Physiological Limits and Climate Vulnerability in Tropical Freshwater Fishes.

Tropical freshwater fishes evolved under historically stable thermal, oxygen, and chemical regimes that favored physiological specialization within narrow environmental ranges. Rapid climate warming is now compressing these already limited safety margins by simultaneously increasing metabolic demand, reducing oxygen availability, altering ionoregulatory balance, and intensifying exposure to pollutants. Evidence from Amazonian systems shows that physiological limits emerge from interacting constraints rather than single stressors, and that tolerance measured under controlled conditions often overestimates persistence in nature. As safety margins erode, energetic trade-offs propagate from molecular stress and metabolic disruption to impaired growth, reproduction, and population stability, ultimately altering ecosystem function. This integrative perspective reveals that vulnerability is defined not by absolute thermal limits alone, but by how multiple constraints accumulate and scale across levels of biological organization. Understanding these scaling relationships is essential for predicting resilience and for advancing physiology from documenting responses toward anticipating ecological outcomes in a rapidly warming world.