Experimental studies of the population processes in the vole Microtus townsendii
Creators: name: Boonstra, Rudy
Publisher: name: University of British Columbia
Year: 2010

description: A number of field studies on small mammals have suggested that aggressive behavior may limit breeding density. To investigate how the presence of one individual affects another's chances for survival, reproduction, and growth, I carried out a series of experimental studies on Microtus townsendii near Vancouver, Canada. In the first experiments, population density was reduced by removing voles before or during the spring decline. Drastic artifical reduction during a spring decline improved female survival but not male survival; similar reduction in the fall prior to a decline improved both male and female survival. Survival was not density-dependent in males, but was in females during one decline. To explain these results, a behavioral model is proposed in which females compete for nest sites and males compete for females. In a second experiment, a population predicted to experience a decline was enclosed. I wanted to see if preventing emigration would stop the decline. Unfortunately the control area remained at peak densities and neither area suffered a spring decline. The enclosed population had higher rates of increase, reached higher densities, and had higher survival rates than those on the control area. This resulted in severe overgrazing and a sharp population decline within the enclosure. These results indicate that movements play a necessary role in population regulation when voles are at peak, densities. In a third experiment, the role of predation in causing population changes was measured. Predation was not necessary to initiate a decline or to maintain it. Of the tagged voles known to have been eaten, there was no consistent selection by predators for either of the saxes, for any weight class, or for voles with any other characteristic that could be measured by live-trapping. Of the total number of voles known to have been eaten, avian predators consistently chose animals that were smaller than those in the tagged population. This indicates that either the live-traps selectively caught large animals, or avian predators selectively caught small animals, or that both biases were present. In the fourth experiment, the role of resident adults in determining juvenile survival was tested by removal of adults and by adding juveniles to experimental populations. Survival of young was improved in the absence of all adults, but not in either intact populations, in reduced populations, or in female populations. Survival of young was better in periods of reduced adult breeding. Height at sexual maturity tended to be higher in the presence of adults of the same sex. Growth in young males tended to be reduced in the presence of adult males. These results indicate that females reduce survival of young, and suggest that emphasis be placed on the study of female behavior as a factor affecting microtine numbers. In the final experiment, the survival and dispersal rates of very young voles were examined to determine where the enormous loss between birth and recruitment occurred. A high density vole population was trapped concurrently with live-traps and pitfall traps. Capture of a large number of young in pitfalls indicated that mortality was higher among post-weanlings than among any other age group. The pitfall traps enumerated up to twice as many animals as the live-traps, and over half of the 1100 animals caught first in pitfalls were never caught in live-traps. Trapping solely with live-traps may severely underestimate numbers (at least when densities are high) and give inaccurate population statistics. Populations of J. townsendii fluctuate in size and Chitty's behavior hypothesis predicts that spacing behavior underlies these density changes. My experimental results are consistent with this general view and add specific details to the mechanism by which density is regulated. My experiments pinpoint post weaning survival as a critical phase determining population changes in this vole and focus attention on two critical areas: adult female - postweanling interactions and adult female-adult female interactions.