Effects of dispersion and parent population density on enclosed populations of Microtus montanus (Peale)

Graduation date: 1964This field investigation was designed to further\ud the understanding of the significant influence which\ud parent population density, as well as dispersion, have\ud on the dynamics of enclosed populations of the montane\ud vole, Microtus montanus (Peale). The study was conducted\ud in Klamath County, Oregon from June, 1963 to February,\ud 1964. The objectives were to investigate possible effects\ud of parent population density on such characteristics of\ud the ensuing generations as density, reproduction, survival,\ud and movement, and to determine the effects of dispersion\ud on a population. Four one-quarter acre enclosures were\ud used during this study. Two contained voles from a 1962\ud high parent population density (E 4 and E 6) and two\ud contained voles from a 1962 low parent population density\ud (E 5 and E 7). A means for mice to disperse was provided\ud in two enclosures (E 5 and E 6), one enclosing mice from\ud a low parent population, the other, mice from a high\ud parent population. All population characteristics were\ud determined by live trapping from three to five days at\ud two to six week intervals.\ud The enclosed populations had comparable peak densities\ud during December, with the exception of E 4 which\ud was significantly lower than E 7. The ratio of increase\ud was greatest in E 4 and E 6. E 6 supported the largest\ud number of mice and E 4 the smallest. Males were dominant\ud in E 4 and E 7 through most of the study. Females were\ud dominant in E 5 throughout the study and in E 6 until\ud December. E 5 and E 6 supported a larger percentage of\ud young animals than did E 4 or E 7. E 4 had the highest\ud average percent of females perforate, pregnant, and with\ud mammary glands large or lactating; E 6 had the next highest\ud percentages and E 5 and E 7 the smallest. Reproduction\ud stopped in all enclosures after the November trapping\ud period. All enclosures had very good survival through\ud December but poor survival through January. The poor\ud January survival was probably due to the presence of\ud tularemia within the enclosures. E 4 and E 6 had consistently\ud better cohort survival throughout the study than\ud did E 5 and E 7. Juveniles survived best in all enclosures\ud until August, after which time the sub-adults and adults\ud had the better survival rate. E 6 had statistically better mean survival through December than did E 4 or E 7,\ud and through January than either E 5 or E 7. The dispersal\ud ramps captured 2.73 times more animals from E 6 than\ud from E 5. During the study juvenile and sub-adult females\ud and sub-adult and adult males were the only age classes\ud of mice captured in the ramps until November when adult\ud females were first caught. Most sacrificed dispersal\ud males were found to be in breeding condition while only a\ud few of the females had bred. Captures per ramp day were\ud generally density dependent.\ud Some differences observed in the population characteristics\ud of the four enclosures can be attributed to parent\ud population density and /or dispersion. High parent population\ud density did not adversely affect reproductive potential\ud or the survival of a vole after it had become established\ud in a population. Prenatal mortality and the amount\ud of wounding, an indication of intraspecific strife, was\ud greatest in the enclosure with mice from a high parent population\ud density and where dispersion was not allowed.\ud Where dispersion was allowed recruitment was high, survival\ud was good, and the amount of wounding was low regardless of\ud parent population density. It is suggested that animals\ud from a high parent population are selected for aggressiveness\ud which eventually causes a disruption of the social\ud structure. Dispersal tends to maintain a stable social\ud structure until populations become dense, at which time\ud dispersion is a less effective regulatory mechanism