The Distribution and Dispersal of Pikas: Consequences of Insular Population Structure

The population structure of a small boreal mammal, the pika (Ochotona princeps), was analyzed from demographic data and the observed spacing of populations on discrete islands of habitat in the Sierra Nevada of California. These islands of habitat varied in size and distance from one another. Not all of the islands were occupied and most seemed to contain fewer individuals than they could hold. The populations apparently represent dynamic equilibria between extinction, which was directly related to island size, and recolonization, which was inversely related to interisland distance. Distances greater than 300 m appeared to pose difficult barriers to dispersing juveniles. The study site is near the distributional boundary of pikas in this area. The birth rate is higher, adult death rate lower, and juvenile death rate higher than for other pika populations found near the center of the altitudinal or geographic range of the species. It appears that the probability of successful colonization decreases with progressively lower elevations, until ultimately colonization is unable to offset extinction and the limit of the species range is determined.