EFFECTS OF TRACKING BY ARMORED VEHICLES ON TOWNSEND GROUND-SQUIRRELS IN THE ORCHARD TRAINING AREA, IDAHO, USA

Maintaining raptor populations is a primary objective of the legislati on that designates the Snake River Birds of Prey National Conservation Area. Army training activities could influence habitat quality for ra ptors by changing the density, productivity, or behavior of their Town send's ground squirrel (Spermophilus townsendii) prey. These changes c ould occur directly or as a result of changes in the vegetation availa ble as food and cover for the ground squirrels. We assessed the effect s of long-term tracking by armored vehicles by comparing 9-ha areas in sagebrush (Artemisia tridentata)-dominated shrubsteppe and bluegrass (Poa secunda)-dominated grasslands subjected to low-intensity tracking for similar to 50 years with others that had not been tracked. We did not detect any effect on ground squirrel population dynamics associat ed with long-term tracking. Although densities of adults and juveniles tended to be higher in the areas exposed to such tracking, we attribu te this difference to other factors that varied spatially. To determin e short-term (two-year) effects, we experimentally tracked two sagebru sh and two grassland sites with an M-1 tank after animals had begun th eir inactive season. In the following two active seasons we monitored squirrel demography and behavior and vegetative characteristics on the experimentally tracked sites and compared the results with control si tes. Although we experimentally tracked similar to 33% of the surface of each of four sites where ground squirrel densities were assessed, t he tracking had a detectable effect only on some herbaceous perennials and did not influence ground squirrel densities or behavior significa ntly during the subsequent two active seasons. We conclude that tracki ng after the start of the inactive season is likely to influence groun d squirrel demography or behavior only if vegetation cover is substant ially changed by decreasing coverage of preferred food plants or incre asing the coverage of annual grasses and forbs that are succulent for only a short time each year.