Population Responses to Environmental Change: Life History Variation, Individual-based Models, and the Population Dynamics of Short-lived Organisms

SYNOPSIS. We review two potentially important approaches topredicting the consequences of environmental change for populationsof short-lived organisms. First, we examine the concepts of"feasible life histories" and "feasible demographies" and presentthe results of a set of simulations in which the effects onpopulation growth rate of varying one of the demographic variables(average nest survival, average juvenile survival rates, averageannual adult survival rates, or age-specific fecundity) overa broad range of values while the others are maintained at long-termpopulation average values for the Grapevine Hills, Texas populationof the short-lived lizard Sceloporus merriami. The results ofthese simulations are compared to an analogous set of simulationsfor a Michigan population of the relatively long-lived snappingturtle (Chelydra serpentina, Congdon et al., 1994). The implicationsof differences in feasible demographies and life histories suchas described for these two species are discussed. We also discuss the approach of using individual-based, physiologicallystructured models to predict population response to environmentalvariation and present the results of simulations using a modeldeveloped for predicting population-level effects of operativeenvironmental variation in the lizard S. merriami under twodifferent climate change scenarios. This individual-based, physiologicallystructured model incorporates population-specific data on ecologicalenergetics, thermal and size dependence of digestive physiologyand metabolic rates, energetics of individual growth, allometricrelationships, social structure and mating system, and the dependenceof mortality rates on age, size, and social status of individuals.The data necessary to such models of population response toenvironmental variation can come only from detailed long-termstudies of individual populations.