Climate-Induced Physiological Stress Drives Rainforest Mammal Population Declines.

Climate change is a major driver of global biodiversity loss, yet the precise mechanisms linking climate change to population declines remain poorly understood. We developed a novel, broadly applicable framework that integrates biophysical, nutritional, and population modeling to capture fundamental physiological constraints on mammalian herbivores and applied it to investigate the causes of declines in ringtail possums of the Australian Wet Tropics (Pseudochirops archeri and Hemibelideus lemuroides). Our approach bridges the gap between mechanistic ("bottom-up") models, which simulate species' responses based solely on their traits and local microclimates, and the more common ("top-down") statistical models, which infer species' responses from occurrence or abundance data and standard environmental variables. We quantified population dynamics over a 30-year period by generating species-specific estimates of temperature and water stress, foraging limitations, and linking these with annual monitoring and nutritional quality within an open population model. Our findings demonstrate that climate change has impacted populations through physiological stress, but in a species-specific manner. Both species have experienced population collapses at lower elevations and in low-nutritional sites. For P. archeri, we found evidence that population changes were driven by reduced survival due to overheating and dehydration, alongside diminished recruitment from limited foraging. In contrast, our model suggests that H. lemuroides populations were primarily affected by foraging constraints, emphasizing the importance of considering climate-driven limitations on foraging activity in addition to direct physiological stress. These mechanistic insights offer a foundation for targeted conservation strategies to mitigate the impacts of climate pressures on wild populations.