Effects of Added Food on Some Attributes of an Enclosed Vole Population

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Abstract
Attributes of grey-sided vole (Clethrionomys rufocanus bedfordiae) populations were compared between a 1-ha control grid and a 1-ha grid on which the voles were fed in a 2.1-ha outdoor enclosure in Hokkaido, Japan. Voles moved between grids and were monitored by live trapping from 1984 to 1986. Population density was 2–7 times greater on the experimental grid to which food was added than on the control grid, although the difference in density shrank to <2 fold in both summer and autumn. The sex ratio of adults was more strongly female-biased on the experimental than control grid. The combined survival rate (male and female) was similar between grids. Movement from the experimental to the control grid frequently occurred in spring and summer, whereas more voles moved from the control grid to the experimental grid in winter 1985–1986. Many more males than females relocated on both grids. The population on the experimental grid had a greater reproductive output and longer breeding season than that on the control grid. Comparisons among these factors showed that reproductive output was most effective as a factor causing the difference in density between grids.