EFFECTS OF DROUGHT AND PROLONGED WINTER ON TOWNSENDS GROUND-SQUIRREL DEMOGRAPHY IN SHRUBSTEPPE HABITATS

During a mark-recapture study of Townsend's ground squirrels (Spermoph ilus townsendii) on 20 sites in the Snake River Birds of Prey National Conservation Area, Idaho, in 1991 through 1994, 4407 animals were mar ked in 17 639 capture events. This study of differences in population dynamics of Townsend's ground squirrels among habitats spanned a droug ht near the extreme of the 130-yr record, followed by prolonged winter conditions. Townsend's ground squirrels have a short active season (a pproximate to 4 mo) in which to reproduce and store fat for overwinter ing. Their food consists largely of succulent grasses and forbs in thi s dry shrubsteppe and grassland habitat. The drought in the latter hal f of the 1992 active season produced early drying of Sandberg's bluegr ass (Poa secunda) and was associated with low adult and juvenile body masses prior to immergence into estivation/hibernation. The following prolonged winter was associated with late emergence of females in 1993 . Early-season body masses of adults were low in 1993 relative to 1992 , whereas percentage of body fat in males was relatively high. These w eather patterns in spring 1992 and winter 1993 also resulted in reduce d adult persistence through the approximate to 7-mo inactive period, e specially for adult females, and near-zero persistence of > 1200 juven iles. Consequently, densities of Townsend's ground squirrels across th e 20 livetrap sites declined. The demographic effects of drought and p rolonged winter lasted at least through the subsequent breeding season . Adult females that survived these weather extremes produced fewer em ergent young per female than did adult females prior to the event. Pri or to the drought/prolonged winter, yearling female body masses were h igher than, or indistinguishable from, those of adults. Females produc ed in 1993 had lower body masses as yearlings than did adult females. Demographic response to the drought and prolonged winter varied with h abitat; ground squirrels in sagebrush habitat showed less decline in p ersistence and density and produced more young per female during the n ext active season following the drought (1993) than did ground squirre ls in grassland habitat, where densities had been significantly higher prior to the drought and prolonged winter. Studies involving habitat comparisons of animal demography should always be placed in the contex t of long-term weather patterns, because habitat quality rankings base d on density, reproduction, and survival may differ with environmental conditions. Physiological effects of environmental ''crunches'' on co nsumers may persist beyond the period of influence on food resources, reducing reproductive success and growth rates of future offXspring.