Seasonal regulation in fluctuating small mammal populations: feedback structure and climate

We studied fluctuating populations of six small mammal species in the Appalachian Plateau of Pennsylvania, USA for 20 yr. We analyzed the feedback structure of these species using statistical time series models for spring and autumn abundances. All species showed a seasonal density-dependent structure, and in five of them first-order feedbacks were dominant in winter and summer. Instead, southern red-backed voles (Clethrionomys gapperi) showed a different feedback structure during winter and summer. In three species (C. gapperi, Napaeozapus insignis and Peromyscus maniculatus), environmental factors were more important during summer, while the opposite pattern was found in Blarina brevicauda and Tamias striatus. Snowfall influenced positively the winter population growth rates of southern red-backed voles, whitefooted mice, woodland jumping mice and eastern chipmunks. We found seasonal differences in the effects of the small mammals assemblage on population growth rates of the two Peromyscus species. The common feedback structure between seasons observed in most of the species, particularly among voles and mice, points to a different feedback structure from northern cyclic small mammals. We conclude that a seasonal feedback structure dominated by intra- and inter-specific competitive interactions may be at the basis of the population dynamics of these species.