Research Question: The purpose of the article was to examine whether competition between the sympatric populations of M. montanus and M. pennsylvanicus resulted in habitat segregation?
Study design: A scientifically rigorous field study was undertaken between 1961 and 1962 at the National Bison Range in western Montana. Two live-trap plots of 25-ft were set in surrounding dry grassland and hydrosere where both species of vole inhabited. The plots were set 0.75 miles apart covering an area of 2.48 and 3.26 acres. The captured voles were marked for individual identification by tie clipping and by ear tagging. From September 1961 to May 1962 trapping was undertaken in a mesic habitat in smaller plot or experimental plot and on the adjacent mesic 25-ft strip of the xeric habitat of a larger plot or control plot. The M. pennsylvanicus captured on the experimental plot were removed continually. The population density was estimated using Schnabel method and the average distance between successful points of capture performed to determine the effect of M. pennsylvanicus removal on the movement of M. montanus.
Summary of Results: The results of the study have been clearly presented. Removal trapping reduced the population density of meadow voles to nearly 24 animals per acre. The average distribution for the six M. montanus formerly identified in xeric habitat and captured in mesic habitat was 102-ft. The number is much greater than the average distribution for M. montanus throughout the whole study and declined to 60-ft, signifying that the movements were beyond the range of the usual movement patterns. The six marked and 70 unmarked M. montanus that were captured in the mesic habitat shows that the animals shifted their centers of activity into areas previously dominated by M. pennsylvanicus.
In the article, the specific environmental or ecological issue that the authors have clearly managed to examine is that the removal of Microtus pennsylvanicus from mesic grasslands in western Montana allowed Microtus montanus to move into the vacated habitat—the latter was formerly restricted to adjacent xeric habitats. As such, the authors are in a position to elaborate their postulation that competition is responsible, at least in part, for the habitat segregation of M. pennsylvanicus and M. montanus microtines.
Despite the clear research finding, the one thing the authors could improve upon in terms of the layout of their study is that they stated the purpose of the research was to find out how competition between Microtus pennsylvanicus and Microtus montanus caused habitat segregation, but they did not stress on the mechanism itself and how it works to cause and maintain habitat segregation. Most of the study focused on how the niche overlap, during normal activities between the two, measured for the intensity of the competition. I feel that the authors should have placed a greater emphasis on the measurement of the intensity of the interactions (competition exclusion) to fully address the topic of this study. The rationale should be that the subordinate species (Microtus montanus) might show higher emigration rates, lower survival rates, and smaller or negative weight gains in the presence of the dominant species than when alone, which, therefore, causes it to live mostly in xeric habitats than mesic; vice versa—habitat segregation caused by competition.
In conclusion, the study presents an inadequate test of the competition hypothesis, because of differences in densities and conditions of the plots. The experimental plot in this study was fenced and not grazed by bison and had 4 to 5 times more animals than the control. Replications of emigration experiments at two densities of the species by increasing the area of study will provide significant information concerning the effect of population size on species interactions.
Koplin, J., & Hoffmann, R. (1968). Habitat Overlap and Competitive Exclusion in Voles (Microtus). American Midland Naturalist,, 80(2), 494-507.