The effects of temperature on bighorn population estimates in Yellowstone National Park

Abstract

The effects of temperature on bighorn population estimates for Yellowstone National Park's Mt. Evert's winter range were examined. Based on work by Houston (1982), a bipartite relationship was hypothesized, whereby the effects of temperature upon the observability of sheep was predicted to be significantly different in those portions of the temperature spectrum where snowmelt occurs. Analyses of the regressions of bighorn numbers observed on mean daily ground temperature supported this hypothesis. At temperatures less than 0°C, counts increased significantly with temperature at a rate of 3.15∶1, but were found to underestimate bighorn numbers and be unreliable as estimators of population trend. Counts conducted at 0° to 7°C increased with temperature at a significantly greater rate of 18.53∶1. However, correction for population increases suggested this relationship reflected the intercorrelation of temperature and year; a result of selecting increasingly better census conditions over time. It was concluded that no additional benefit accrued from further increases in temperature once snowmelt occurred and that counts conducted at 0° to 7°C (the period between snowmelt and spring migration) were equally reliable. The importance of regarding snowmelt as a threshold value was underscored by the vastly different conclusions regarding bighorn population trends which have been drawn by different researchers from very nearly the same data. While previous research concluded that apparent population increases were the result of temperature-biased data, this study concluded that the apparently sigmoidal population growth was real.