Regulatory Mechanisms and Dynamics of an Island Population of Clethrionomys glareolus (Schreber, 1780)1

A scheme of density regulation in an island population of Clethrionomys glareolus (Schreber) is presented. The key factor for the changes in numbers was mortality of untrappable voles. A peak in population numbers followed low mortality of the spring generation, and, in peak years, mortality of the autumn generation was always high. An experimentally enriched food supply resulted in low mortality of both the spring and autumn generations. Natality, which is usually stable, increased slightly as a consequence of the additional food supply. This was due to an earlier start of the breeding season (6 weeks earlier) and to an increase in numbers of mature (i.e. capable of reproduction) females.