Genetic determinants of the expansion of eastern grey squirrel populations across Europe

Abstract

Invasive alien species have long been recognized as a major global threat to biodiversity and ecosystems. A key question related to biological invasions is whether some species or populations within species are more invasive than others and if so, what are the biological and ecological factors responsible for such differences. In this project, I focus on a highly invasive mammalian species, the American eastern grey squirrel, introduced to Europe multiple times, a competitor to the native red squirrel and a forest pest. I investigate whether factors such as genetic variation, inbreeding, genetic drift, and founder effects can interfere with the¬¬ grey squirrel dispersal processes. I used 12 highly polymorphic microsatellites as a tool to address my questions and examined 60 populations from England, Scotland, Wales, Ireland, Italy and the USA, with a total of 1434 individuals. The results indicate that there is a trend whereby larger founder sizes had greater genetic diversity, less local genetic differentiation, and faster rate of range expansion. Genetic diversity is positively correlated to rates of spread. The results also provide evidence that human-mediated translocations have played a major role in the spread of this species across the UK, and that there remains localized genetic structure reflecting the historical genetic differentiation of original propagules and subsequent genetic drift. Given the existing genetic structure across Europe, I explored the potential application of DNA forensics to identify the sources of new translocations, examining how the approach might be adapted from conservation applications. These findings have important implications for the managing of grey squirrels and other invasive species: good practice should focus on preventing future translocations and the subsequent merging of currently genetically distinct populations.