Ecological and Genetic Determinants of the Expansion of Grey Squirrel Populations in Italy and Britain

Invasive species are a major threat to global biodiversity and are widely associated with human introductions. The American Grey squirrel, following multiple introductions into Europe, has had a massive impact on native red squirrel populations in the UK, Italy and Ireland, causing major shifts in the red squirrel's range. In addition, grey squirrels in the UK have had an important impact on agroindustry by debarking trees in plantations. The species is seen as likely to disperse widely from Italy, displacing red squirrels and harming agroindustry across Europe. Under a worst-case scenario, models predict the expansion of Italian grey squirrels into France and Switzerland within 20-30 years. Hypotheses. However, models of grey squirrel expansion have been parameterized approximately, and Italian, British and Irish populations of grey squirrels have expanded at substantially different rates for unknown reasons. These populations originated from very different numbers of introduced individuals (and hence have different genetic diversity), and are subject to different ecologies. Reduced red squirrel body mass and recruitment has been linked to the presence of greys. Associated declines in red populations due to competition with greys are accentuated by a poxvirus disease lethal to reds, but carried by and not harmful to greys. The poxvirus is present in the UK and absent from Italy, suggesting the hypothesis that UK expansion is faster than in Italy because poxvirus kills local reds, eliminating a competitor that otherwise would slow the greys' advance. However, competition could well be asymmetrical; it has not been demonstrated that competition with reds reduces the vital rates of greys. An alternative hypothesis is that low genetic variation and inbreeding depression slows the growth of the Italian grey squirrel population, relative to British populations. In a large study of invasive species, founder population size was identified as a key factor in invasive success; the founder population sizes of grey squirrels in the UK were much larger than those in Italy. The main goal of the research is to test these hypotheses, restated here: 1) Different expansion rates of grey squirrels in Italy and Britain are due primarily to different competitive environments faced by the greys, partly because competition from reds is eliminated or reduced by pox virus in the UK. 2) Different expansion rates are due primarily to different levels of genetic diversity in the grey squirrel populations in Italy and the UK. The relative contributions of both factors to expansion rates will be assessed. Results may have major management implications. Methods. Methods will include microsatellite analysis of DNA samples from Italian and British squirrels, and trapping programs and other field work at two Italian and two British sites, one in each country with greys only, and one with greys and reds. Management implications. If expansion rate of grey squirrels in Italy is strongly limited by low genetic diversity, it will be important to prevent the three separate Italian populations from merging and thereby increasing diversity and speeding expansion rate. This could be accomplished by eradicating the two populations which are still small enough for eradication. It would also be essential to prevent all further introductions, even of a few squirrels. If expansion rate is limited by competition from red squirrels, it will be essential to prevent the introduction of poxvirus into Italy. Model-based estimates of expansion patterns are an important tool for conservation policy. The proposed research may help to parameterize models more precisely and increase effectiveness of future conservation actions.