Dispersal, survival, and population regulation of the vole Microtus townsendii

A number of field studies on voles have suggested that dispersal is an important mechanism of population regulation. To investigate if voles disperse from declining populations, I made two vole-proof enclosures, each having an area into which the voles could disperse. These two fenced populations and two unfenced control populations were trapped from Hay 1976 until June 1978 near Vancouver, Canada. All four Microtus townsendii populations were low during the 1976 summer, and the population sampled by live-traps increased during the fall and winter non-breeding season, owing to delayed capture in live-traps. Male and female minimum survival rates during the 1976 winter were 0.92 per two weeks. all four vole populations declined for three months after breeding began in spring 1977, the declines being concurrent with dispersal of the smaller voles. More males dispersed than females during this spring decline, and male minimum survival dropped to 0.76 per two weeks, whereas female minimum survival remained high at 0.90. Voles were 2-15% heavier in the peak 1977 spring than during the decline spring of 1978. Mounding increased sharply when breeding began and reached a peak six weeks later. Dispersal of subadult voles was common in the peak populations of summer 1977, with these individuals more often in reproductive condition than were resident subadults. I suggest that aggressive behavioural interactions between smaller animals attaining sexual maturity and larger sexually-mature adults prompted the dispersal of the smaller individuals. All four vole populations declined for a second time in October 1977 for eight months, with the decline spanning both the non-breeding and subsequent breeding season. Most of the losses occurred in the non-breeding season, when the voles were neither wounded nor dispersing. The smaller males survived especially poorly and both sexes lost body weight. Minimum survival of males was 0.81 per two weeks, while female minimum survival was 0.82. The rate of decline accelerated rapidly when breeding began in spring 1978, and within four weeks the populations virtually disappeared, with minimal wounding and no dispersal. Male minimum survival dropped to 0.47 per two weeks, while female minimum survival was 0.65. I suggest that interference competition between large and small voles for a limited food supply may have accounted for some of the non-breeding season losses. The known length of time spent on the grids (lifetime) and dispersal tendency were non-randomly distributed among litters in increasing and peak populations, which indicates a heritable basis for these' traits. Spring and summer-born dispersers leave at the same age, when they become sexually mature. In the fall of 1977, avian predators accounted for a minimum of 15% of the loss from the tagged vole populations, and this percentage declined during the winter of 1977. The predators selected males and small individuals, and mortality accounted for by avain predation was density-dependent in the vole populations; denser vole populations suffered higher predation rates. Juvenile survival was highest in increasing populations and lowest in declining ones, whereas nestling survival was lowest in peak populations. Nestling mortality contributed little or nothing to population declines. Apparent adult survival was dependent upon the trapping technique. The concurrent use of live-traps and pitfalls showed that live-traps failed to enumerate completely any size or sex category of voles. Moreover, the estimates of demographic parameters based solely on live-trap data may be inaccurate owing to dispersal of young, sexually-mature individuals before they enter live-traps. Live-traps sample the dominant individuals preferentially, pitfalls the subordinates. This study indicates that although dispersal from increasing and peak vole populations may stop them f rota increasing indefinitely, it can not account for the subsequent decline. According to the Chitty hypothesis only one explanation is required to account for these declines: voles at the beginning of the two declines have been subjected to different selection pressures, and differences between the two declines are due to changes in the properties of the individuals. The characteristics of the 1978 decline partially support the Chitty hypothesis and also the view that changes in food quality or quantity were involved in population regulation at least in this particular instance.Science, Faculty ofZoology, Department ofGraduat