Effect of density reduction on Uinta ground squirrels: Analysis of life table response experiments

The effects of natural or experimental environmental perturbations on popul ations can be diverse, simultaneously affecting several life history variab les. Population-level responses to such influences frequently are measured as changes in projected population growth rate (lambda). Sensitivity and el asticity analyses can be used to quantify the potential influence of small changes in different life history variables on lambda. When a population is subjected to an experimental treatment, life table response experiment (LT RE) analysis allows decomposition of changes in lambda into contributions d ue to observed changes in individual life history variables. We investigate d the potential and actual influence of demographic characteristics (age at maturity, juvenile and adult survival, fertility, and age at last reproduc tion) on lambda of Uinta ground squirrels (Spermophilus armatus) in the Was atch Mountains of Utah, USA. Ground squirrels were studied in three differe nt habitats, before and after an experimental reduction of population size. Survival and reproduction of squirrels increased in response to a reductio n in population density. Consequently. lambda increased by at least 21% in two of the three habitats. Population growth rate was potentially most sens itive to changes in age at maturity (alpha) and fertility. LTRE analysis re vealed that at did not change and contributed nothing to changes in lambda, but changes in fertility were large and contributed most to observed chang es in lambda. Age at last reproduction (omega) increased after density redu ction but contributed little to observed changes in lambda because of low s ensitivity of lambda to changes in omega. Thus, there was little correspond ence between potential influence and actual contributions to observed chang es in lambda. We concluded that some demographic variables, notably alpha, had little environmental or phylogenetic scope for reduction, whereas ferti lity and to some degree survival rates were considerably more plastic under the experimental treatment. Because LTRE analysis incorporates observed ch anges in life history variables and also sensitivity of lambda to these cha nges, it worked well for quantifying the response of Uinta ground squirrel populations to density manipulation and holds promise for evaluating altern ative management strategies in conservation biology.