Predation and Noncyclicity in a Microtine Population in Southern Sweden

Abstract

Field data on density changes in field voles and consumption by eight predator populations suggested that the noncyclic pattern in the vole population in southern Sweden was attributable to predation. I tested experimentally whether the spring decline in this non-cyclic population of Microtus agrestis, could be ascribed to predation. In a grassland area of 0.5 ha, avian and mammalian predators (apart from weasels) were excluded by fencing. Voles could move freely in and out of the exclosure. Changes in vole numbers were estimated by mark-recapture. The spring decline was significantly less pronounced in the exclosure than in the control area and this difference was produced by a higher survival rate of voles in the exclosure. The small decline that occurred in the exclosure could be explained by weasel predation and by the fact that voles at the border of the fenced area were exposed to all predators as they moved outside. The results confirm earlier observations that predation is a primary cause of the spring decline in vole populations in southern Sweden. From a graphic analysis of a predator-prey model, Rosenzweig and MacArthur predicted that the following conditions would promote stability in a predator-prey system: 1) access to alternative food that can sustain predators when preferred prey becomes scarce, 2) predators characterized by low prey-capture efficiency, and 3) predator populations limited by other factor(s) than food (e.g., territorial behaviour). These predictions were in agreement with data from our study area. The vole population showed a high degree of stability (no multi-annual cyclicity). Predators of primary importance (certain generalist predators) had a rich supply of alternative food and these generalist predators fed on voles mainly when these were abundant and easily available. Finally, the size of these generalist predator populations was probably limited by territorial behaviour.