Summary1. Voles in northern Europe have been shown to exhibit cyclic population dynamics, with a latitudinal gradient in cycle length, amplitude and interspecific synchrony. 2. Previous studies have been based on a relatively sparse network of sampling sites. In the absence of spatially comprehensive long-term records of vole dynamics, we analysed a proxy of vole density, bird-ringing data on vole-eating avian predators, Tengmalm's owl ( Aegolius funereus L.), the Ural owl ( Strix uralensis Pall.), the long-eared owl ( Asio otus L.) and the rough-legged buzzard ( Buteo lagopus Pontoppidan) to study spatial population dynamics of voles. 3. We demonstrate that the breeding success of the avian predators is highly dependent on the abundance of voles, which is also reflected in the numbers of nestlings ringed in a particular area in each year. 4. Our results show the expected increase in cycle length from south to north in Finland, but also from west to east, and in contrast to previous studies, increasing irregularity of the cyclic dynamics towards the north. 5. Fluctuations of vole populations have been synchronous over large distances, up to several hundred kilometres. Such large-scale synchrony is more likely to be caused by movements of vole-eating predators and/or by climatic perturbations than by dispersal of voles. 6. We could not conclusively verify the recent suggestion that vole population dynamics have become less regular across Finland, although certain long-term changes are apparent.

Small rodents are key species in many ecosystems. In boreal and subarctic environments, their importance is heightened by pronounced multiannual population cycles. Alarmingly, the previously regular rodent cycles appear to be collapsing simultaneously in many areas. Climate change, particularly decreasing snow quality or quantity in winter, is hypothesized as a causal factor, but the evidence is contradictory. Reliable analysis of population dynamics and the influence of climate thereon necessitate spatially and temporally extensive data. We combined data on vole abundances and climate, collected at 33 locations throughout Finland from 1970 to 2011, to test the hypothesis that warming winters are causing a disappearance of multiannual vole cycles. We predicted that vole population dynamics exhibit geographic and temporal variation associated with variation in climate; reduced cyclicity should be observed when and where winter weather has become milder. We found that the temporal patterns in cyclicity varied between climatically different regions: a transient reduction in cycle amplitude in the coldest region, low-amplitude cycles or irregular dynamics in the climatically intermediate regions, and strengthening cyclicity in the warmest region. Our results did not support the hypothesis that mild winters are uniformly leading to irregular dynamics in boreal vole populations. Long and cold winters were neither a prerequisite for high-amplitude multiannual cycles, nor were mild winters with reduced snow cover associated with reduced winter growth rates. Population dynamics correlated more strongly with growing season than with winter conditions. Cyclicity was weakened by increasing growing season temperatures in the cold, but strengthened in the warm regions. High-amplitude multiannual vole cycles emerge in two climatic regimes: a winter-driven cycle in cold, and a summer-driven cycle in warm climates. Finally, we show that geographic climatic gradients alone may not reliably predict biological responses to climate change.

Most small rodent populations in the world have fascinating population dynamics. In the northern hemisphere, voles and lemmings tend to show population cycles with regular fluctuations in numbers. In the southern hemisphere, small rodents tend to have large amplitude outbreaks with less regular intervals. In the light of vast research and debate over almost a century, we here discuss the driving forces of these different rodent population dynamics. We highlight ten questions directly related to the various characteristics of relevant populations and ecosystems that still need to be answered. This overview is not intended as a complete list of questions but rather focuses on the most important issues that are essential for understanding the generality of small rodent population dynamics.

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We investigated the functional response of the least weasel (Mustela nivalis nivalis) in a series of experiments conducted in large outdoor enclosures (0.5 ha). Radiocollared Microtus voles were released in the enclosures at different densities (4, 8, 16 and 100/ha) three days before the release of a radiocollared weasel. During the three‐day experiment every vole killed was replaced with another one as soon as possible to retain constant prey density. The results demonstrated type II functional response with the predation rate reaching 50% of the asymptotic rate at a vole density of 15 individuals per ha. More voles were killed at the highest densities than would be expected from the known energy demands of weasels. Female and male voles were killed in proportion to their abundances in the enclosures, and no difference in predation risk was detected between voles released in the enclosure before the weasel (residents) and during the experiment (transients).