Growth, Reproduction and Residency in a Declining Population of Microtus agrestis

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Abstract
The population dynamics of M. agrestis (L.) inhabiting a young larch plantation in south-west Britain [UK] were studied from spring 1967-spring 1969. As the larch trees grew they shaded out the ground layer vegetation and there was a resultant decline in vole density from 97/ha-6/ha during the 2 main years of study. By the summer of 1972, this species had disappeared altogether. Vole growth rates were significantly correlated with an index reflecting the abundance and rate of growth of the grasses which formed their main source of food. Despite a good start to the breeding season in 1968, conditions deteriorated rapidly as a result of shading, and there was a considerable rise in perinatal mortality. Rates of residency (minimum survival) were lowest during 2 main periods of the year. The 1st, in spring, involved mainly males and coincided with the onset of sexual activity. The 2nd, in summer, involved mainly females and coincided with the replacement of overwinter residents by animals of the year. Decreases in the rates of residency were correlated with increases in the distances moved between consecutive captures by male voles, reflecting a concomitant increase in the size of their home ranges. Previously published records of male spring body weights from several parts of Britain were significantly correlated with the standing crop of a meadow at Moor House Experimental Station and with the hay yields of the unfertilized control plots in the Rothamsted Park Grass Experiment. These correlations suggest a link between vole population fluctuations and climate. The effects of the quality or quantity of the spring food supply upon vole growth and reproduction may play an important part in this link.