INFLUENCE OF HABITAT, SEX, AGE, AND DROUGHT ON THE DIET OF TOWNSENDS GROUND-SQUIRRELS

We assayed diets of Townsend's ground squirrels (Spermophilus townsend ii) in a semi-arid shrub steppe in Idaho by analyzing 1,432 fecal samp les during 1991-1994. Dietary composition differed between adults and juveniles but not between males and females. Sandberg's bluegrass (Poa secunda), a perennial bunchgrass, was the most common item in diets. Diets varied among habitats that differed in plant cover. Diets also b roadened progressively during the active season of this hibernating sp ecies (February-June) to include more shrub material, annuals, and oth er foods. A drought at the end of the active season in 1992 caused low er body mass and survival of individuals because there were fewer annu al plants during the mid-and late-season periods and fewer seeds in th e late-season than in non-drought years. Sandberg's bluegrass was cons umed in higher proportions during the late-season of the drought year, but most of it was senescent with reduced levels of essential fatty a cids and other nutrients. Among native shrubs, winterfat (Ceratoides l anata) was preferred over big sagebrush (Artemisia tridentata). Adults in habitats that lacked shrubs had a lower probability of being recap tured the year following the drought. We conclude that preferred nativ e shrub species provided a relatively constant food source under droug ht conditions, compared with perennial grasses and annual plants. Pere nnial grasses were the most commonly eaten food, and habitats dominate d by them provided a good food resource except under drought condition s. Low germination rates of annual plants under drought conditions and their short period of succulence make them a poor food source, and ha bitats dominated by them are unlikely to sustain viable Populations of ground squirrels.