Do founder size, genetic diversity and structure influence rates of expansion of North American grey squirrels in Europe?

Abstract

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.

Genetic patterns reveal an old introduction event and dispersal limitations despite rapid distribution expansion

“…Similar spatiotemporal patterns of genetic diversity have also been observed in other invasive species that rapidly expand their range (Dong et al 2013, Schulte et al 2013, Signorile et al 2014.…”

Geographical range expansion of alien birds and environmental matching

“…One of the most consistent is the numbers of individuals introduced, or propagule pressure (Duncan et al 1999, Cassey 2001, Signorile et al 2014. One of the most consistent is the numbers of individuals introduced, or propagule pressure (Duncan et al 1999, Cassey 2001, Signorile et al 2014.…”

Mixture or mosaic? Genetic patterns in UK grey squirrels support a human‐mediated ‘long‐jump’ invasion mechanism

“…The observed population structure in the UK and Ireland appears mostly to be a consequence of genetic drift: it should be expected that the translocated propagules should, on average, constitute the same proportions of diversity as the source, except where stochastic processes such as drift have occurred. Our results indicate that significant levels of differentiation arise from the translocations from a single introduced population, confirming previous studies (Signorile et al ., ). Genetic drift increases the level of differentiation among locations by randomly changing allelic frequencies; its effects are especially strong for populations with severe bottlenecks (Kidd & Cavalli‐Sforza, ).…”

“…Our study of the grey squirrel invasion of in the UK sheds new light on its role as an invasive species, with the surprising suggestions that it may not have the invasive potential one would predict by looking at the overall historic pattern of spread on its own. However, our results also indicate a potential threat, were subpopulations able to mix and increase their genetic diversity, leading to greater invasive potential (Signorile et al ., ).…”