PREDATION AND INTERSPECIFIC COMPETITION IN 2 MICROTUS VOLES

We studied predation and interspecific competition in two Microtus spe cies (M. agrestis and M. epiroticus) using long term trapping and data on predator diet in western Finland. In the study area, Microtus dens ities fluctuated from ten to 100-fold and predator densities fluctuate d more than ten-fold. M. epiroticus suffered more from predation than M. agrestis, probably because densities in M. epiroticus patches were higher than densities in M. agrestis patches making M. epiroticus more profitable prey for patch-searching predators. The proportion of M. e piroticus among all voles declined during the crash phase of the vole cycle, at least partly as a result of predation. Breeding female voles showed little spatial overlap and presumably competed for space at hi gh densities. At low densities, interference competition through preda tors may have been more important than space competition. The proporti on of the smaller species M. epiroticus in the total number of all vol es increased rapidly with increasing vole densities. This could be due to more aggregated way of life which gives an advantage in competitio n for space. Predation reduced the time for space competition to short periods, and decreased periodically the proportion of the more succes sful competitor. Thus, the coexistence of the two vole species seemed to be predator-induced. In this sense predation was a more important f actor.than interspecific competition.