Aim This study investigates how founder size may affect local genetic diversity and spatial genetic structure of the invasive American eastern grey squirrel (Sciurus carolinensis) in European areas. It also examines whether dispersal propensity and invasion rate may be related to founder size, genetic diversity and structure.Location Piedmont, Italy; Northern Ireland, Northumberland and East Anglia, UK.Methods Across the invaded range in Europe, 315 squirrels from 14 locations, grouped in four areas, were sampled and examined at 12 highly polymorphic microsatellite loci. We estimated both genetic variation and population structure using AMOVA, Mantel tests and Bayesian analysis. We also estimated migration rates and range expansion rates.Results Genetic diversity varied in accordance with numbers of founders across populations. For instance, the Italian population had the smallest founder size and lowest genetic variability, whereas Northumberland had high values for both. Significant levels of genetic differentiation were observed in all the examined regions. Gene flow, migration and population range expansion rate were also higher in England and Ireland than in Italy.Main conclusions Populations descending from human-mediated releases of few individuals were more genetically depauperate and more differentiated than populations established from a greater number of founders. Propagule pressure is therefore a significant factor in squirrel invasions. There is a trend whereby larger founder sizes were associated with greater genetic diversity, more dispersal, less local genetic differentiation and faster range expansion rate in squirrels. These findings have important management implications for controlling spread rate of squirrels and other invasive species: good practice should prioritize preventing further releases and the merging of genetically distinct populations as these events can augment genetic diversity.

A new population of the invasive American Eastern grey squirrel (Sciurus carolinensis) has recently settled in central Italy from an accidental release in Perugia, Umbria in the early 2000s. The grey squirrel is known to compete with and exclude native red squirrels (S. vulgaris) in the British Isles and Northern Italy, so it represents a potentially important new conservation threat to the red squirrel subspecies of south and central Italy, S. vulgaris italicus and S. v. meridionalis, which are endemic to peninsular Italy. The grey squirrel population range in Perugia is currently expanding at a rate of about 0.29 km/year (SD 0.19), slower than grey squirrel invasions elsewhere in Europe. Nuclear DNA analysed at 12 different microsatellite loci revealed that the grey squirrels in Perugia have extremely low genetic diversity, consistent with a small founder size. Genetic assignment tests indicate that the Perugia population was founded by translocations from an established population in Piedmont, Italy. No genetic substructure is evident yet in the Perugia population. These results together have serious consequences for the management of the grey squirrel invasion in Perugia and the conservation of the red squirrel subspecies: the Perugia grey squirrel population should be eradicated if politically feasible; otherwise new releases of grey squirrels, especially from sources other than the Piedmont population, should be prevented because such introductions could increase genetic diversity, thereby potentially increasing population range expansion rate to the much higher levels seen for more diverse grey squirrel populations elsewhere in Europe.

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