Competition Between Peromyscus maniculatus and Microtus townsendii in Grasslands of Coastal British Columbia

Abstract

(1) Microtus sp. were removed from two grassland areas south of Vancouver, British Columbia, over three years in order to test the hypothesis that Microtus sp. competitively exclude Peromyscus maniculatus from grasslands. Social organization of Microtus on a third area was disrupted for two years and the demographic fate of P. maniculatus on these manipulated areas was compared. (2) Peromyscus populations increased dramatically to sixty-one mice/ha within two years of the continuous removal of Microtus from one grassland area. Peromyscus populations also increased dramatically to thirty-four mice/ha in an area where the social organization of the resident Microtus townsendii was disrupted by manipulation of the sex-ratio of Microtus toward a shortage of females. Virtually no Peromyscus lives on the control area which had a normal Microtus population. (3) In the absence of Microtus the trappability of male Peromyscus was 9% higher than in the presence of Microtus. (4) Survival of Peromyscus was remarkably high on all study areas, especially in winter and spring. There were no indications of a decline in survival rates with the onset of breeding. (5) Reproduction of Peromyscus appeared to start earlier in the year and last later in autumn in the absence of Microtus. (6) When Microtus were allowed to recolonize an experimental grid, the number of Peromyscus dropped dramatically within six weeks. The cause of the decline in the number of Peromyscus was traced to an almost complete exclusion by Microtus of newly recruiting Peromyscus and an increase in mortality or dispersal in adult Peromyscus.