Journal of Animal Ecology
© 2007 The Authors. Journal compilation © 2007 British Ecological Society
Blackwel Publishing Ltd
The interaction of parasites and resources cause
crashes in a wild mouse population
Amy B. Pedersen
* and Timothy J. Greives
Department of Biology, University of Virginia, Charlottesville, VA 22904, USA; and
Department of Biology and Center for
the Integrative Study of Animal Behaviour, Indiana University, 1001 E. 3rd Street, Bloomington, IN 47401, USA
Populations of white-footed mice
and deer mice
increase dramatically in response to food availability from oak acorn masts. These populations
subsequently decline following this resource pulse, but these crashes cannot be explained solely by
resource depletion, as food resources are still available as population crashes begin.
We hypothesized that intestinal parasites contribute to these post-mast crashes;
infected by many intestinal parasites that are often transmitted by density-dependent contact and
can cause harm to their hosts. To test our hypothesis, we conducted a factorial experiment in natural
populations by supplementing food to mimic a mast and by removal of intestinal nematodes with
the drug, ivermectin.
Both food supplementation and the removal of intestinal nematodes lessened the rate and
magnitude of the seasonal population declines as compared with control populations. However, the
combination of food supplementation and removal of intestinal nematodes prevented seasonal
population crashes entirely.
We also showed a direct effect on the condition of individuals. Faecal corticosterone levels, an
indicator of the stress response, were signi±cantly reduced in populations receiving both food
supplementation and removal of intestinal nematodes. This effect was observed in autumn, before
the overwinter crash observed in control populations, which may indicate that stress caused by the
combination of food limitation and parasite infection is a physiological signal that predicts low
winter survival and reproduction.
This study is one of the few to demonstrate that the interaction between resource availability and
infectious disease is important for shaping host population dynamics and emphasizes that multiple
factors may drive oscillations in wild animal populations.
glucocorticoids, host–pathogen interactions, macroparasites,
, resource pulses.
Relatively few empirical studies have attempted to assess the
role of infectious disease in natural populations. This is
despite the fact that diseases often exhibit density-dependent
transmission and have harmful effects on host ±tness
(Anderson & May 1992). Theoretical models suggest that
macroparasites can cause oscillations in abundance
(Anderson & May 1979; Anderson 1980), and a manipulative
±eld experiment has strongly supported this hypothesis as
an explanation for population cycles in red grouse
(L.) (Hudson, Dobson & Newborn 1998). It