Water Quality Affects Larval Performance in a Near Threatened Neotropical Toad

ABSTRACT Amphibians are experiencing substantial population declines due to environmental stressors such as habitat degradation, disease, pollution and climate change. Melanophryniscus montevidensis , a near‐threatened anuran, experiences regional population declines, although the influence of water quality on its reproductive success remains understudied. We evaluated the effects of in situ biotic and abiotic variables on the development of larvae across three ephemeral ponds in a protected area in Uruguay. We placed eggs from natural breeding events (25 clutches) in semi‐permeable enclosures and monitored them for 34 days. We recorded physicochemical and biotic variables and modelled their effect on survival and body size at three development stages. During early stages (Gosner < 25), no successful hatching occurred in the pond with the highest ammonia concentrations and lowest dissolved oxygen levels. In middle stages (Gosner 26–30), survival decreased with increasing turbidity and larval density, and body size increased with age, but decreased with conductivity; while in late stages (Gosner 31–42), greater depth and larger body size improved outcomes. Our results suggest that natural hydroperiod fluctuations and decomposition processes can generate physicochemical conditions severe enough to compromise recruitment, even within protected areas and under limited anthropogenic eutrophication. These findings underscore the importance of continuous monitoring and adaptive management of water quality and hydroperiod dynamics, particularly under increasing local climatic variability.