Spatial dynamics of cyclic field vole, Microtus agrestis, populations
1. Many ecologists have recently advocated the study of spatial patterns of abundance or growth rates as a means of better understanding population dynamics. In this study I described the spatial pattern of abundance of the field vole, Microtus agrestis, which has previously been shown to have cyclic temporal fluctuations of abundance in my study area in Kielder Forest, northern England. A combination of techniques was then used to investigate which processes determine the spatial pattern of dynamics in these vole populations.2. Previous analysis of spatial patterns in density of field voles from an area of approximately 80 km2 within Kielder forest over a 13-year period indicate that density varies as a periodic travelling wave moving across the landscape. I collected data on vole density from a larger area (approximately 600 km2) over a 2.5-year period and used the same analysis to show that the spatio-temporal variation in density was well explained by a travelling wave of density moving across the entire region. Estimates of the wave's speed of 14 kmyr-1 and direction of 66° from north were consistent with the estimates obtained from the data set covering a smaller spatial extent but spanning a longer temporal scale.3. Processes that determine population abundance do so by acting on vital demographic rates. Spatial patterns in demographic rates were therefore described to help identify potential causes of the spatial patterns of density. Eight 0.3 ha live-trapping grids were established within the study area and animals were trapped at monthly intervals during a period of extended low density and during a period of increasing densities. Capture-mark-recapture techniques were used to estimate abundance, juvenile recruitment rate, apparent survival rate and immigration rate at these sampling sites. The synchrony of changes in these parameters between sites was measured as the cross-correlation coefficient between the time series of each parameter and the spatial patterns in the synchrony of each demographic rate were described using Mantel correlations. Field vole abundance, juvenile recruitment rate and a derived estimate of mortality rate were most synchronous between the closest sites, however no directionality was found in these patterns and they could not therefore be related to the travelling wave pattern of density. I concluded that the process responsible for the wave probably only acted during the period of declining density and that therefore the factors causing spatial patterns in abundance could differ at different stages of the population cycle.