POPULATION RESPONSES TO ENVIRONMENTAL-CHANGE - LIFE-HISTORY VARIATION, INDIVIDUAL-BASED MODELS, AND THE POPULATION-DYNAMICS OF SHORT-LIVED ORGANISMS

We review two potentially important approaches to predicting the conse quences of environmental change for populations of short-lived organis ms. First, we examine the concepts of ''feasible life histories'' and ''feasible demographies'' and present the results of a set of simulati ons in which the effects on population growth rate of varying one of t he demographic variables (average nest survival, average juvenile surv ival rates, average annual adult survival rates, or age-specific fecun dity) over a broad range of values while the others are maintained at long-term population average values for the Grapevine Hills, Texas pop ulation of the short-lived lizard Sceloporus merriami. The results of these simulations are compared to an analogous set of simulations for a Michigan population of the relatively long-lived snapping turtle (Ch elydra serpentina, Congdon et al., 1994). The implications of differen ces in feasible demographies and life histories such as described for these two species are discussed. We also discuss the approach of using individual-based, physiologically structured models to predict popula tion response to environmental variation and present the results of si mulations using a model developed for predicting population-level effe cts of operative environmental variation in the lizard S. merriami und er two different climate change scenarios. This individual-based, phys iologically structured model incorporates population-specific data on ecological energetics, thermal and size dependence of digestive physio logy and metabolic rates, energetics of individual growth, allometric relationships, social structure and mating system, and the dependence of mortality rates on age, size, and social status of individuals. The data necessary to such models of population response to environmental variation can come only from detailed long-term studies of individual populations.