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

Abstract

1. The regional synchrony hypothesis (RSH) states that synchrony in microtine abundance over large geographical areas is caused by nomadic avian predators 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 natural experiment to evaluate their influence on synchronizing voles at different spatial scales. Field vole populations fluctuated on a 3–4‐year cycle of 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 with a decrease in synchrony. We could find no change in synchrony during the period of the greatest decline in kestrel and short‐eared owl numbers (1980–97). 4. In Kielder, vole abundance has been shown to change in a wave‐like manner, 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 abundant vole‐eating raptor in our study area, and might have an equalizing influence on vole abundance over smaller areas if they foraged in a density‐dependent manner and responded functionally to increasing vole density. If this was the case, spatial variability in vole density should have been less in occupied than unoccupied owl territories, especially in years of low vole density when owls could take a larger proportion of the standing crop of voles. Even though tawny owls caught a significant proportion of the vole population, 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 unlikely to be caused by avian predators. Instead, it is more likely to be related to the pattern of clear‐cutting that has developed in Kielder, which restricts vole dispersal. If this assumption is correct, we would predict more widespread synchrony in vole abundance in first‐generation forests when extensive areas are planted over short periods of time, and this is supported by anecdotal evidence. These conclusions indicate that foresters may be able to manipulate the spatial dynamics of voles and vole predators by varying patch sizes within forests.

Landscape effects on temporal and spatial properties of vole population fluctuations

“…Avian predators have been suggested to be capable of inducing regional synchrony among small rodent populations (Ydenberg 1987;Korpimäki and Norrdahl 1989;Ims and Steen 1990; see also Petty et al 2000); this suggestion also has received experimental support (Norrdahl and Korpimäki 1996;Ims and Andreassen 2000). Heikkilä et al (1994) also suggested that stoats might be capable of synchronizing vole population fluctuations.…”

Spatially structured population dynamics in feral oilseed rape

“…Three potential causes of synchrony in population fluctuations over wide geographical areas have been identified: (i) environmental forcing (the 'Moran effect'; see Koenig 2002); (ii) local, non-random dispersal (Royama 1992;Ranta et al 1999;Bjornstad & Grenfell 2001); and (iii) predator-prey dynamics involving highly mobile, aggregating predators (Hassell 2000;Petty et al 2000). Environmental stochasticity causes temporal fluctuations in the intrinsic rate of increase and/or carrying capacity of local populations that are correlated in space (but not time), with the environmental correlation typically decreasing with distance (Lande et al 1999).…”

Generation of periodic travelling waves in cyclic populations by hostile boundaries

“…These are a major predator: across the UK as a whole, birds are responsible for about 30 per cent of vole predation [61], and data for Kielder Forest itself shows that tawny owls alone remove 10-15% of the vole population per annum [62]. To the best of my knowledge, there are no data on the spatial distribution of avian predation, but its expected localization near the reservoir edge would make this habitat boundary very hostile for voles, suggesting that this might be the cause of the observed PTW in vole abundance.…”