Population Regulation in the Wood Mouse, Apodemus sylvaticus. II. Density Dependence in Spatial Distribution and Reproduction

Abstract

SUMMARY (1) Density dependence in spatial and temporal changes in abundance of wood mice (Apodemus sylvaticus) was examined on forty grids, each with an effective trapping area of approximately 1 ha, in representative forest compartments of 700 ha of continuous mixed woodland and plantations. The grids were trapped ten times over 33 months. Mean number of mice did not vary significantly among the four groups of ten grids trapped during each trapping occasion. (2) Spatial variation in abundance between grids was frequently as great as temporal change in mean population size. Overall dynamics conformed to those of previous studies. (3) The pattern of dispersion of adult males and juveniles and the overall population, was aggregated throughout the study. Aggregation of juveniles was most marked when they first appeared in the trappable population. Adult females were generally aggregated but were distributed randomly across the forty grids during the latter half of the breeding seasons. Dispersion of reproductively active females was less heterogeneous than that of overwintered females in June of two of the three breeding seasons. (4) There was a high level of correlation between trapping sessions in the number of individuals per grid. This was apparent even where trapping sessions were separated by 2 years. Spatial variation in habitat quality may have an important influence on the spatial variation in abundance of wood mice. (5) Mean instantaneous rate of population change per week varied considerably between grids although mean values fluctuated positively with overall population size. Sex differences in rates of population change were inconsistent. Male and female rates of population change were positively associated. There was, however, no overall difference between male and female rates of population change. (6) There were negative relationships between instantaneous rates of population change and overall abundance across the forty grids throughout all annual cycles of abundance. Spatial density dependence was most consistent within subdivisions of the population, e.g. adult males. (7) During the late spring, summer and autumn, spatial density dependence was evident in reducing female reproductive activity and recruitment of young. Spatial density dependence in reproduction is potentially the major density-dependent process during periods when numbers normally increase and must have an overall effect on population size. (8) Density dependence in the absence of reproduction and a decrease in overall population size, was probably due to differential rates of dispersal which redistribute both males and females relative to resources such as food.