Investigating the patterns and processes that determine the geographic distributions of organisms is a central goal of ecology and evolutionary biology, and an understanding of these processes is essential for scientists to predict how species ranges will respond to environmental change. This study uses modern ecoinformatic databases and spatially-explicit species distributional modeling to resolve important relationships between the environment and stress. The work will include measuring physiological stress in ways that are well established in biomedical research, but until now have not been applied to natural populations within an explicit geographic context. Measures of genetic variability (e.g., inbreeding and heterozygosity) will also be measured, as previous work has shown that populations at the edge of the species range have reduced genetic variability that is associated with lower quality habitats and reduced ability to survive or adapt to changes in environmental conditions. Thus, this project will be one of the first to unify physiological, genetic, and environmental assessments of stress in wild populations. While positive correlations among risk of population extinction, inbreeding and isolation, and physiological stress are widely assumed, direct comparisons among these ecological, genetic, and physiological measures of fitness have never been directly compared. The research focuses on the wood frog as a model system because amphibians are sensitive to environmental change, and they are the most threatened group of vertebrates on the planet. The result will be a statistical assessment of the relationship between environmental quality and stress across geographic space within the range of the wood frog. The goal is to provide a toolkit of fitness indicators for conservation biologists and biogeographers to use when predicting the geography of population health to make better predictions about whether populations will persist given the threats of climate change or habitat destruction.

This project aims to ground-truth a new technique that may become a useful tool in conservation biology to help assess population health and sub-lethal environmental stress in amphibians. This project provides the first integration of environmental endocrinology, population genetics, and spatially-explicit distributional modeling in a landscape-level stress assessment. This research will also enhance the education of both undergraduate and graduate students and train them to be integrative biologists who can use powerful bioinformatics databases to design studies and ask important conservation questions. Finally, in addition to research publications, presentations, and adding museum specimens to a natural history collection, a workshop on the Biogeography of Stress will be conducted to bring together research scientists, conservation biologists, and students to enhance dissemination of findings and synthesis of future directions in this field.

Project Report

This project focused on understanding the relationships between our estimates of environmental quality and indicators of physiological stress across a species range. Our goal was to develop methods to measure the health of populations in the field and determine how natural variation in stress hormone levels, or the ability to respond to a stressor, are related to environmental and geographic parameters that vary in space over time. Our study was unique because we measured multiple indicators of population health (hormone assays, foraging behavior, body size and condition, disease prevalence, genetic diversity) across multiple populations of wood frogs, a cold-adapated species whose eastern range extends from Alabama to Nova Scotia. We assessed habitat quality by measuring local features of breeding ponds and geographic information systems (GIS)-based analyses to tease apart the relative influence of climate, human disturbance, and geography on physiological measures of stress across the landscape. We also conducted large-scale ecological experiments in which we either standardized environments in a common garden or reciprocally transplanted animals from different environments to ultimately understand how environmental conditions shape the ability of animals to response to stressors. We focused our work on the wood frog because it has the one of the largest range of any amphibian in North America, and because it is cold-adapted, it is one of the few species of frogs that can be found in northern climates as far as Alaska. This feature also makes this species particularly vulnerable to global warming trends, and the more we understand about the physiology, genetics, and ecology of this species will help us understand how it might respond to climate change. Our work has revealed that the measurement of glucocorticoid hormones to assay stress responsiveness in populations across the eastern range, which spans the most dramatic climate gradient in the species range, can identify populations that are experiencing stress at both local and regional scales. On the regional scale, we found wood frog populations at the southern edge of the range have elevated resting levels of glucocorticoid hormones and reduced stress responsiveness, and they have the least amount of genetic variation, likely suffering from the effects of isolation from other populations compounded by climates hovering at the species tolerance. Yet, populations within the center of this range in highly suitable habitat and the greatest amount of genetic variation harbor the highest prevalence of ranavirus infection. Our analysis at a smaller scale within this region shows that wood frogs from roadside ponds exhibit higher glucocorticoid levels and edema, and tadpoles from these ponds experience higher ranavirus exposures and reduced growth and survival likely due to the run-off of salt and other contaminants from roads. Therefore, we’ve characterized two very different dimensions of population health that have different gradients within the eastern wood frog range. This is the first study to generate such large-scale, multidimensional patterns of population health, which are of interest to wildlife management of this species, as well as other amphibians. These patterns generates brand new hypotheses and a deeper understanding of the impact of both climate and land use on populations, which will undoubtedly fuel future research to uncover the ecological and physiological mechanisms that have generated these patterns. This work was instrumental in bringing together researchers from diverse disciplines including community phylogenetics, disease, physiology, biogeography, and population dynamics in a teaching and research symposium at the International Biogeography Society meeting. These interactions will hopefully inspire this model of interdisciplinary science to the study of other animal and plant systems. Broader Impacts also focused on collaborations and interactions with students, technicians, and researchers at multiple field stations, wilderness areas, and national forest lands across parts of the eastern United States and Canada. As part of our work multiple graduate students and undergraduates at multiple institutions (principally The University of Alabama and Washington State University) were involved in field and laboratory work, and we have presented this work at over 30 different venues. Research also extended to 17 separate natural history museums where we measured collections of the wood frog from across North America. In summary this research should advance the field of biogeography by linking neuroendocrinology, population genetics, and ecology through a better understanding of how stress is partitioned in populations distributed across space in time.

National Science Foundation (NSF)
Division of Behavioral and Cognitive Sciences (BCS)
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Thomas J. Baerwald
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Washington State University
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