How Predation and Landscape Fragmentation Affect Vole Population Dynamics

Abstract

BackgroundMicrotine species in Fennoscandia display a distinct north-south gradient from regular cycles to stable populations. The gradient has often been attributed to changes in the interactions between microtines and their predators. Although the spatial structure of the environment is known to influence predator-prey dynamics of a wide range of species, it has scarcely been considered in relation to the Fennoscandian gradient. Furthermore, the length of microtine breeding season also displays a north-south gradient. However, little consideration has been given to its role in shaping or generating population cycles. Because these factors covary along the gradient it is difficult to distinguish their effects experimentally in the field. The distinction is here attempted using realistic agent-based modelling.Methodology/Principal FindingsBy using a spatially explicit computer simulation model based on behavioural and ecological data from the field vole (Microtus agrestis), we generated a number of repeated time series of vole densities whose mean population size and amplitude were measured. Subsequently, these time series were subjected to statistical autoregressive modelling, to investigate the effects on vole population dynamics of making predators more specialised, of altering the breeding season, and increasing the level of habitat fragmentation. We found that fragmentation as well as the presence of specialist predators are necessary for the occurrence of population cycles. Habitat fragmentation and predator assembly jointly determined cycle length and amplitude. Length of vole breeding season had little impact on the oscillations.SignificanceThere is good agreement between our results and the experimental work from Fennoscandia, but our results allow distinction of causation that is hard to unravel in field experiments. We hope our results will help understand the reasons for cycle gradients observed in other areas. Our results clearly demonstrate the importance of landscape fragmentation for population cycling and we recommend that the degree of fragmentation be more fully considered in future analyses of vole dynamics.

How do variations in seasonality affect population cycles?

“…In particular, if one restricts attention to breeding season lengths of six to eight months, the results in figure 3 predict little variation in the dynamics. This is consistent with the results of Dalkvist et al [22]. They studied data on Fennoscandian voles with breeding season lengths of six to eight months, and found that the difference between these season lengths was not a statistically significant factor in differences in cyclicity.…”

“…The north-south gradient in Fennoscandia corresponds to a curve in this parameter plane. There is currently insufficient data to enable a detailed formulation of this curve, but it is likely to be nonlinear, especially when factors such as landscape fragmentation [22,35,36] are taken into account. Our results suggest that more information on the form of this nonlinear curve would be a key step in understanding the complexities in vole dynamics, such as the sudden change from annual to multi-year cycles that occurs in mid-Fennoscandia [7].…”

“…This contrasts with the pronounced differences in the pattern of seasonality between the southern and northern parts of Fennoscandia, and the effect of this on the predation hypothesis has received almost no attention (a notable exception being Dalkvist et al [22] who consider three different breeding season lengths). In southern Fennoscandia, the breeding season can be over seven months long, whereas in northern areas it is usually three to four months [16,22,23]. Furthermore, there is evidence to suggest that the vole cycles have been reducing in period length because of climate change [24,25].…”

Food resources and intestinal parasites as limiting factors for boreal vole populations during winter

“…A standard was prepared by combining 2 ml from each sample over all trapping occasions. Duplicate standard concentrations of 200,150,100,50,25,10,5 and 0 were run on each plate simultaneously with samples. Plates containing samples and standards were then incubated for 3 hours at room temperature.…”

“…Although delayed density-dependent predation is often considered the principle driver of cyclic dynamics [6][7][8][9][10], regulatory processes are likely to be multifactorial and geographically variable [1,11]. Hence, consensus on causal factors behind cyclicity has not been reached despite several decades of research [1,[10][11][12][13][14].…”