Comparative Demography of Pikas (Ochotona): Effect of Spatial and Temporal Age‐Specific Mortality

The structure of 4 populations of a small boreal mammal, the pika (Ochotona princeps), is examined to explore the causal mechanisms of evolution of life—history features of the species. Litter size reflects potential and realized fecundity and is largest at Bodie (low latitude, low altitude), intermediate at Sierra and Colorado (low latitude, high altitude), and smallest at Alberta (high latitude). The age specificity of mortality largely determines the number of sites available for settlement by juveniles, which dictates the feedback to liter size. Populations are stable and habitats continuously saturated at Alberta; mortality of adults is predictable. The Sierra and Colorado sites possess unstable population. At these sites, snowmelt in spring (the harbinger of emergent vegetation and successful weaning) is relatively unpredictable compared to Alberta; adult mortality may be high in years of early or late snowmelt. The unpredictability also selects for asynchronous breeding such that more sites are available in any one year to offspring from certain successfully weaned litters. The habitat at Bodie is temporally and spatially unsaturated. Largely because of poor vagility of pikas, much of the habitat is vacant. Colonization is unable to completely offset extinction of populations on patches of habitat. Here, the selective factor determining litter size is juvenile production and not adult mortality.