Climate is likely to be a major driver of wildlife population dynamics in extreme (arctic and alpine) environments. However, the influence of climate on demography cannot be understood without determining it’s importance relative to intrinsic factors like density dependence, or comparing its effects across multiple demographic parameters. Most studies of climatic effects on alpine and arctic vertebrates have focused primarily on survival, and to a lesser extent, reproductive phenology. Our objective was to determine the influence of climate relative to population density on reproduction by Hoary Marmots (Marmota caligata), an obligate alpine herbivore distributed throughout northwestern North America. Hoary Marmots live in a harsh environment where the severity and duration of winter can strongly influence survival, and individuals may lose up to 40% of their body mass during hibernation. Marmot family groups hibernate together, and may benefit from group thermoregulation. Individuals in larger family groups may therefore emerge from hibernation in better physical condition, which could increase reproductive performance. However, group size may also negatively impact fertility, as dominant females may suppress reproduction by subordinates. We built multi-state models in program MARK to test whether age, group size, winter climate (indexed by Pacific Decadal Oscillation values), and breeding history influenced current breeding state using a 6-year mark-recapture dataset. We also used generalized linear mixed models to model juveniles per family group and litter size as functions of group size, number of breeding females per group, and winter PDO.
Results/Conclusions
Our mark-recapture analyses suggested that breeding state did not have a strong influence on over-winter survival. In addition, females did not consistently reproduce in alternate years, as reported in other studies. Finally, the probability that an individual would breed was influenced by age and to a lesser extent, population density, but not by climate. On the other hand, both climate and the number of breeding females significantly influenced the number of juveniles produced per family. Litter size did not vary with climate, family group size, density of breeding females, or mother’s age. Climatic variation clearly does not affect all demographic parameters equally. To further explore the relative importance of climate and population density, we will conduct a Life Table Response Experiment, in which matrix population models are used to identify the relative contributions of individual demographic parameters to variation in projected population growth rate.
