Population Ecology and Energy Relationships of Clethrionomys gapperi in a Colorado Subalpine Forest

Abstract

Small mammals were live-trapped on a 12 by 12 study grid (1.2 ha) located in a subalpine forest on the eastern slope of the Colorado Front Range at an elevation of 3,120 m. The area was characterized by a continental climate, and snow covered the ground for about 7½ months of the year. Subnivean activity of Clethrionomys gapperi was monitored by use of live-traps located within trap chimneys. The population was monitored at 2-week intervals from January 1974 to September 1975. Population density of mice increased during summer, peaked in late November and then gradually declined to a low in May during spring thaw. Two periods of hardship existed for red-backed voles in the study area—the autumn freeze and spring thaw. The autumn freeze occurred before snow accumulation, which reduced survival. The spring thaw produced extensive flooding, which caused high mortality. Red-backed voles commenced breeding in late March below a continuous snow cover (150 to 200 cm deep). The breeding season lasted about 7½ months. Pregnant females caught in June produced another litter that summer. Voles concluded breeding in September and lactating females were caught in October. Winter-caught red-backed voles weighed significantly less and also showed a lower per gram metabolic rate than the larger summer mice. Winter weight loss seemed advantageous in reducing food requirements as verified by metabolic studies. Small mammals in the subalpine forest ecosystem are not simply primary consumers, their role is complex and derived from numerous “strategies” adaptive in coping with the harsh subalpine environment.