Predation as a Regulating Factor on Small Rodent Populations in Southern Sweden

Abstract

Predation rate on non-cyclic Microtus agrestis and Apodemus sylvaticus populations was calculated from data on numbers, food spectra, and food demands of the mammalian and avian predators present. Annual predation on rodents was of the same magnitude as the calculated annual production for each of the rodent populations. A marked decrease in rodent numbers from autumn to spring equalled the numbers eaten in the same period. A high predation rate at the beginning of the breeding season delayed and reduced the rodent populations' annual recovery. At peak density in autumn predation rate was density-dependent, caused by a marked switch in the predators' diet, especially in some facultative rodent predators. The data strongly support the hypothesis that predation is the primary cause of non-cyclicity in the examined rodent populations. Predation pattern found over the year in this study was similar to that described for various phases of cyclic microtine populations, but the predators' response to increased vole numbers in this study was mainly a functional one (switching) with a short time delay, whereas for cyclic populations the numerical response with a considerable time delay predominates. Important prerequisites for the important role of predation seem to be: 1) a rich supply of alternative prey sustaining a high and constant predator density (facultative rodent predators played a predominating role), 2) availability of small rodents for most of the year, 3) a heterogeneous environment influencing dispersal and availability of rodents.