The Distribution and Dispersal of Pikas: Influences of Behavior and Climate

Daily and seasonal behavior in relation to local climate was analyzed for two populations of pikas, (Ochotona princeps), representing the extremes of the altitudinal range of the species in the Sierra Nevada of California. The goal of this work was to determine the relationship of a propagule to its environment at the time of dispersal, in order to evaluate the probability of successful colonization in different parts of the species range. Pikas at the high—altitude site (3,400 m) were active throughout the day, whereas at the warmer low—altitude site (2,550 m) they concentrated their activity in the morning and late afternoon. Field experiments suggest that when pikas are deprived of the opportunity to retreat to favorable microclimates in rockslides, they are unable to tolerate the high diurnal temperatures found at lower elevations. Thus, high environmental temperatures apparently constitute a major stress on dispersing individuals at low altitudes. The onset of hay storage, vocalization, and parturition, three measures of seasonality, occurred about 6 wk earlier at the low—altitude site. The timing of haying and vocalization there was correlated with the amount of precipitation during the previous winter: during dry years with little snow and an early spring pikas began and terminated their haying earlier than in wet years with a late spring. The reproductive season appeared to remain the same from year to year at the same site. The timing of dispersal of juveniles is related to the reproductive season. Most juveniles at the low—altitude site disperse during midsummer when temperatures are high and their chance of success is low. Also, during dry years most of the period suitable for gathering hay ends before a majority of the juveniles can disperse. Temperature and seasonality apparently have little effect on the dispersal of pikas at high altitude. At high altitude the pikas are wider ranging than at low altitude, and they occupy a greater proportion of available sites.