Cyclic dynamics in field vole populations and generalist predation

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 significant 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 field 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 significant 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 field 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 modified to account for the regular population cycles occurring in northern Britain.