Insights From Observations and Large-Scale Field Experiments on Vole Population Cycles in Northern Europe: A 40-Year...

The mechanisms driving 3-5-year population cycles of voles involve delayed density-dependent feedback on vole populations. The key drivers of this feedback include prolonged periods of food depletion or predation mortality over more than one phase of the cycle. We review observational and experimental data gathered between the 1970s and 2010s on vole population fluctuations and the responses of their avian and mammalian predators in west-central Finland, focusing on studies that have investigated these drivers. Least weasels and stoats were the main predators of voles, causing 77% of all kills, while 22% were killed by avian predators. The numbers of least weasels tracked vole densities with a 9-12-month lag, which resulted in delayed density-dependent kill rates of voles in winter. Experimental reduction of small mustelids and avian predators in unfenced areas (each 2.5-3 km²) prevented the cyclic decline of vole densities in the subsequent summer, whereas in areas with only least weasel reduction and in control areas, a decline in vole densities occurred. In another field experiment, the reduction of both mustelid and avian predator densities increased the autumn density of Microtus voles fourfold during the low phase of the cycle, accelerated the increase twofold, increased the autumn density of voles twofold in the peak phase, and delayed the initiation of decline. Our unique experimental results suggest that the collective impact of both mustelid and avian predators is a likely mechanistic explanation for high-amplitude population cycles of voles in North Europe. In these highly seasonal environments with short summers, a shortage of high-quality winter food may be the directly density-dependent factor stopping the growth of vole populations. This allows predators to catch up with prey densities and impose population decline and prolong the low phase of the cycle in a delayed density-dependent manner.