Parasites as weapons of mouse destruction.

Epidemiological models illustrate that parasites can drive fluctuations in host populations, but small-mammal ecologists have tended to reject parasitism as an important factor. In one of the few experimental tests of the impact of parasites on wildlife population dynamics, Pedersen and Greives reduced prevalence of intestinal nematodes in free-ranging populations of Peromyscus mice with and without access to supplemental food. In isolation, parasite removal and food addition reduced rates of population decline only modestly, but the two factors acted synergistically to strongly alleviate declines. It is tempting to disregard factors that, in isolation, have weak effects, but the results from Pedersen and Greives’ multifactor experiment remind ecologists that complex factor interactions pervade real ecosystems.