Alien species and interspecific competition: effects of introduced eastern grey squirrels on red squirrel population dynamics
Article first published online: 6 JAN 2004
Journal of Animal Ecology
Volume 73, Issue 1, pages 26–35, January 2004
How to Cite
Gurnell, J., Wauters, L. A., Lurz, P. W. W. and Tosi, G. (2004), Alien species and interspecific competition: effects of introduced eastern grey squirrels on red squirrel population dynamics. Journal of Animal Ecology, 73: 26–35. doi: 10.1111/j.1365-2656.2004.00791.x
- Issue published online: 6 JAN 2004
- Article first published online: 6 JAN 2004
- Received 3 December 2002; accepted 16 June 2003
- demographic processes;
- invasive species;
- Sciurus carolinensis;
- Sciurus vulgaris
- 1Throughout much of Britain, Ireland and north Italy, red squirrels (Sciurus vulgaris L.) have been replaced by alien grey squirrels (S. carolinensis Gmelin) introduced from North America.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.
- 2We 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.
- 3Litter 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.
- 4Fecundity 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.
- 5Overall, 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.