Rare animals are difficult to study, but understanding their habitat selection and comparative physiology is essential for conserving populations and promoting biodiversity. Extreme environments often harbor high proportions of rare and endemic species. We challenge the assumption that rare species endemic to harsh conditions are physiologically adapted to these environments and thrive there. The Narrow-foot Hygrotus Diving beetle (Hygrotus diversipes) is a rare aquatic beetle found in only a handful of intermittent streams in Wyoming. Hygrotus diversipes have been petitioned for listing under the US Endangered Species Act several times due to our limited knowledge of the species. The beetles experience many harsh environmental pressures in Wyoming’s basins, including desiccation, flash floods, and very high conductivity. We hypothesize H. diversipes are physiologically stressed in the extreme environments where they occur, but they select these habitats due to a competitive disadvantage in less harsh ecosystems. My research integrates habitat selection analyses, survival and choice experiments, and hydrology and climate data to assess the habitat preferences of H. diversipes, their physiological limits, and how environmental change may impact their restricted distribution. Hygrotus diversipes were detected in conductivities as high as 64,000μS/cm, and the highest recorded conductivity at a known site was 110,000μS/cm! Alternatively, our survival experiment found beetles in 30,000μS/cm had a 30% survival rate after just five days, and they were 75 times more likely to die each day compared to beetles in conductivities less than 10,000μS/cm. If these beetles are already living near their physiological limits, increases in salinity due to agriculture, anthropogenic water use, and rising temperatures could increase mortality rates and even lead to the extinction of the species. It is critical to confront the assumption that rare species are physiologically adapted to the extreme environments they occur in because they may be especially vulnerable to habitat change.