Scaling from individual physiological measures to population-level demographic change: Case studies and future directions for conservation management

Loss of biodiversity is a leading conservation issue and, accordingly, a central topic in ecological research is to predict how organisms respond to natural and anthropogenic environmental stressors. Proactive conservation science involves management strategies that rely on early identification and monitoring of threats before demographic instability is reached and may provide a more cost- and time-effective method for managing risks in an increasingly uncertain world. Using physiological measurements to predict organismal responses to environmental perturbations has historically been uncommon in the wild, despite the promise they hold as a tool to support management decisions. We provide an overview of interdisciplinary research that investigates environmental variation in conjunction with physiological processes to understand, and potentially predict, population-level demographic responses, which we refer to as Environment-Physiology-Demography, or EPD, studies. Using four EPD case studies (common murre, Cape mountain zebra, Daphnia magna, and common lizard) of the 29 we discovered during our literature review, we demonstrate how physiological biomarkers can be used as indicators of population change and/or stability to aid resource managers in the decision-making process. Of the 29 EPD studies we found, 72% were successful in connecting physiology to both an environmental and demographic change. Further, we outline geographic, taxonomic, and physiological biases observed across EPD studies, and the importance of considering the context-dependency of physiological traits when linking them to environmental variation and demographic processes. We encourage researchers to consider the EPD approach when investigating if and how the responses of individuals to environmental stressors translate into population-level consequences