Europe's grey squirrels (Sciurus carolinensis) originated in North America, where they are known as eastern gray squirrelsBertolino, S., & Genovesi, P. (2003). Spread and attempted eradication of the grey squirrel (Sciurus carolinensis) in Italy, and consequences for the red squirrel (Sciurus vulgaris) in Eurasia. Biological Conservation, 109(3), 351–358. https://doi.org/10.1016/s0006-3207(02)00161-1 (to differentiate from western gray squirrels). They eat large seeds, flowers, buds, fruits, fungi, some insects and occasionally bird eggs.Sandro, B. (2008). Introduction of the American grey squirrel (Sciurus carolinensis) in Europe: a case study in biological invasion. Current Science, 95(6), 903–906. https://www.jstor.org/stable/24103188
They were first introduced into England, in a concerted way, in 1876,Hickman, L. (2012, September 5). “If you want red squirrels, you have to kill greys.” The Guardian. https://www.theguardian.com/environment/2012/sep/05/red-grey-squirrels-cornwall and through rapidly growing population and further introductions they spread to the rest of Great Britain by the early to mid-20th century.Okubo, A., Maini, P. K., Williamson, M. H., & Murray, J. D. (1989). On the spatial spread of the grey squirrel in Britain. Proceedings of the Royal Society of London. Series B, Biological Sciences, 238(1291), 113–125. https://doi.org/10.1098/rspb.1989.0070 The eastern grey squirrel was introduced to Continental Europe in 1948 and has quickly taken advantage of Europe's food sources, habitats and lack of predators for grey squirrels. Genetic studies have shown that human intervention—released pets and intentional dispersal—may play a much larger role in the spread of grey squirrels than previously believed.Dunning, H. (2016, January 26). Don’t blame grey squirrels: their British invasion had much more to do with us. Imperial. https://www.imperial.ac.uk/news/170448/dont-blame-grey-squirrels-their-british/
The native squirrel in Europe is the red squirrel (Sciurus vulgaris). Both species have similar diets, use similar habitats and have similar activity patterns; however, there is no evidence of noticeable aggression between the two species. These eastern grey squirrels are considered an invasive species in Europe because of their presence contributing to the displacement of the red squirrels.
The invasion of the grey squirrel in the United Kingdom can be classified as a Disease Mediated Invasion (DMI).Strauss, A., White, A., & Boots, M. (2012). Invading with biological weapons: the importance of disease‐mediated invasions. Functional Ecology, 26(6), 1249–1261. https://doi.org/10.1111/1365-2435.12011 More specifically, the invasion of the grey squirrels and displacement of the red squirrels can be described by "spillover DMI", which is defined as a non-native species bringing diseases with them into the new area that will infect the native species. Spillover is especially an issue when the invading species is morphologically similar to the native one. This can have devastating effects.
Introduction of grey squirrel to Europe Grey squirrels have been introduced either on purpose or by accident in many places throughout the world.
They were first introduced into England, in a concerted way, in 1876, and through rapidly growing population and further introductions they spread to the rest of Great Britain by the early to mid-20th century.
The first reported introduction of grey squirrels in continental Europe was in 1948, and they were taken to Stupinigi, Italy, where there is still a growing population. In Ireland, however, Lord Longford was said to have introduced the American squirrels into his estate in 1911. Then in 1966, five squirrels were introduced to Nervi, Italy in the park of Genoa. Later in 1994, three pairs were again released to a park in Trecate but were captured and removed in 1996. As stated before, not all grey squirrel introductions were intentional. For example, in 1966 two females and one male escaped from a game farm on Vancouver Island.Simberloff, D. (2008). Rats are not the only introduced rodents producing ecosystem impacts on islands. Biological Invasions, 11(7), 1735–1742. https://doi.org/10.1007/s10530-008-9400-5
Almost 80% of known grey squirrel introductions worldwide resulted in successful establishment in the new area.Lawton, C., Cowan, P., Bertolino, S., Lurz, P., & Peters, A. (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 De L OIE, 29(2), 287–298. https://doi.org/10.20506/rst.29.2.1983 Typically, grey squirrels do not need large numbers to start a new population. From studying all of the known successful introductions of this species, 71.4% used fewer than 10 individuals. This efficiency in the spread of grey squirrels can be explained by the enemy release hypothesis, which is when the invading species has better survival because its natural predators are not present. Also, natural parasites typically are not present in the area that the species has been introduced or has invaded. Because the grey squirrel poses such a threat to the trees and red squirrel in Europe, there is great effort to stop the further spread of the grey squirrels.
