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

We studied fluctuating populations of six small mammal species in the Appal achian Plateau of Pennsylvania, USA for 20 yr. We analyzed the feedback str ucture of these species using statistical time series models for spring and autumn abundances. All species showed a seasonal density-dependent structu re, and in five of them first-order feedbacks were dominant in winter and s ummer. 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 f actors were more important during summer, while the opposite pattern was fo und in Blarina brevicauda and Tamias striatus. Snowfall influenced positive ly the winter population growth rates of southern red-backed voles, whitefo oted mice, woodland jumping mice and eastern chipmunks. We found seasonal d ifferences in the effects of the small mammals assemblage on population gro wth rates of the two Peromyscus species. The common feedback structure betw een seasons observed in most of the species, particularly among voles and m ice, points to a different feedback structure from northern cyclic small ma mmals. We conclude that a seasonal feedback structure dominated by intra- a nd inter-specific competitive interactions may be at the basis of the popul ation dynamics of these species.