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The decline in amphibian populations is one of the starkest examples of the biodiversity crisis. For stream breeding amphibians, alterations to natural flow regimes by dams, water diversions, and climate change have been implicated in declines and extirpations. Identifying drivers of amphibian declines requires long time series of abundance data because amphibian populations can exhibit high natural variability. Multiple population viability analysis (MPVA) models integrate abundance data and share information from different populations to estimate how environmental factors influence population growth. Flow alteration has been linked to declines and extirpations in the Foothill Yellow‐legged Frog (Rana boylii), a stream breeding amphibian native to California and Oregon. To date, no study has jointly analyzed abundance data from populations throughout the range of R. boylii in an MPVA model. We compiled time series of egg mass counts (an index of adult female abundance) from R. boylii populations in 36 focal streams and fit an MPVA model to quantify how streamflow metrics, stream temperature, and surrounding land cover affect population growth. We found population growth was positively related to stream temperature and was higher in the years following a wet year with high total annual streamflow. Density dependence was weakest (i.e., carrying capacity was highest) for streams with high seasonality of streamflow and intermediate rates of change in streamflow during spring. Our results highlight how altered streamflow can further increase the risk of decline for R. boylii populations. Managing stream conditions to better match natural flow and thermal regimes would benefit the conservation of R. boylii populations. [ABSTRACT FROM AUTHOR]

Chytridiomycosis, caused by the pathogenic fungus Batrachochytrium dendrobatidis (Bd), has contributed to amphibian declines globally, but drivers of outbreaks vary locally. Here, we explore the role of drought in population and host‐disease dynamics of the endangered stream‐breeding foothill yellow‐legged frog (Rana boylii). In central California (USA) where severity of seasonal drought is increasing, we observed the non‐native, Bd‐tolerant and lentic‐adapted North American bullfrog (Lithobates catesbeianus) extend into streams when flood disturbance was minimal. Analysis of skin swabs revealed that prevalence and load of Bd infection among bullfrogs was low. Yet, among the native frogs, prevalence and load intensified as the seasonal drought progressed and surface flow became intermittent. When temperatures decreased in autumn and frogs concentrated at a reduced number of water points, we found dozens of dead foothill yellow‐legged frogs (2018–2019). Necropsies suggested chytridiomycosis as the likely cause of death. Despite recent lethal outbreaks, foothill yellow‐legged frog population abundance appeared resilient based on comparison to prior decades when no die‐offs were observed. Wet–dry mapping of the stream channel and retrospective analysis of hydrologic records revealed that the native frogs spawn away from perennial pools, a behaviour that may allow them to avoid bullfrogs and predatory fish. In an ecological trade‐off, tadpoles face the risk of the stream drying before metamorphosis. Fluctuations in population size thus corresponded to extremes of inter‐annual variation in streamflow that limit recruitment rather than disease outbreaks. We conclude that hydrologic constraints, which climate change may exacerbate, appear to override the stressors of non‐indigenous species and chytridiomycosis. [ABSTRACT FROM AUTHOR]

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