Mortality factors in a cyclic vole population.

The causes of cyclic fluctuations in microtine rodent populations are still a bone of contention. In particular, the actual causes of mortality in the different phases of the 3-4-year vole are an enigma. We present results from studies of radio-collared voles (Microtus agrestis, M. rossiaemeridionalis and Clethrionomys glareolus), which show that small mustelid predation was the major mortality factor of voles in the decline phase, but had less importance in the increase phase of the 3-year population cycle. After the initial decline in the non-breeding season (winter), vole-kill rate from predators increased to a point where mortality substantially exceeded the reproductive capacity of microtine prey. Our results suggest that predators may alone cause a decline in the density of these vertebrate-prey populations.