Population Ecology of Snowshoe Hares in the Central Rocky Mountains

Abstract

Measurements of natality rates, survival rates, movements, and densities were integrated to describe the mechanics of population balance for snowshoe hares (Lepus americanus) in Colorado (1969-71) and Utah (1972-73), near the southern extreme of this species' range. Adult home-range size averaged eight hectares in spruce (Picea spp.)-fir (Abies spp.) forest, the preferred habitat. Juveniles used unforested habitat more frequently than adults. Population density remained stable throughout the duration of both studies; historical accounts also indicate population densities have remained fairly stable in the Rocky Mountain region. Hares commenced breeding from mid-April to mid-May. Females surviving an entire reproductive season produced an average of 2.1-2.3 litters annually. Later-litter sizes, averaging 4.8 in Colorado and 5.9 in Utah, were larger than most litter sizes reported from higher latitudes. The annual natality rate was 8.2 and 11.5 young per female surviving an entire reproductive season in Colorado and Utah, respectively. Annual survival rate for adults in Colorado was 0.45. About 16 percent of juveniles born each summer needed to survive until the following breeding season to balance the population density. Second-litter juveniles had a lower survival rate than did first-litter juveniles. We surmise the long-term stability of the hare population in the Rocky Mountains to be related to dispersal of juveniles into poor (open) habitats where survival is low. The discontinuity of preferred spruce-fir habitat allows dispersal into open habitat to occur readily, and this may be the main mechanism averting the potential population increase. J. WILDL. MANAGE. 39(3):535-549 The snowshoe hare is indigenous to boreal forests throughout North America. This species has long interested population ecologists because of its population fluctuations (Keith 1963); however, mechanisms of population regulation are still poorly understood. In particular, little is known about population characteristics of hares in the southernmost extension of its range in the Rocky and Appalachian Mountains in the United States. This paper helps fill this void by presenting information on the population ecology of snowshoe hares in the Rocky Mountains of central Colorado (1969-71) and northern Utah (197273). The study had three objectives: (1) to estimate local densities of snowshoe hares, (2) to examine natality rates, survival rates, and movements, and to describe the immediate mechanics of population balance and fluctuation, and (3) to compare these populat on parameters with those published from snowshoe hare studies conducted at higher latitudes in an effort to clarify mechanisms causing cyclic fluctuations. We gratefully acknowledge J. E. Gross and M. L. Wolfe for their guidance and interest. D. A. Hein, F. H. Wagner, F. A. Glover, H. S. Donoho, S. Dolbeer, and M. Laderach also provided timely assistance.