Spatial synchrony in field vole Microtus agrestis abundance in a coniferous forest in northern England: The role of vole-eating raptors

1. The regional synchrony hypothesis (RSH) states that synchrony in microti ne abundance over large geographical areas is caused by nomadic avian preda tors that specialize on small mammals for food. This has proved a difficult hypothesis to test because experiments at an appropriate scale are almost impossible. 2. We used the decline of the most abundant, nomadic vole-eating raptors in an extensive conifer forest in northern England (Kielder Forest) as a natu ral experiment to evaluate their influence on synchronizing voles at differ ent spatial scales. Field vole populations fluctuated on a 3-4-year cycle o f abundance, similar to the periodicity in central Fennoscandia. 3. Over a 23-year period, the combined numbers and density of kestrels and short-eared owls significantly declined. If these raptors were responsible for synchronizing vole abundance, the decline should have been associated w ith a decrease in synchrony. We could find no change in synchrony during th e period of the greatest decline in kestrel and short-eared owl numbers (19 80-97). 4. In Kielder, vole abundance has been shown to change in a wave-like manne r, with synchrony in the direction of the wave being 5-10-fold smaller than that reported in Fennoscandia. Tawny owls are sedentary and the most abund ant vole-eating raptor in our study area, and might have an equalizing infl uence on vole abundance over smaller areas if they foraged in a density-dep endent manner and responded functionally to increasing vole density. If thi s was the case, spatial variability in vole density should have been less i n occupied than unoccupied owl territories, especially in years of low vole density when owls could take a larger proportion of the standing crop of v oles. Even though tawny owls caught a significant proportion of the vole po pulation, we could find no difference in variation in vole density between owl territories that were unoccupied, occupied with no breeding attempt, or occupied with a successful breeding attempt. 5. We conclude that the small-scale synchrony in field vole abundance is un likely to be caused by avian predators. Instead, it is more likely to be re lated to the pattern of clear-cutting that has developed in Kielder, which restricts vole dispersal. If this assumption is correct, we would predict m ore widespread synchrony in vole abundance in first-generation forests when extensive areas are planted over short periods of time, and this is suppor ted by anecdotal evidence. These conclusions indicate that foresters may be able to manipulate the spatial dynamics of voles and vole predators by var ying patch sizes within forests.