Identifying mechanisms responsible for spatial synchrony in the dynamics of populations is the focus of much recent research (reviews in Bjørnstad, Ims & Lambin 1999; Koenig 1999; Caldow & Racey 2000). Specifically, research has focused on the contribution of climatic factors (Grenfell et al. 1998), dispersal (Kaitala & Ranta 1998; Baillie et al. 2000) and patterns of predation (Ims & Steen 1990) to patterns of synchrony, often extending over large areas (Ranta, Kaitala & Lindén 1995). The topic is of both fundamental and applied interest (Ormerod & Watkinson 2000). Indeed, the extent to which populations are synchronized impinges on the viability of fragmented populations and on the dynamics of pathogens.
In boreal regions, the hypothesis that predation by mustelids is responsible for the periodic high amplitude fluctuations in vole abundance has gained much support (reviews in Stenseth & Ims 1993; Korpimäki & Krebs 1996; Turchin & Hanski 1997). Spatial synchrony in vole abundance may be a consequence of predation by wide-ranging raptors (the regional synchrony hypothesis; RSH; Ydenberg 1987) that can respond both functionally and numerically to increasing vole abundance without a time-lag (Galushin 1974; Korpimäki 1985; Korpimäki 1986; Ydenberg 1987; Korpimäki & Norrdahl 1991; Korpimäki 1991a,b; Steen 1995; Norrdahl & Korpimäki 1996).
Support for the RSH comes from both spatially explicit models of predator–prey interactions and from experimental observations. Ims & Steen (1990) used a model to show that nomadic avian predators could cause population synchrony if they killed approximately 25% of the summer prey population and responded numerically, without a time lag, to spatial variations in vole abundance. Steen (1995) recorded the mortality rates of radio-collared root voles Microtus oeconomus, in a cyclic population still at peak density after the density in neighbouring areas had declined. He found that predators accounted for 82% of the mortality, of which approximately half was due to avian predators, and concluded that predation by nomadic avian predators was sufficient to cause population synchrony. Norrdahl & Korpimäki (1996) attempted to test the RSH experimentally by removing nest sites of some avian predators in five (four in some years) 3-km2 areas, 4–15 km apart, over 4 years in western Finland. Subsequently, spatial variations in prey density were higher among predator-reduction areas than among similar-sized controls, which suggested that avian predators might contribute to spatial synchrony. Further studies demonstrating that reductions in nomadic predator density can lead to higher variation in vole densities across a landscape would provide useful collaborative evidence for the RSH, particularly if the density of nomadic predators was reduced over large areas.
In this paper we focus on the role of vole-eating raptors in synchronizing field vole Microtus agrestis L. abundance in an extensive conifer forest in northern England. Here field voles inhabit the grassy vegetation in young conifer crops and numbers fluctuate on a 3–4-year cycle (Petty 1992, 1999; Petty & Fawkes 1997; Lambin, Petty & MacKinnon, 2000), similar to those in central Fennoscandia (Hanski, Hansson & Henttonen 1991). However, 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 (Lambin et al. 1998). We show that the most abundant nomadic vole-eating raptors have been in long-term decline, hence providing a natural experiment to investigate the role of these raptors in regulating vole synchrony. If these species are able to synchronize vole abundance over extensive areas, the RSH predicts that the decline should be associated with a decrease in spatial synchrony in vole abundance. We also investigated the ability of a sedentary vole-eating raptor to synchronize vole abundance over smaller areas.