Competitive Interactions between Microtus californicus and Reithrodontomys megalotis during Two Peaks of Microtus Abundance

Abstract

The responses of a population of Reithrodontomys megalotis to fluctuations in density of Microtus californicus were examined at Russell Reservation, Contra Costa Co., California. Reithrodontomys increased in abundance on four permanently established grids during years of low Microtus density but declined to very low numbers or local extinction when vole populations irrupted. The annual spring decline in abundance of harvest mice noted at Russell Reservation also occurred three to four months earlier in the year when vole numbers were rapidly increasing. Reproductive activity was found to be seasonally complementary in the two species. The hypothesis that Microtus density is negatively correlated with Reithrodontomys reproductive activity could not be tested due to the small number of harvest mouse captures during the period of peak vole abundance. However, the only female Reithrodontomys captured during this time that was reproductively active was taken from a 3- × 4-m area from which Microtus had been excluded. Analyses of space use patterns indicated a random association between the two species at low Microtus density, but an increasingly negative association as vole density increased. These data support the hypothesis that interference between Microtus and Reithrodontomys may be relaxed during most years. Behavioral interactions intensify when Microtus density increases, however, and interspecific competition during vole irruptions is severe.