Competitive interactions between Microtus californicus and Reithrodontomys megalotis during two peaks of Microtus abundance.

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 also occurred 3-4 months earlier in the year when vole numbers were rapidly increasing. Reproductive activity was seasonally complementary in the 2 species. Data support the hypothesis that interference between Microtus and Reithrodontomys may be relaxed during most years. Behavioral interactions intensity when Microtus density increases, however, and interspecific competition during vole irruptions is severe. -from Authors