A grey future for Europe: Sciurus carolinensis is replacing native red squirrels in Italy.
Introduced mammals can cause extinction of native species due to replacement competition, disease, predation or hybridization. We studied the colonization of Piedmont (NW-Italy) by American grey squirrel (Sciurus carolinensis) and its effect on the native red squirrel (Sciurus vulgaris). Presence/absence data (2×2 km2), of both species were (re)constructed using questionnaires, literature, existing databases, unpublished information, and direct monitoring with hair-tubes. In 1970 red squirrels were still widespread and greys were restricted to forests near the introduction site. By 1990, grey squirrels had increased their range to 220 km2, which coincided with the disappearance of native squirrels from 33 squares inside this range. The invasive species continued its spread occupying an area of 2,016 km2 in 2010; within this area red squirrels went extinct in 88 squares. Overall, from 1970 to 2010 red squirrel went extinct in 62% of 2×2 km2 (ca. 1,689 km2), and were replaced by grey squirrels. The spread of the alien species was slow in the first 20 years, but doubled in the successive two decades. Nevertheless spread was slower than in Ireland and England. Grey squirrel adapt to climate and habitats in both North and South Europe, causing extinction of the native red squirrel. A EU LIFE co-funded project with the aim to control the grey squirrel in North Italy and recent trade-restrictions and trade-ban are a first step in reducing the risk of grey squirrels invading other countries, but their effectiveness will have to be evaluated.