Population cycles in small mammals: The alpha-hypothesis

Causes of cyclic fluctuations in abundance (population cycles) of some smal l-mammal populations remain poorly understood despite 6 decades of research and >20 hypotheses. Population cycles are demographic processes and cannot be fully explained without considering demographic mechanisms that underli e cyclic fluctuations in abundance. From simulation studies, we have recent ly shown that phase-related, density-dependent changes in age at maturity, abetted secondarily by changes in juvenile survival, are likely the main de mographic causes of cyclic fluctuations in population size. The suggested m echanism of population cycles is based primarily on changes in age at matur ity tot (alpha); we refer to this idea as the alpha -hypothesis. Here, we f ully develop the alpha -hypothesis and present a testable, demographically based, mechanistic explanation of population cycles. The alpha -hypothesis identifies the demographic basis of population cycles and provides a mechan istic explanation of how changes in key demographic variables (age at matur ity and juvenile survival) might cause cyclic fluctuations in abundance and biologic attributes of the cycles. The alpha -hypothesis is supported by, and logically consistent with, empirical patterns of life history and dynam ics of cyclic populations of small mammals. Future research should focus on empirically determining causes of phase-related changes in age at maturity and juvenile survival.