Effect of density reduction on uinta ground squirrels: analysis of life table response experiments.

Abstract

The effects of natural or experimental environmental perturbations on populations can be diverse, simultaneously affecting several life history variables. Population level responses to such influences frequently are measured as changes in projected population growth rate (l). Sensitivity and elasticity analyses can be used to quantify the potential influence of small changes in different life history variables on l. When a population is subjected to an experimental treatment, life table response experiment (LTRE) analysis allows decomposition of changes in l into contributions due to observed changes in individual life history variables. We investigated the potential and actual influence of demographic characteristics (age at maturity, juvenile and adult survival, fertility, and age at last reproduction) on l of Uinta ground squirrels (Spermophilus armatus) in the Wasatch Mountains of Utah, USA. Ground squirrels were studied in three different habitats, before and after an experimental reduction of population size. Survival and reproduction of squirrels increased in response to a reduction in population density. Consequently, l increased by at least 21% in two of the three habitats. Population growth rate was potentially most sensitive to changes in age at maturity (a) and fertility. LTRE analysis revealed that a did not change and contributed nothing to changes in l, but changes in fertility were large and contributed most to observed changes in l. Age at last reproduction (v) increased after density reduction but contributed little to observed changes in l because of low sensitivity of l to changes in v. Thus, there was little correspondence between potential influence and actual contributions to observed changes in l. We concluded that some demographic variables, notably a, had little environmental or phylogenetic scope for reduction, whereas fertility and to some degree survival rates were considerably more plastic under the experimental treatment. Because LTRE analysis incorporates observed changes in life history variables and also sensitivity of l to these changes, it worked well for quantifying the response of Uinta ground squirrel populations to density manipulation and holds promise for evaluating alternative management strategies in conservation biology.