Spatial organization of a bank vole population in years of small or large numbers

Abstract

Examination was made of the effect of differences in the numbers of a free-living island population of Clethrionomys glareolus (Schreber, 1780), ort the elements of its spatial organization. Material obtained from studies on this population during the periods from 1966—1970 and 1972—1973 was used (In the second series the food supply was increased artificially for experimental purposes). Years of low level (1967, 1969), high (1966, 1968, 1970) and very high (1972, 1973) level of numbers were found to occur. It was found that in years with large numbers of bank voles individual home ranges are smaller than in years with low numbers. Both in peak years and years with low population numbers the home ranges of males are larger than those of females, and the hierarchy of successive entries into the population of generations is maintained in respect of the size of home ranges. Ten-dency to clumping distribution is always observed in spring, whereas in summer and autumn distribution may be either clumping or random. A positive correlation was found between; the proportion of the youngest group of individuals and tendency to clumping distribution. The average number of individuals per trapping station does not significantly differ between years of high and low numbers. In all the study years (including those with supplementary food supply) higher density of bank voles is observed on the station in the optimum habitat, in comparison with the suboptimal habitat, although the increase in the number of individuals in the latter, together with increase in population numbers, was significantly greater than in the optimum habitat.