POPULATION DYNAMICS OF PIKAS: EFFECTS OF SNOWPACK AND VEGETATION (OCHOTONA PRINCEPS MORTALITY, COLORADO)

The population ecology of a small alpine mammal, the pika (Ochotona princeps), was studied in the Niwot Ridge Biosphere Reserve, located approximately 45 km west of Boulder, Boulder County, Colorado. Environmental and population parameters were monitored over a four year period (1980-1984) in order to determine the effect of snowpack and plant growth on the overwinter survival and reproductive success of pikas. Snowpack was manipulated in the last year of the study to experimentally test the hypothesis that snowpack influences overwinter mortality and reproductive success of pikas by determining the initiation of new plant growth and thus, the timing of nutrient and energy availability.;An analysis of habitat characteristics (meadow-talus interspersion and standing crop) was also performed to determine whether these variables influence the "carrying capacity" of specific sites and could account for the observed differences in densities between sites. A step-wise regression revealed that 63% of the variation in density was attributable to meadow-talus interspersion and an additional 23% to standing crop.;In years with greater snowpack, late snow cover, and delayed plant growth, survivorship and reproductive success were lower. Overwinter survival decreased by 67% on the snow fence site following erection of the fence and the subsequent four-fold increase in snowpack. These results indicate that snowpack and the date of snowmelt influence overwinter survival and reproductive success in pikas as hypothesized. Overwinter survival and reproductive success appear to influence population recruitment as well by (1) determining the number of available sites for juvenile establishment; and (2) determining the number of juveniles competing for these sites within any given year.