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

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Abstract
SYNOPSIS. We review two potentially important approaches to predicting the consequences of environmental change for populations of short-lived organisms. First, we examine the concepts of “feasible life histories” and “feasible demographies” and present the results of a set of simulations in which the effects on population growth rate of varying one of the demographic variables (average nest survival, average juvenile survival rates, average annual adult survival rates, or age-specific fecundity) over a broad range of values while the others are maintained at long-term population average values for the Grapevine Hills, Texas population 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 (Chelydra serpentina, Congdon et al., 1994). The implications of differences 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 population response to environmental variation and present the results of simulations using a model developed for predicting population-level effects of operative environmental variation in the lizard S. merriami under two different climate change scenarios. This individual-based, physiologically structured model incorporates population-specific data on ecological energetics, thermal and size dependence of digestive physiology 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.

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