Influence of population density and climate on the demography of subalpine golden-mantled ground squirrels

Abstract

Temporal fluctuation in abundance is common in many wildlife populations, but the causes and consequences to population dynamics of these fluctuations remain poorly understood. We used long-term (1990–2008) field data to investigate the influence of population size and environmental factors (climatic variables and predation) on the demography of golden-mantled ground squirrels (Callospermophilus lateralis). Survival varied by sex and age class, with highest survival for adult females (W 5 0.519, 95% confidence interval [95% CI] 5 0.462– 0.576) and lowest survival for juvenile males (W 5 0.120, 95% CI 5 0.094–0.152). Population size negatively influenced survival with a time lag of 1 year. Among environmental factors, current year’s rainfall and intensity of predation substantially influenced survival. Probability of successful reproduction (probability that a female weans 1 pups) was higher for older females (Y 5 0.816, 95% CI 5 0.734–0.877) than for yearlings (Y 5 0.313, 95% CI 5 0.228–0.412). Rainfall negatively influenced probability of successful reproduction of both older and yearling females with a time lag of 1 year. Litter size ranged from 1 to 8 pups, with a mean of 4.8 (95% CI 5 4.5–5.1). We found no evidence that litter size varied among age classes or over time, or was influenced by population size or environmental factors. Our results suggest that population size and environmental factors do not affect all demographic variables in the same way, and that both density-dependent and environmental factors influence the size of our study population.