CR 23:81-87 (2002)  -  doi:10.3354/cr023081

ABSTRACT: The potential for anthropogenic changes in climate has raised questions about how these changes might affect wildlife populations. We fit the Ricker population model to 25 years of data on the population sizes of white-tailed ptarmigan in Rocky Mountain National Park (RMNP), Colorado, USA, using 12 different weather variables as covariates. The best approximating model for population growth rates of the ptarmigan was selected using the Akaike information criterion and Aikaike weights. Our linear regression results suggest that median hatch dates advanced significantly from 1975 to 1999 in response to increases in April and May temperatures. Our best approximating population model indicated that high winter minimum temperatures retarded the growth rate of the population. Our data also had reasonable support that high mean winter monthly minimum temperatures and high mean winter monthly maximum temperatures might lower the population growth rate of the ptarmigan. We simulated the effects of future warming on the ptarmigan population in RMNP using our best ptarmigan population model and future climate scenarios projected by the Canadian Climate Center and Hadley Centre models. Our simulation results suggested that future warming would accelerate declines in ptarmigan abundance. Although our results showed a clear population level response to variation in climate, we did not detect a density-dependent effect in the ptarmigan population, and we cannot completely explain the inverse relationship between winter temperatures and population growth rates. A process-oriented modeling approach is needed for future studies to elucidate the mechanisms of the effects of climate change on the population dynamics of the white-tailed ptarmigan.

KEY WORDS: Breeding phenology · Climate · Climate warming · Population dynamics · Population model · White-tailed ptarmigan