Summary

1. The causes of cyclical fluctuations in animal populations remain a controversial topic in ecology. Food limitation and predation are two leading hypotheses to explain small mammal population dynamics in northern environments. We documented the seasonal timing of the decline phases and demographic parameters (survival and reproduction) associated with pop-ulation changes in lemmings, allowing us to evaluate some predictions from these two hypotheses. 2. We studied the demography of brown lemmings (Lemmus trimucronatus), a species show-ing 3- to 4-year population cycles in the Canadian Arctic, by combining capture–mark–recap-ture analysis of summer live-trapping with monitoring of winter nests over a 10-year period. We also examined the effects of some weather variables on survival. 3. We found that population declines after a peak occurred between the summer and winter period and not during the winter. During the summer, population growth was driven by change in survival, but not in fecundity or proportion of juveniles, whereas in winter popula-tion growth was driven by changes in late summer and winter reproduction. 4. We did not find evidence for direct density dependence on summer demographic parame