Expansion of the grey squirrel in Italy

Sandro Bertolino

The grey squirrel in Italy: species expansion and threats for neighbouring countries

Origin and spread of the grey squirrel in Italy

The grey squirrel was introduced into Piedmont (north-western Italy) in 1948, when two pairs were released at Stupinigi (province of Turin) (Bertolino et al., 2000). In 1966, 5 animals imported from Norfolk (Virginia, USA) were released into the park of Villa Groppallo at Genoa Nervi. The Piedmont population has shown a rapid increase of its range in the last decades. From their introduction until 1970 the grey squirrel was recorded only close to the release site, occupying an area of about 25 Km2 (Wauters et al., 1997). Subsequently, the species started to spread into the surrounding area: in 1990 the species’ range arrived to 243 Km2, in 1997 to 380 Km2. After 1997 the range showed a dramatic increase, and in 1999 the grey squirrel was present in an area of 880 Km2 (Wauters et al., 1997; Bertolino and Genovesi, submitted).

Competition with red squirrel and damage

The grey squirrel was introduced into Great Britain and Ireland several times from the end of the last century, causing the progressive disappearance of the native red squirrel from a wide portion of the two islands (Reynolds 1985, Gurnell and Pepper 1993), and severe damage to forests and commercial tree plantations (Rowe and Gill 1985, Dagnall et al. 1998). Bark-stripping activity inflicts wounds that severely degrade timber quality and can facilitate the penetration of insects and fungi (Kenward 1989). It is remarkable that gray squirrel produces little damage to tree in its native area in north America (Kenward 1989). Italian data confirm the competitive exclusion of the red squirrel from the grey squirrel range. A distribution survey in the area where grey squirrels are present, showed a reduction of 46% in the range of the red squirrel from 1970 to 1990, and a further decrease of 55% from 1990 to 1996 (Wauters et al. 1997a).

Eradication proposal and failure of the project

In respect to the urgency of remove the grey squirrel from Italy, in 1997 the NWI (National Wildlife Institute), in co-operation with the University of Turin, produced an action plan finalised to the eradication of the alien species. One of the first steps of the plan was the experimental removal of the small population present in the Racconigi park, in order to produce a pilot eradication scheme. The trial eradication started in May 1997, and the preliminary results were very encouraging, but some animal right groups took the NWI to court, managing to halt the project. In July 2000 the Appeal Court of Turin discharged the two officers.


The three years legal struggle determined the failure of the campaign. Since then, as a consequence of the suspension of the eradication campaign, the species has significantly expanded its range and nowadays the complete eradication is considered not feasible anymore (Genovesi and Bertolino 2000). In fact, the range of the grey squirrel has reached the continous wood belt of the Alps, and of the hilly system of the eastern Piedmont. The dramatic expansion of the species’ range in the last two years, and the homogeneous and continuous broadleaf woodlands characterising the Alpine region, make a colonisation of the entire alpine arch in a few years very probable. Such pessimistic prediction is also supported by the results of a model developed by Lurz et al. (submitted), that predicts that grey squirrel spread is likely to speed up once they are in the pre-alpine forest due to reduced fragmentation, and indicate two possible scenarios, with the greys across Alps and in France in 30-50 years. The NWI to produce a strategy for the control of the Grey squirrel, and after an assessment of the range expansion, the NWI proposed a strategy aimed to: 1) identify and protect key areas for the conservation of viable populations of red squirrels, 2) contain the grey squirrel in order to delay its expansion to neighbouring countries and to the mountain system of Italy (Genovesi and Bertolino 2000).

Threats to other countries

The grey squirrel has colonised in Italy a mean of 17.2 Km2 per year, similarly to the values reported for Great Britain (18 Km2/year, Okubo et al., 1989). The colonisation of new areas has not been constant since 1970, ranging from 1.1 Km2/year in the first phase, when the spread outside the wooded area of Stupinigi was hindered by the presence of extended cultivated fields with a very reduced and fragmented woods, to 10-20 Km2/year, once the species started spreading along the rivers, and recently increased to 250 Km2/year, when the grey squirrel reached the hilly areas of eastern Piedmont and the Po river (Bertolino & Genovesi, submitted). Considering these data and the model developed by Lurz et al. (submitted), a scenario with the grey squirrel through the Alps and in France in the next decades is likely.


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