Long-term population fluctuations of small mammals at the UWM Field Station

Population sizes of four small mammals, Peromyscus leucopus, Microtus pennsylvanicus, Blarina brevicauda, and Sorex cinereus, were monitored for over twenty years at the UWM Field Station. F\ leucopus had moderate fluctuations in size, but showed no evidence of cyclic or regular fluctuations. M. pennsylvanicus exhibited dramatic fluctuations with cycles of 3-5 years. Neither shrew species appeared to have cyclic fluctuations. The population sizes of the two shrew species were positively correlated with each other. No other significant correlations were found between species. Population sizes were probably influenced by a number of factors including predation, competition and weather. INTRODUCTION The population dynamics of small mammals have received considerable attention, primarily because of an interest in the occurrence of regular multiannual or cyclic population fluctuations (Krebs and Myers, 1974; Keith and Windberg, 1978; Taitt and Krebs, 1985; Hanski, 1987; Lidicker, 1988; Ostfield, 1988). Despite this interest, long-term studies (twenty years or longer) of fluctuations of small mammal population sizes are relatively rare. Here, we report on annual population size fluctuations over a twenty-two year period of four small mammal species at the UWM Field Station; two rodents (Rodentia): Peromyscus leucopus (white-footed mouse), Microtus pennsylvanicus (meadow vole); and two insectivores (Insectivora): Blarina brevicauda (short-tailed shrew), and Sorex cinereus (masked shrew). Peromyscus leucopus is a habitat generalist, being found in habitats ranging from grasslands to mature forest (Adler and Wilson, 1987), however, the species shows a preference for wooded habitats with complex vertical structure, including a definite shrub layer and fallen trees, stumps and logs (M'Closkey and Lajoie, 1975; Kaufman, et al., 1983). White-footed mouse populations have traditionally been thought to be relatively stable (Terman, 1968). Recent studies indicate, however, that populations of Peromyscus leucopus may exhibit