Grae - Journal of Animal Ecology Alien species and...

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Journal of Animal Ecology 2004 73 , 26–35 © 2004 British Ecological Society Blackwell Publishing Ltd. Alien species and interspecific competition: effects of introduced eastern grey squirrels on red squirrel population dynamics JOHN GURNELL*, LUC A. WAUTERS*, PETER W. W. LURZ† and GUIDO TOSI‡ * School of Biological Sciences, Queen Mary, University of London, London E1 4NS, UK; Centre for Life Sciences Modelling, Porter Building, University of Newcastle, Newcastle upon Tyne NE1 7RU, UK; and Department of Structural and Functional Biology, University of Insubria, Varese, Via Dunant, I-21100 Varese, Italy Summary 1. Throughout much of Britain, Ireland and north Italy, red squirrels ( Sciurus vulgaris L.) have been replaced by alien grey squirrels ( S. carolinensis Gmelin) introduced from North America.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 competi- tion for food and space between red and grey squirrels and have showed that grey squir- rels 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 popu- lation 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 popu- lation size and eventually population extinction. Key-words : demographic processes, fecundity, invasive species, Sciurus carolinensis , Sciurus vulgaris . Journal of Animal Ecology (2004) 73 , 26–35 Introduction When resource availability is limited, one organism will have negative effects upon another by controlling access to, or by consuming, this resource. This defini- tion of competition by Keddy (1989), implies that (1) an important resource must be limited for competition to occur, and (2) the effects of competition operate primarily on the individual. Thus, at the level of the individual, it can affect reproduction negatively (e.g.
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