A new perspective on amphibians under global change: Detecting sublethal effects of environmental stress as agents of silent population declines

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Amphibians are at the forefront of the worldwide biodiversity crisis due to disturbances associated with anthropogenic global change such as habitat destruction, ongoing urbanization, and agricultural expansion. But biodiversity is also lost in habitats unaffected by such direct disturbances, possibly caused by environmental stressors such as chemical pollution, pathogens, and ongoing climate change resulting in steady but silent population declines. Exposure to sublethal environmental stressors increases the stress-hormone level but might also affect morphology, physiology, and immune functioning with important carry-over effects to performance, health, and survival. In global change reality, amphibians at all life stages are exposed to simultaneously occurring stressors but studies investigating interactive effects of stressors are rare and long-term studies on various life stages of amphibians are completely missing but needed to estimate sublethal stress effects on population scale.The proposed project aims to clarify the role of sublethal environmental stressors in enigmatic amphibian declines and to develop an early warning tool for detecting if populations are already chronically stressed but not yet or insidiously declining by noninvasive stress-hormone measurements in amphibian breeding ponds. In particular, this project will investigate short- and long-term responses to thermal stress and nitrate pollution by analyzing morphological, physiological, endocrinological, immunological, and genetic biomarkers as suitable proxies for stress. The study will focus on a common, but slowly declining anuran species, the common frog (Rana temporaria). Also, this study aims to determine the acclimation potential of this species to stressful conditions and will help to assess the future threat of R. temporaria in the face of global change. The study is based on three consecutive components: (A) a meta-analysis, studying the empirical knowledge on the interactive effects of aquatic contaminants and temperature on amphibians, (B) a field study over three years, focusing on the impact of different nitrate contaminations on stress-hormone levels in larval and adult R. temporaria, and (C) a laboratory study, investigating the effects of temperature fluctuations and nitrate pollution on various biomarkers of stress during and after metamorphosis. The results of this study will enable us to understand stress responses to changing environments more holistically by detecting multiple biomarkers of stress and will provide insights into sublethal effects of global change which affect amphibian populations insidiously resulting in either decline or adaptation. Further, the results of this study might serve as a basis for appropriate conservation strategies before populations of R. temporaria proceed to decline dramatically across its distribution range.