Spatial Regulation and Population Growth in the Wood Mouse Apodemus sylvaticus: Experimental Manipulations of Males and Females in Natural Populations

Abstract

1. The role of male and female Apodemus sylvaticus (L.) in population regulation was investigated by their removal in two independent, replicated, field experiments, carried out in mixed woodland during the breeding season. 2. The removal of adult male A. sylvaticus during the breeding season resulted in a decline in male numbers but an increase in the number of adult females in manipulated areas. The effect of treatment was influenced by density at the start of the experiment. During the breeding season, adult male and female A. sylvaticus may compete for food. Thus, the removal of male A. sylvaticus may reduce competition for food with a concomitant increase in female numbers. 3. There was no evidence that adult male aggression influenced recruitment of young as recruitment was unaffected by male density or removal during the breeding season. The role of males in population regulation in A. sylvaticus therefore is limited to their impact on females and possibly through infanticide of nest young, rather than having a direct effect on the survival of weaned young. 4. The removal of female A. sylvaticus resulted in a fall in numbers of females and a concomitant increase in immigration of males and females on manipulated grids. Reproductive success of female immigrants was enhanced on grids where females were removed but did not result in an increase in the overall proportion of females that were reproductively active. It is suggested that reproductive activity in immigrant females is inhibited by resident females. 5. The enhanced dispersal of the females into areas from where resident females have been removed suggests that there are dispersing females in the population that may establish themselves in areas with few females and, in time, become reproductively active. 6. Male immigration during the female removal experiment was lowest on control grids where female density and numbers of reproductively active females were highest. Male A. sylvaticus may orientate towards areas that offer the greatest opportunity for copulation, regardless of absolute numbers of reproductively active females. 7. The results of the present experimental manipulations are consistent with the hypothesis that spatial density-dependent inhibition of breeding in female A. sylvaticus regulates population increase during the increase phase of the annual cycle. This mechanism may be overridden by superabundance of food in some years.