Demography of a Townsend Ground Squirrel Population in Southwestern Idaho

The demography of a Townsend ground squirrel (Spermophilus townsendi) population was investigated during a 7—yr study in southwestern Idaho, where this species serves as a major food source for raptors and other predators. Squirrels were identified as residents, transients, or immigrants based upon the frequency of capture. Immigration peaked in late February and early March, shortly after the termination of the breeding season. Squirrels identified as transient appeared throughout the trapping season (February though June). A majority of the yearling females and nearly all of the adult females produced a single litter annually. Intersexual differences in dispersal and perhaps in mortality accounted for a significantly greater survival of juvenile and adult females. Survival rates of yearling males and females were similar. This ground squirrel has evolved a reproductive strategy adapted to its unpredictable desert environment. Successful reproduction is related to the availability of a fresh growth of grasses and ultimately to the amount of rainfall in the preceding fall and early winter. With suspension of reproduction during the drought in 1977, the study population entered torpor 4—6 wk earlier than usual. Body mass at immergence was not affected. Population density was reduced by more than one—half as a result of the drought. An increase in the pregnancy rate of yearling females was the only process operating during the post—drought period to restore an equilibrium density. There were no significant changes in immigration rate, litter size, or overwinter survival rate in response to density reduction. Both dispersal and predation operated to reduce density. In addition, disease in the form of sylvatic plague (Yersinia pestis) was present within the study population. A plague—caused reduction in density over a wide region would have a significant effect on the reproductive success of a variety of reptilian, avian, and mammalian predators.