Eastern grey squirrels in Europe

Europe's Grey squirrels (Sciurus carolinensis), originated in North America, where they are known as eastern gray squirrels (to differentiate from western gray squirrels). They eat large seeds, flowers, buds, fruits, fungi, some insects, and occasionally bird eggs. 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. 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 Britain by the early-to-mid 20th century. 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. 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). 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. In fact, grey squirrels as a DMI species could be classified as "biological weapons".

Grey squirrels have been introduced either on purpose or by accident in many places throughout the world. 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 on purpose. For example in 1966, two females and one male escaped from a game farm on Vancouver Island.

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