Unforeseen Disruption of Wood Mouse Population Dynamics after Food Reduction: A Field Experiment

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Abstract
A high-density population of wood mouse (Apodemus sylvaticus L.), living in the absence of predators and other small mammal competitors in a relatively small, isolated area on the southern coast of Iceland, was subjected to a food reduction experiment. The study plot was covered by Angelica archangelica L., which constituted a major autumn and winter food for the mice, which had been followed from 1973. In summer 1976 all Angelica flowering umbels were removed and the mouse population was followed (live-trapping and individual marking); subsidiary trapping plots serving as controls. The autumn numbers declined due to reduced breeding intensity and lack of juvenile recuritment. The low-density autumn population was dominated by post-reproductive individuals. Subsequent overwinter success and survival rates were high with few signs of a food shortage. Between March and June 1977 immigration occurred and during the next summer reproduction was good, while adult male survival rate was lower than for females. In August sex ratio among reproductives was as usual female-biased (contrary to 1976) and the autumn increase led to an extremely high number in November. The results are interpreted as follows: The food reduction did not cause starvation, increased social stress, mortality or dispersal, but affected population structure by markedly reducing juvenile recruitment. The consequence of this (combined with the scarcity of Angelica seeds) was a winter with a low density, a low frequency of trespassing, and subdued social interactions. This stable social environment favoured survival and next years'' reproduction and accounted for a high peak number in autumn. In conclusion, social interactions are important in regulating numbers.