Impact on other species Competition for food One of the biggest concerns about the grey squirrel as an invasive species is the effect introduction has on populations of red squirrels. Grey and red squirrels compete for resources, and greys will pilfer food caches of red squirrels.Bertolino, S., Lurz, P. W., Sanderson, R., & Rushton, S. P. (2008). Predicting the spread of the American grey squirrel (Sciurus carolinensis) in Europe: A call for a co-ordinated European approach. Biological Conservation, 141(10), 2564–2575. https://doi.org/10.1016/j.biocon.2008.07.017Gosso, A., La Morgia, V., Marchisio, P., Telve, O., & Venturino, E. (2012). Does a larger carrying capacity for an exotic species allow environment invasion? - Some considerations on the competition of red and grey squirrels. Journal of Biological Systems, 20(03), 221–234. https://doi.org/10.1142/s0218339012500131 Reduction in red squirrel individual growth, juvenile recruitment, and reproductive success has been linked to competition between grey and red squirrels. In areas of large core overlap between red and grey squirrel populations, a reduction in body mass of red squirrels during the spring has been observed. This is most likely due to the fact that reds and greys participate in direct competition for limited resources, suggesting there is no niche partitioning occurring. This direct competition also explains a decrease in juvenile recruitment and reproductive success, as immature reds will die due to lack of resources before reaching sexual maturity.
Poxvirus In addition to competition for resources, the spread of the squirrelpox virus from grey squirrels to reds is thought to be a major factor in the decline of red squirrel populations. Grey squirrels do not die from the squirrelpox virus and can infect red squirrels causing the appearance of scabs and lesions on the face, feet, and genitals and eventually death. In Great Britain, grey squirrels have been able to spread 17 to 25 times faster through competitive exclusion of the red squirrel due to increased mortality of reds from the squirrelpox virus. In fact, the virus works so quickly in killing its host after infection that seeing a red squirrel with poxvirus is uncommon even though the disease may be highly prevalent in a red squirrel population. The full consequences of these interactions cannot be completely determined, though the difference in population density and weight of the two species may have an effect on energy flow in ecosystems.
Woodland and songbird species Grey squirrels also bear some of the responsibility for the decline of select woodland and songbird bird species as they sometimes feed on bird eggs and chicks.Bertolino, S., & Lurz, P. W. W. (2011). Callosciurus squirrels: worldwide introductions, ecological impacts and recommendations to prevent the establishment of new invasive populations. Mammal Review, 43(1), 22–33. https://doi.org/10.1111/j.1365-2907.2011.00204.x Although the grey squirrel is often blamed in part for this decline in the UK, this is probably not the case for populations introduced to Italy. In those populations, the greys mainly feed on "seeds, fruits, buds, flowers and, occasionally, on insects, switching from one resource to another according to seasonal availability".
Bark stripping Certain behaviour of the grey squirrel, such as bark stripping, also has negative effects on trees and timber plantations. This is thought to be biggest effect introduced squirrels have on island ecosystems. For example, Garry oak trees on Vancouver Island have become endangered due to urban development and the presence of invasive species including the grey squirrel. Scatter-hoarding of seeds by grey squirrels causes notching in the acorns of white oaks. Notching prevents germination, which can hinder natural forest regeneration. Bark stripping makes branches weaker and more susceptible to being broken off by wind or animals and can also cause secondary infection that may lead to death. Bark stripping makes trees more susceptible to red band needle blight (RBNB), which is caused by the asexual stage of the fungus Dothistroma septosporum. RBNB results in a necrotrophic infection that kills needle tissue in pine species. Phytophthora ramorum is another fungus affecting tree health.Shuttleworth, C. M., Lurz, P. W., Geddes, N., & Browne, J. (2012). Integrating red squirrel (Sciurus vulgaris) habitat requirements with the management of pathogenic tree disease in commercial forests in the UK. Forest Ecology and Management, 279, 167–175. https://doi.org/10.1016/j.foreco.2012.05.019 Trees between 10 and 40 years of age and certain species, including beech and sycamore, are more vulnerable than others. Grey squirrels also damage trees by eating their sap, suggesting this behaviour provides some nutritional benefit.
Predicted spread
Predicted spread throughout Europe Spatially explicit population dynamics models were used by Bertolino et al. 2008 to predict the spread of the grey squirrel populations present in Italy in 1996: the Piedmont, Genoa and Ticino populations. The models examined potential corridors that would allow grey squirrels to invade France and Switzerland and the time scale of such invasions.
In a scenario based on a random pattern of good, poor and mast-year seed crops (vegetation produces significant abundance of food resources), in the "best-case" scenario, grey squirrels will reach the Western Alps between 2026 and 2036, France between 2066 and 2071, and Switzerland between 2051 and 2066. In a scenario where poor-year seed crops were not included, the "worst-case" scenario, grey squirrels will reach the Western Alps by 2015, France by 2026, and Switzerland between 2031 and 2041.