SPATIAL REGULATION AND POPULATION-GROWTH IN THE WOOD MOUSE APODEMUS-SYLVATICUS - EXPERIMENTAL MANIPULATIONS OF MALES AND FEMALES IN NATURAL-POPULATIONS

1. The role of male and female Apodemus sylvaticus (L.) in population regulation was investigated by their removal in two independent, repli cated, field experiments, carried out in mixed woodland during the bre eding season. 2, The removal of adult male A. sylvaticus during the br eeding season resulted in a decline in male numbers but an increase in the number of adult females in manipulated areas. The effect of treat ment was influenced by density at the start of the experiment. During the breeding season, adult male and female A. sylvaticus may compete f or food. Thus, the removal of male A. sylvaticus may reduce competitio n for food with a concomitant increase in female numbers. 3. There was no evidence that adult male aggression influenced recruitment of youn g as recruitment was unaffected by male density or removal during the breeding season. The role of males in population regulation in A. sylv aticus therefore is limited to their impact on females and possibly th rough 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 s uccess of female immigrants was enhanced on grids where females were r emoved but did not result in an increase in the overall proportion of females that were reproductively active. It is suggested that reproduc tive 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 fema les and. in time, become reproductively active. 6. Male immigration du ring the female removal experiment was lowest on control grids where f emale density and numbers of reproductively active females were highes t. Male A. sylvaticus may orientate towards areas that offer the great est opportunity for copulation, regardless of absolute numbers of repr oductively 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 pop ulation increase during the increase phase of the annual cycle. This m echanism may be overridden by superabundance of food in some years.