Mapping Habitat Suitability Rangewide for Headwater Stream-Associated Torrent Salamanders

Abstract

In the moist, coniferous forests of Oregon and Washington, a dense network of headwater streams supports high levels of amphibian species endemism and diversity, including the torrent salamanders (Rhyacotriton spp.). The Columbia and Cascade torrent salamanders (R. kezeri and R. cascadae, respectively) are species of greatest conservation need, but current information is insufficient for an adequate status assessment. We aimed to determine the current distributional extent, occurrence, and abundance of both species across their respective ranges, and to characterize watershed-scale habitat suitability. We conducted rangewide surveys for both species and used a novel, robust model development and refinement process to characterize habitat suitability based on climatic, topographic, forest-structure, hydrological, and water-balance drivers of their distributions. Results supported interspecific differences in rangewide occurrence and abundance patterns, as well as ecological associations that provide insights into potentially divergent patterns of resilience. Topographic, water-balance, and streamflow metrics were top predictors of Columbia torrent occurrences, whereas metrics of seasonal temperature and atmospheric moisture were top predictors of Cascade torrent occurrences. Our findings can inform regional conservation planning efforts to identify likely climate refugia and habitat-connectivity pathways for gene flow and watershed-scale ecological resilience.