Small Rodents with Social and Trophic Interactions in a Seasonally Varying Environment

Abstract

Using a simulation model based on a consumer-resource set of differential equations, we have analyzed Hansson's hypothesis on rodent cycles, according to which both microtine-plant and predator-microtine interactions play essential roles in the rodent cycles of northern Fennoscandia, with interference competition (social regulation) for high-quality food limiting population growth at high densities, general forage depletion initiating the decline and specialist predators sustaining it. Our results suggest that Hansson's premises are sufficient for generating a multiannual predator-volevegetation cycle, provided that interference competition persists in all seasons. Without strong and persistent interference competition at high population densities, herbivores will, however, severely overexploit their winter resources and crash to virtually zero density. The system will then collapse into an essentially pure microtine-plant interaction. The role of interference competition is not to generate fluctuations but to keep them within the bounds required for the persistence of predators. Our success in generating a microtine cycle by the simulation was only partial. The four-year predator-microtine-plant cycle generated by the model consisted of three consecutive years of seasonal microtine-plant fluctuations and a fourth year when predation turned the normal spring rise into a further decline which was sustained until late autumn. Microtine fluctuations of roughly this kind have been reported from Alaska and northern Russia, although it is unclear whether they are regular. The typical Fennoscandian cycle has more distinct peaks and longer periods of low microtine densities. We suggest that spatial heterogeneity is essential for generating the kind of cycles which are normally seen in northern and central Fennoscandia.