Being high is better: effects of elevation and habitat on arctic ground squirrel demography

We investigated the effect of local environment on the demography and population dynamics of arctic ground squirrels (Spermophilus parryii plesius ) by comparing reproduction, survival, and population trends of squirrels living in low elevation boreal forest and high elevation alpine tundra sites in southwestern Yukon Territory, Canada. Contrary to the trend for most birds and mammals, reproduction was significantly lower at the lower elevation and females living at higher elevation did not delay the age at which they first reproduced. Even though survival in the boreal forest was lower in summer than in the alpine, it was higher over winter so annual adult female survival was similar between sites. Sensitivity analysis of model parameters revealed that in the forest, population growth rate (l) was most sensitive to small changes in adult active season survival whereas for the alpine population, l was most sensitive to changes in juvenile winter survival. In their respective habitats, these parameters also showed high year to year variation and thus contributed greatly to the population trends observed. Even though ground squirrels persisted in the boreal forest, the measured demographic rates indicate the forest was sink habitat (lB/1) and may have relied on nearby grassy meadows for immigrants. In contrast, the alpine habitat maintained a ground squirrel population in the absence of immigration (l"/1). The variation in demographic rates between ground squirrels living at high and low elevation may arise from phenotypic responses of squirrels to different habitat structure. Arctic ground squirrels rely on sight to detect predators from a safe distance, and the boreal forest, with its lower visibility and higher predator density, appears to be suboptimal habitat.