Water Temperature and Availability Shape the Spatial Ecology of a Hot Springs Endemic Toad (Anaxyrus williamsi)

Desert amphibians are limited to exploiting ephemeral resources and aestivating or to inhabiting scarce refuges of permanent water, such as springs. Understanding how amphibians use these resources is essential for their conservation. Dixie Valley Toads (Anaxyrus williamsi) are precinctive to a small system of cold and hot springs in the Dixie Valley, Nevada, USA. The toads have been petitioned for listing under the US Endangered Species Act, and information about how they use terrestrial and aquatic resources will help managers to conserve the toads and identify threats like geothermal energy development that might affect these toads. We used radiotelemetry to study the seasonal home ranges, movements, and habitat associations of Dixie Valley Toads in autumn 2018 and spring 2019. We found that toads were very closely associated with water in both seasons, with most observations occurring in water, especially for males in spring and all toads in the autumn. Even when found in terrestrial habitat, toads were a median distance of 4.2 m (95% credible interval = 3.3–5.3) from water; 95% of the time in spring and autumn, toads were within 14 m of water. Dixie Valley Toad habitat selection indicated a similar pattern, with selection in both spring and autumn for locations closer to water and for warmer water and substrates than at nearby available locations. In autumn, toads also avoided bare ground and terrestrial graminoids. Dixie Valley Toads selected brumation sites in, over (within dense vegetation), or near water, often near springs where water depths and temperatures are likely stable through the winter. The reliance of Dixie Valley Toads on water in spring, autumn, and during brumation suggests that alteration to historical flows and water temperatures are likely to affect the toads. Changes to the hydrothermal environment when toads are brumating could be particularly detrimental, potentially killing inactive toads.