The Regulation of Wood Mouse (Apodemus sylvaticus) Numbers in Wytham Woods, Berkshire

The population dynamics of the wood mouse, in deciduous woodland in Wytham Woods, Berkshire, were investigated using data collected by several authors. The studies included three where numbers present were estimated at approximately monthly intervals over periods of 2 years, one where an index to the numbers present was obtained twice a year for 12 years, and two where numbers caught were recorded at 2-monthly intervals over 4 years and 20 months respectively. From the analysis the following main points emerged. (1) Over a period of 4 years overwinter survival was bad when the acorn crop failed and good when it was plentiful. Over a period of 12 years overwintering success was strongly correlated with the size of the acorn crop the previous autumn. This probably reflects overwinter survival. (2) In the spring of most years there was a drop in density due to a sudden deterioration in survival. The size of this drop was density-dependent, being slightly greater following higher initial densities. The poorer survival, in one spring at least, was not associated with any deterioration in food supply. It is thought to be due to an increase in strife within the population at the start of the breeding season, but this is speculation. (3) Density usually remained stationary during the summer. This was shown to be mainly due to an extremely poor survival of juveniles. (4) The stable summer density was apparently not associated with the food supply at the time. (5) Density usually increased rapidly in the autumn. This was due to a great improvement in the survival of juveniles together with some improvement in adult survival. (6) The poor survival of juveniles in the summer may be due to antagonistic behaviour of established adults, and the better juvenile survival in autumn to change in social behaviour, but these explanations are speculative. (7) Between the start and the end of the breeding season the results indicated that a very efficient density-dependent process was operating. This was identified as a control over the start of the autumn increase in numbers; the higher the spring density, the later the increase began. As a result spring densities varied greatly (depending on the overwintering success), but autumn densities were similar in most years. It is through this mechanism that the main regulation of Apodemus numbers takes place. It was concluded that Apodemus populations in oak woodland in southern England are self-regulating. Nevertheless, because of t