Yellow-Bellied Marmot Population Dynamics: Demographic Mechanisms of Growth and Decline

Multiple environmental factors may act synergistically to influence demographic characteristics, and ultimately the dynamics, of biological populations. Using prospective and retrospective analyses of demographic data from a 40-year study of individually marked animals, we investigated the demographic mechanisms of the temporal and spatial dynamics of a yellow-bellied marmot (Marmota flaviventris) population. Prospective elasticity analyses indicated juvenile survival (Pj) would have the largest relative influence on the projected population growth rate (X). Relative magnitudes of elasticities did not differ between years characterized by positive (X > 1.0) and negative (X < 1.0) population growth. However, retrospective analyses of a life table response experiment (LTRE) revealed that changes in fertility (F), followed by age of first reproduction ((x) made the largest contributions to observed annual changes in X. Changes in F and a made the largest contributions to annual declines in X, whereas changes in Pj also were important to cause increases in X. Population dynamic differences among marmot colonies were due primarily to spatial variations in a and Pj. Our results indicate that changes in reproductive parameters (ol and F) primarily drive the temporal dynamics of our study population, and that demographic mechanisms of population increases might differ from those of population declines. Studies of the regulation of yellow-bellied marmot populations should focus on the factors or processes influencing reproductive parameters.