Synchronous short‐term population fluctuations of some birds and mammals in Fennoscandia ‐ occurrence and distribution

We examined population fluctuations of the voles Microtus agrestis L. and Clethrionomys glareolus Schreb. by biannual trapping; black grouse Tetrao tetrix L., mountain hare Lepus timidus L., and red fox Vulpes vulpes L. by using bag records from all over Sweden, questionnaires from south‐central Sweden, and population indices from Grimsö Wildlife Research Area in south‐central Sweden. Synchronous population fluctuations between voles as a group and the other species conformed to a 3–4 year periodic pattern in both autumn and spring populations only in northern and central Sweden (the boreal forest region). Spring populations of boreal forest grouse, hare, and probably also fox, lagged one year behind the voles. The northerly areas also formed geographical units of co‐fluctuation within each game species. Using our own data as well as reviewing previous studies in Fennoscandia, we conclude that synchronized 3–4 year population fluctuations of voles and medium‐sized herbivores are confined to the central and northern part of Fennoscandia, although voles may exhibit short‐term population fluctuations further south. The synchronizing link between the herbivores could be (1) food plant quality and/or quantity and/or (2) predation. We could not reject either of these two plausible mechanisms as a cause of interspecifically synchronous fluctuations.