Suggestions of collapse in small herbivore cycles since the 1980s have raised concerns about the loss of essential ecosystem functions. Whether such phenomena are general and result from extrinsic environmental changes or from intrinsic process stochasticity is currently unknown. Using a large compilation of time series of vole abundances, we demonstrate consistent cycle amplitude dampening associated with a reduction in winter population growth, although regulatory processes responsible for cyclicity have not been lost. The underlying syndrome of change throughout Europe and grass-eating vole species suggests a common climatic driver. Increasing intervals of low-amplitude small herbivore population fluctuations are expected in the future, and these may have cascading impacts on trophic webs across ecosystems.

Summary 1.A geographical gradient in the relative impact of generalist and specialist predators on small rodent populations has been hypothesized to be responsible for the gradient in cyclicity found in Fennoscandia. Population oscillations resulting from weasel±vole interactions are said to be dampened by the increasing stabilizing impact of generalist predators in southern Fennoscandia resulting from: (i) a greater abundance and diversity of predators sustained by alternative prey; (ii) the absence of signi®cant snow cover leading to constant exposure of voles to generalist predators; and (iii) a heterogeneous habitat that makes dispersing voles more vulnerable to predators. 2. Changes in the abundance of ®eld voles (Microtus agrestis L.) in a man-made spruce forest in northern England were recorded during 1984±98 using sign indices at 14±18 sites calibrated with capture±recapture estimates of vole density. 3. Field vole populations exhibited cyclic dynamics which were in many ways similar to those reported from Fennoscandia, including population declines taking place during the breeding season and long periods with no recovery in numbers following population crashes. 4. The density dependence structure of the time series was explored by means of partial autocorrelation functions, which suggested second-order density dependence. Analyses based on two density estimates per year (spring and autumn) reveal signi®cant negative values for lags of 1, 1´5 and 2 years, suggesting that the time-lag might be somewhat shorter than 2 years. 5. Estimates of predation on ®eld voles by red foxes and tawny owls at high vole density were above the value predicted for this site and for the whole generalist predator community by a published model assuming that predation by generalist predators stabilizes vole populations. However, empirical estimates of the parameter used both for designing and testing the model are inherently imprecise. 6. A qualitative evaluation of the three variables (see 1) correlated to the Fennoscandian gradient and assumed to contribute to variations in generalist predation pressure did not support the hypothesis that low predation rates by generalist predators are necessary for vole dynamics to be dominated by the destabilizing impact of weasel±vole interactions. The specialist/generalist predation hypothesis must therefore be modi®ed to account for the regular population cycles occurring in northern Britain.

We demonstrate evidence for the presence of travelling waves in a cyclic population of field voles in northern Britain by fitting simple, empirical models to spatially referenced time series data. Population cycles were broadly synchronous at all sites, but use of Mantel correlations suggested a strong spatial pattern along one axis at a projection line 72 degrees from North. We then fitted a generalized additive model to log population density assuming a fixed-form travelling wave in one spatial dimension for which the density at each site was offset in time by a constant amount from a standard density-time curve. We assumed that the magnitude of this offset would be proportional to the spatial separation between any given site and the centroid of the sampling sites, where separation is the distance between sites in a fixed direction. After fitting this model, we estimated that the wave moved at an average speed of 19 km yr-1, heading from West to East at an angle of 78 degrees from North. Nomadic avian predators which could synchronize populations over large areas are scarce and the travelling wave may be caused by density-dependent dispersal by field voles and/or predation by weasels, both of which act at a suitably small spatial scale.

Tawny owls, Strix aluco, laid female-biased clutches on territories with more abundant prey (field voles) in June, the month that chicks fledge. This appeared to enhance the subsequent reproductive success of fledglings, as in 1995 there was a significant correlation between the number of chicks fledged by adult females and the June vole abundance in the territory on which they were reared as chicks. This relationship did not hold for males. Since tawny owls lay eggs in March, these results indicate that owls are able to predict the June vole numbers on their territory, and respond by producing more of the sex most likely to gain a long-term benefit when resources are good.