EXPERIMENTAL REDUCTION OF PREDATORS REVERSES THE CRASH PHASE OF SMALL-RODENT CYCLES

Abstract

The mechanisms driving short-term (3–5 yr) cyclic fluctuations in densities of boreal small rodents, and especially, those causing a crash in numbers, have remained a puzzle, although food shortage and predation have been proposed as the main factors causing these fluctuations. In the first large-scale vertebrate predator manipulation experiment with sufficient replication, densities of small mustelids (the least weasel Mustela nivalis and the stoat M. erminea) and avian predators (mainly the Eurasian Kestrel Falco tinnunculus and Tengmalm’s Owl Aegolius funereus) were reduced in six different areas, 2–3 km2 each, in two crash phases (1992 and 1995) of the 3-yr cycle of voles (field vole Microtus agrestis, sibling vole M. rossiaemeridionalis, and bank vole Clethrionomys glareolus). The reduction of all main predators reversed the decline in density of small rodents in the subsequent summer, whereas in areas with least weasel reduction and in control areas without predator manipulation, small rodent densities continued to decline. That only reduction of all main predators was sufficient to prevent this summer crash was apparently because least weasels represent <40% of vole-eating predators in western Finland. These results provide novel evidence for the hypothesis that specialist predators drive a summer decline of cyclic rodent populations in northern Europe.