Alien species and interspecific competition: effects of introduced eastern grey squirrels on red squirrel population dynamics

Abstract

Summary1. Throughout much of Britain, Ireland and north Italy, red squirrels ( Sciurus vulgaris L.) have been replaced by alien grey squirrels ( S. carolinensis Gmelin) introduced from North America. We have studied squirrels in two mixed woodlands in north Italy and two conifer forests in north England. In each country, one site was occupied by red squirrels and one site by both species. 2. We have previously considered interference competition and exploitation competition for food and space between red and grey squirrels and have showed that grey squirrels caused reduced body growth in juvenile and subadult red squirrels, and compete for tree seeds cached by adult red squirrels in spring. Here we report on the effects of grey squirrels on three fitness components in red squirrels that have consequence at the population level: fecundity, residency and recruitment. 3. Litter production peaked in the spring and summer, but fewer females bred in the summer with grey squirrels present. In addition, fewer individual red squirrel females produced two litters per year in the sites with grey squirrels. Moreover, red squirrel recruitment rate and, in the mixed broadleaf sites, red squirrel juvenile residency, decreased with increasing grey squirrel density. 4. Fecundity of individual female red squirrels was lower in red-grey than in red-only sites because they had a lower body mass in sites with grey squirrels. 5. Overall, there was no significant effect of grey squirrels on residency of adult red squirrels or on population turnover rate. However, the presence of grey squirrels resulted in a reduction in red squirrel fitness which was evident by lower population summer breeding and a lower recruitment. Over time, this will result in a decline in population size and eventually population extinction.

Coexistence and Spread of Competitors in Heterogeneous Landscapes

“…Since there is great variation in landscape structure between urban, agricultural and wooded areas, one may wonder why coexistence seems impossible. Gurnell et al (2004) conclude that the grey squirrel can outcompete the red squirrel in every life stage, and that it is more resistant to certain diseases. Hence, landscape heterogeneity does not translate into patches where the red squirrel would fare considerably better.…”

Locating species range frontiers: a cost and efficiency comparison of citizen science and hair-tube survey methods for use in tracking an invasive squirrel

“…The eastern grey squirrel (Sciurus carolinensis), native to eastern North America, has been frequently introduced to locations worldwide (Bertolino 2009) where it often easily establishes (Wood et al 2007) to the detriment of native sciurid populations (Kenward and Holm 1993;Gurnell et al 2004b;Sainsbury et al 2008). The grey squirrel was introduced to one site in the Republic of Ireland in 1911 (Watt 1923;O'Teangana et al 2000a), via populations in Great Britain (Fig.…”

Feeding of grey squirrels (Sciurus carolinensis) with the contraceptive agent DiazaCon™: effect on cholesterol, hematology, and blood chemistry

“…In areas where both species are present, grey squirrels raid red squirrel caches of seed, leading to reduced reproductive and juvenile recruitment rates for red squirrels (Wauters et al 2002;Gurnell et al 2004). Grey squirrels also carry Squirrelpox virus, which is lethal to red squirrels but has little effect on grey squirrels (Rushton et al 2000;Sainsbury et al 2000;Tompkins et al 2002;Thomas et al 2003;McInnes et al 2006).…”