until the mid-1990s, then increased steadily thereafter. This resulted in an overall significant doubling of the index between 1961 and 2009. This reflects the ongoing range expansion and increasing abundance of this introduced species.

Change in grey squirrel bags over time, with 95% confidence limits (see statistical methods):

Country Sites Start
year
End
year
Change (%)
1961-2009
Change (%)
1984-2009
Change (%)
1995-2009
England 839 1961 2009 97*
54 to 160
112*
73 to 156
58*
39 to 77

* significant at P < 0.05

Scotland

Index of bag density from 1977 to 2009 (see statistical methods and interpretational considerations).
Error bars represent 95% confidence intervals.

Grey squirrel trend Scotland

Between 1961 and 1976, there were just nine reports of bags from six sites. Since 1977, there has been a significant threefold increase in the bag index. This reflects the ongoing range expansion and increasing abundance of this introduced species.

Change in grey squirrel bags over time, with 95% confidence limits (see statistical methods):

Country Sites Start
year
End
year
Change (%)
1961-2009
Change (%)
1984-2009
Change (%)
1995-2009
Scotland 98 1977 2009 311*
76 to 784
205*
142 to 369
100*
58 to 171

* significant at P < 0.05

Wales

Index of bag density from 1961 to 2009 (see statistical methods and interpretational considerations).
Error bars represent 95% confidence intervals.

Grey squirrel trend Wales

Although the bag index dropped initially, it has since built up steadily, with a significant increase between 1995 and 2009. This reflects the ongoing range expansion and increasing abundance of this introduced species.

Change in grey squirrel bags over time, with 95% confidence limits (see statistical methods):

Country Sites Start
year
End
year
Change (%)
1961-2009
Change (%)
1984-2009
Change (%)
1995-2009
Wales 35 1961 2009 87
-12 to 711
107
-20 to 248
88*
13 to 182

* significant at P < 0.05

N Ireland

There are too few bag records of grey squirrel to produce an index graph. Grey squirrel trend N Ireland

 

There are too few bag records of grey squirrel to evaluate rates of change over time

Country Sites Start
year
End
year
Change (%)
1961-2009
Change (%)
1984-2009
Change (%)
1995-2009
N Ireland Too few sites

Environmental zones

Change in grey squirrel bags over time, with 95% confidence limits (see statistical methods):

Environmental zone Sites Start
year
End
year
Change (%)
1961-2009
Change (%)
1984-2009
Change (%)
1995-2009
Easterly lowlands (England/Wales) 539 1961 2009 88*
34 to 157
113*
63 to 167
54*
36 to 76
Westerly lowlands (England/Wales) 234 1961 2009 112*
46 to 196
94*
60 to 149
59*
22 to 113
Uplands (England/Wales) 100 1961 2009 184
-41 to 449
139*
87 to 222
109*
48 to 178
Lowlands (Scotland) 50 1977 2009 311*
79 to 1015
219*
110 to 480
137*
72 to 305
Intermediate uplands/islands (Scotland) 21 1995 2009 no data no data 35
-16 to 72
True uplands (Scotland) 30 1984 2009 no data 281*
113 to 598
87*
4 to 187

* significant at P < 0.05

Comparison with BBS mammal data

Since 1995, data on grey squirrel abundance have been collected under the Breeding Bird Survey (BBS) organised by the British Trust for Ornithology. Below, the UK trend from the NGC is compared to the one from the BBS (from 1995 onwards).

NGC index of bag density (blue) and BBS index of abundance (red), from 1995 to 2009.
Error bars represent 95% confidence intervals.

Grey squirrel: comparison of UK trends from GWCT and BTO

For all years, the BBS confidence intervals are completely or almost completely within the NGC confidence intervals, indicating good agreement between the two index series.

Long-term trend from the National Gamebag Census

There are too few bag records of grey squirrel to produce a trend starting before 1961.

References and further reading

  • Battersby,J. (2005). UK Mammals: Species Status and Population Trends. Joint Nature Conservation Committee/Tracking Mammals Partnership, Peterborough (JNCC download page).
  • Gurnell,J. (1987). The Natural History of Squirrels. Christopher Helm, London.
  • Harris,S., Morris,P., Wray,S. & Yalden,D.W. (1995). A Review of British Mammals: Population Estimates and Conservation Status of British Mammals Other than Cetaceans. Joint Nature Conservation Committee, Peterborough (JNCC download page).
  • Harris,S. & Yalden,D.W. (2008). Mammals of the British Isles: Handbook, 4th edition. Mammal Society, Southampton.
  • Sheail,J. (1999). The grey squirrel (Sciurus carolinensis) - a UK historical perspective on a vertebrate pest species. Journal of Environmental Management 55: 145-156.
  • Tompkins,D.M., Sainsbury,A.W., Nettleton,P., Buxton,D. & Gurnell,J. (2002). Parapoxvirus causes a deleterious disease in red suirrels associated with UK population declines. Proceedings of the Royal Society of London, Series B 269: 529-533.

This report should be cited as: Aebischer,N.J., Davey,P.D. & Kingdon,N.G. (2011). National Gamebag Census: Mammal Trends to 2009. Game & Wildlife Conservation Trust, Fordingbridge (http://www.gwct.org.uk/ngcmammals).

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