Populations exhibit complex dynamics under the combined influences of climate, densities, and interspecific interactions.  However, little attention has been paid to empirical assessments of the effects of interspecific competition on vertebrate demography.  We evaluated the effects of climate, density-dependent feedback, and elk (Cervus elaphus) on age-specific demographic rates of white-tailed ptarmigan (Lagopus leucurus) in an alpine habitat adjacent to Trail Ridge Road, Rocky Mountain National Park, Colorado, USA.  We fit two-sex, nonlinear matrix population models to 25-year ptarmigan counts by sex and age group, incorporating seasonal temperatures and elk population size as covariates.  We also used cross-correlation functions of de-trended population time series and climate time series to test the tab-hypothesis, which predicts that climate would have more pronounced effects on ptarmigan population fluctuations during the breeding season than the non-breeding season. 

Results/Conclusions

Our results indicated that winter warming reduced the survival of chicks, female yearlings and female adults and brood size, whereas increased elk abundance reduced chick and male adult survival.  Elasticity analysis demonstrated that the finite rate of increase was more sensitive to changes in chick and adult survival.  Therefore, we hypothesize that warming increased elk abundance in alpine habitats during winters, and elk competed for willow buds as winter food with ptarmigan and consequently reduced ptarmigan survival and recruitment.  Additionally, we did not find any evidence to support the tab-hypothesis with cross correlations of ptarmigan population time series and climate time series.  The tab- and tub-hypotheses may not be mutually exclusive when predicting seasonal climate effects on avian species.  Therefore, in addition to direct effects on the physiology and ecology of vertebrates, climatic change imposes indirect effects on trophic interactions in ecosystems, which subsequently affect the demography of interacting vertebrate species.