Journal of Animal
© 2004 British
Blackwel Publishing Ltd.
Alien species and interspecifc competition: eFFects
oF introduced eastern grey squirrels on red squirrel
JOHN GURNELL*, LUC A. WAUTERS*, PETER W. W. LURZ† and
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
Structural and Functional Biology, University of Insubria, Varese, Via Dunant, I-21100 Varese, Italy
Throughout much of Britain, Ireland and north Italy, red squirrels (
L.) have been replaced by alien grey squirrels (
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.
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 Ftness components in red squirrels that have consequence at the popu-
lation level: fecundity, residency and recruitment.
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.
±ecundity 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.
Overall, there was no signiFcant 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 Ftness 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.
: demographic processes, fecundity, invasive species,
Journal of Animal Ecology
When resource availability is limited, one organism
will have negative effects upon another by controlling
access to, or by consuming, this resource. This deFni-
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.