Additional Measurements of the Impact of Carnivores on California Voles (Microtus californicus)

Abstract

Predation by feral cats, raccoons, foxes, and skunks upon Microtus californicus was measured during the collapse of a dense population of voles and was found to agree well with measurements made during the preceding cycle. In 5 months the number of carnivores on the 35-acre study area dropped from 2.16 to 0.36, the number of Microtus present per carnivore varied between 224 and 880 in different months, and the percentage of the Microtus population eaten in each month varied from 33 to 5. The number of Microtus eaten per month per carnivore varied from 109 to 40. Predation pressure, measured by the ratio number of voles per number of carnivores was most intense at the end of the crash. As the vole population declined, the carnivores ate relatively more gophers, harvest mice, rabbits, birds, and reptiles. The data support the theory that carnivore predation during a crash and especially during the early stages of the subsequent population low determines to a large extent the amplitude and timing of the microtine cycle of abundance.