The impact of weasel predation on cyclic field-vole survival: the specialist predator hypothesis contradicted

1. The delayed density-dependent predation of specialists such as weasels (Mustela nivalis L.) may result in cycles in the abundance of their prey. We estimated the demographic impact of weasel predation on field-vole (Microtus agrestis L.) survival using capture-recapture data from a large-scale, replicated predator-manipulation experiment conducted in six unfenced populations in Kielder Forest, northern England. The density of weasels was experimentally reduced through continuous live-trapping in the three removal populations. Field-vole survival was compared between paired removal and control populations during the increase, peak and decline phases of a vole population cycle. Apparent survival and recapture probabilities were estimated using open population Cormack-Jolly Seber models.

2. Field-vole survival varied extensively both spatially between populations and temporally, being highest in the late autumn and over winter and lowest each year in spring and early summer. Patterns of variation in male and female survival were similar between populations over time, but there was independent spatial and temporal variation between adults and juveniles. Variation in weasel abundance explained 18% of this independent spatial and temporal variation between adult and juvenile field-vole survival.

3. The average increase in annual vole survival resulting from weasel removal over the 2-year period was 27% and 25% for adult male and female field voles, respectively. Decreased weasel abundance increased adult field-vole survival. Adult-vole survival in the absence of weasels was predicted to be approximately 8% higher than in the presence of one weasel per 4-5 ha. Surprisingly, weasel removal resulted in lowered juvenile field-vole survival, possibly reflecting increased emigration or mortality due to infanticide. A simple two age-class demographic model indicated that the decrease in juvenile survival in response to reduced weasel predation pressure did not fully compensate for the increase in adult survival.

4. As weasel numbers fluctuated seasonally, with highest weasel densities occurring in late summer and autumn and little spring to spring variation, the impact of weasel removal on field-vole survival was greatest during the breeding season. However, vole population-growth rates were unrelated to adult survival at that time and correspondingly no divergence was observed between the trajectories of control and removal vole populations even when weasel numbers were greatest in control sites. In contrast, vole population-growth rates were closely correlated with survival during the non-breeding season. Nevertheless all control and removal populations experienced a cyclic decline in winter 1999-2000 in spite of the increased adult survival in experimental treatments.

5. We conclude that the impact of weasel predation on field-vole survival was neither sufficient nor necessary to initiate and drive the cyclic decline of field-vole populations in Kielder Forest.