Cyclic dynamics in field vole populations and generalist predation

Abstract

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.

Periodic travelling waves in cyclic populations: field studies and reaction–diffusion models

“…Spatio-temporal patterns can be partly revealed by measuring how the synchrony between the population dynamics of different sites varies with the distance between sites. To illustrate this we present analysis of data on the field vole populations in Kielder Forest (see Lambin et al (1998Lambin et al ( , 2000, Mackinnon et al (2001) and Bierman et al (2006) for more details of this system and more detailed analysis). The raw data consist of average population density estimates from abundance indices (see Lambin et al (2000) for a methodological description) for a range of sites, over the period [1984][1985][1986][1987][1988][1989][1990][1991][1992].…”

“…To illustrate this we present analysis of data on the field vole populations in Kielder Forest (see Lambin et al (1998Lambin et al ( , 2000, Mackinnon et al (2001) and Bierman et al (2006) for more details of this system and more detailed analysis). The raw data consist of average population density estimates from abundance indices (see Lambin et al (2000) for a methodological description) for a range of sites, over the period [1984][1985][1986][1987][1988][1989][1990][1991][1992]. These are the years for which evidence of periodic travelling waves is the strongest: more recent data are not so indicative of a unidirectional wave (see Bierman et al 2006 for details).…”

Resting and daily energy expenditures of free-living field voles are positively correlated but reflect extrinsic rather than intrinsic effects

“…The field vole is common in these ephemeral habitats but completely absent from forested areas, which lack grass cover. Long-term monitoring has shown that field vole populations in the Kielder forest have cyclic dynamics with a 3-to 4-year period (34). Habitat patches are very similar in terms of vegetation and differ primarily in aspect.…”

Experimental evidence that livestock grazing intensity affects cyclic vole population regulation processes

“…In order to estimate density dependent parameters of vole population growth rates we fitted a seasonal log-linear auto-regressive model (Hansen et al 1999), where the proportion of quadrats yielding positive vole signs were used as a proxy for density (Lambin et al 2000). Observation error may cause bias in the estimation of densitydependence.…”

“…In this state-space framework, missing values from spring 2005 were estimated as a by-product of the model fitting process, hence they didn't contribute to inform about model fit. Earlier calibration work established that volesign detection probability is linearly related to vole density estimated by live-trapping (Lambin et al 2000), and it is also known that the log-linear model fitted has some robustness to non-linearities in the proxy for density (Tkadlec et al 2011). We used independent uninformative priors for all the parameters of the model.…”