Eastern grey squirrels in Europe

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A grey squirrel, Sciurus carolinensis, native to North America
A red squirrel, S. vulgaris, native to Europe, which has largely been displaced by the grey squirrel in some areas.

fungi, some insects and occasionally bird eggs.[2]

They were first introduced into England, in a concerted way, in 1876,[3] and through rapidly growing population and further introductions they spread to the rest of Great Britain by the early to mid-20th century.[4] 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.[1] 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.[5]

The native squirrel in Europe is the red squirrel (

Sciurus vulgaris). Both species have similar diets, use similar locations of the environment and have similar activity patterns; however, there is no evidence of noticeable aggression between the two species.[2] 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).[6] 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 of 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,[3] and through rapidly growing population and further introductions they spread to the rest of Great Britain by the early to mid-20th century.[4]

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.[1] 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.[1] 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.[7]

Almost 80% of known grey squirrel introductions worldwide resulted in successful establishment in the new area.

parasites typically are not present in the area that the species has been introduced or has invaded.[6]
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.

niche partitioning occurring.[8] 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

poxvirus is uncommon even though the disease may be highly prevalent in a red squirrel population.[6] 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.[2]

Woodland and songbird species

Grey squirrels also bear some of the responsibility for the decline of select woodland and

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".[11]

Bark stripping

Certain behaviour of the grey squirrel, such as bark stripping, also has negative effects on trees and timber

sap, suggesting this behaviour provides some nutritional benefit.[2]

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

energy requirement than red squirrels.[9] It may be difficult for the grey squirrels to attain their higher energy requirement in this environment because they may have difficulty reaching the small seeds contained in the conifers' cones; for this reason, grey squirrel performance in this habitat will affect the spread of the species.[9] Eventually, if populations are left unchecked, the grey squirrel will spread through a large portion of continental Europe and Eurasia, potentially invading the entire global distribution of the red squirrel.[1]

Predicted spread of the poxvirus and damage to trees

The

fungal growth resulting from the bark-stripping behaviour of the grey squirrel.[12]

Continued spread after attempted eradication

In 1991, eradication of a grey squirrel population in Piedmont was attempted via trapping and euthanasia; however, this effort was suspended for three years after animal rights groups filed a lawsuit.[7] During the three-year suspension, the grey squirrel population had grown so much that it was no longer feasible to attempt eradication by the methods initially used.[7] If the species spreads significantly (the three populations in Italy invade France or Switzerland and establish successful populations outside of Italy) before preventative or eradicative efforts are taken, it will, in all probability, be too late for effective action.[11]

Natural predator

European pine martens have kept grey squirrel populations in check in Ireland

The European pine marten is a natural predator of squirrels, including the eastern grey squirrel, but has been eradicated from large parts of the area affected by the grey squirrel such as England and Wales.[13] The European pine marten's population is currently expanding and there is evidence that grey squirrel populations plummet where it becomes present, with the populations of the co-evolved red squirrel subsequently recovering.[14][15]

Proposed plans for control or eradication

Although the negative consequences of introduction of non-native species are generally known in the scientific community, the majority of the public is unaware of the effects on native ecosystems.[16] Effective control of the spreading of grey squirrels throughout continental Europe will require a coordinated approach of many nations, and ecologists recommend public education and trade restrictions as primary means to prevent new introductions of non-native species.[9][11] Currently, the release of non-native species is illegal in most European countries, but both illegal and allowed importations and escapes of grey squirrels from captivity pose threats to native red squirrel and tree populations.[11] In addition to preventing introduction of grey squirrels, containment and eradication could serve to control the species’ invasive actions. Methods for grey squirrel population control depend on the target of protection. In the presence of red squirrels, cage trapping or shooting can be effective. In the absence of red squirrels, poisoning by the anticoagulant Warfarin can be used in special feeding hoppers.[8] These methods are currently effectively controlling the grey squirrel population in Britain.[2] Selectively timed culls, or removal of a percentage of the breeding individuals, of grey squirrel populations can help control and prevent tree damage. Although grey squirrel populations readily recover from culls, their coincidence with annual periods of the greatest damage to trees, typically between April and September when the squirrels are most active in foraging among broadleaf tree populations, can minimize tree damage.[8] Alternatively, non-lethal methods of population control, such as fertility treatments, are being developed, although the effectiveness of these treatments is unknown.[8] In areas most threatened by spread of grey squirrels, eradication may be more effective than population containment. Eradication of the population of grey squirrels in

ecologists, their success will depend on public opinion, governmental support, and awareness of the situation.[16]

References

  1. ^ a b c d e Bertolino, S. & Piero, G. (2003) Spread and attempted eradication of the grey squirrel (Sciurus caolinensis) in Italy, and consequences for the red squirrel (Sciurus vulgaris) in Eurasia. Biological Conservation, 351-358
  2. ^ a b c d e f Sandro, B. (2008) Introduction of the American grey squirrel (Sciurus carolinensis) in Europe: a case study in biological invasion. Current Science, vol. 95, No. 7, 903-906.
  3. ^ a b Hickman, Leo (2012-09-05). "If you want red squirrels, you have to kill greys". The Guardian. London. Retrieved 2015-01-31.
  4. ^
    S2CID 27529608
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  5. ^ "Don't blame grey squirrels: Their British invasion had much more to do with us | Imperial News | Imperial College London". 26 January 2016.
  6. ^ a b c d e f g Strauss, A., White, A., & Boots, M. (2012) Invading with biological weapons: the importance of disease-mediated invasions. Functional Ecology, 26, 1249-1261.
  7. ^ a b c d e f g Simberloff, D. (2008) Rats are not the only introduced rodents producing ecosystem impacts on islands. Biological Invasions, 1735-1742.
  8. ^ a b c d e f g h i j Lawton, C., Cowan, P., Bertolino, S., Lurz, P.P.W. & Peters, A.R. (2010) The consequences of introducing non-indigenous species: two case studies, the grey squirrel in Europe and the brushtail possum in New Zealand. Rev. sci. tech. Off. int. Epiz., 29 (2), 287-297.
  9. ^ a b c d e f g h i j k l m Bertolino, S., Lurz, P.W.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, 2564-2575.
  10. ^ a b Gossa, A., La Morgia, V., Marchisio, P., et al. (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, 221-234.
  11. ^ a b c d e f Bertolino, S. & Lurz, P. (2011) Callosciurus squirrels: worldwide introductions, ecological impacts and recommendations to prevent the establishment of new invasive populations. Mammal Review, 22-33.
  12. ^ a b Craig M. Shuttleworth, C. M.; Lurz, P.; 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.
  13. S2CID 254286641
    .
  14. ^ George Monbiot, How to eradicate grey squirrels without firing a shot, Guardian, January 30, 2015.
  15. ^ Emma Sheehy & Colin Lawton, Population crash in an invasive species following the recovery of a native predator: the case of the American grey squirrel and the European pine marten in Ireland, Biodiversity and Conservation, March 2014, Volume 23, Issue 3, pp. 753-774.
  16. ^ a b Shine, R. & Doody, J. S. (2011) Invasive species control: understanding conflicts between researchers and the general community. Frontiers in Ecology and the Environment 9: 400–406.
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