California and the western United States are experiencing increases in the frequency and severity of extreme weather events, including droughts, floods, and heatwaves. Such extreme weather events interact with other stressors – including invasive species and disease – to affect amphibians in ways that may be difficult to predict. Amphibians are the most threatened vertebrate taxon on earth, and Department of Defense (DoD) installations in California represent essential refugia to many threatened and endangered amphibian species. Resource managers at DoD installations, who are challenged with managing these species in the face of multiple stressors, require knowledge, tools, and approaches to provide cost-effective stewardship of the natural resources and ecosystems on their lands. The objective of this project is to advance scientific understanding of, and develop management strategies for, the interacting effects of multiple stressors on threatened amphibian species in California. The focal stressors are (a) extreme weather events, including heat waves, droughts, and floods, (b) infectious diseases, and (c) introduced species, but the project team will also obtain information on the effects of other stressors, including livestock grazing and (opportunistically) wildfires. This project focuses on two DoD-relevant threatened amphibian species, the California red-legged frog (Rana draytonii) and the California tiger salamander (Ambystoma californiense), but the results and the tools developed here will also provide information on other DoD-relevant aquatic species, including the southwestern pond turtle (Actinemys pallida) and the tidewater goby (Eucyclogobius newberryi), along with the western spadefoot toad (Spea hammondii).

This project will (i) employ cutting-edge statistical approaches to identify the interactive effects of stressors on amphibians by leveraging a powerful, long-term dataset on amphibian communities from 85 lentic (pond) freshwater sites sampled over 11 years; (ii) extend and enhance this long-term dataset by adding sensor array networks to capture high-resolution information for a suite of stressors and responses; (iii) expand the surveys to the amphibian communities and stressors at Vandenberg Space Force Base and four other DoD installations in California, including both lentic and lotic (stream) systems; (iv) conduct mesocosm experiments to identify the mechanisms of interactions among stressors; and, finally, (v) synthesize information from each task into site-based, mechanistic population models that can be used to predict the effects of interacting stressors and identify effective management strategies. Importantly, the research will consider interactions both across space and through time, thereby identifying whether stressor interactions will manifest at particular spatial extents or temporal lags – an outcome only possible through the uniquely long-term nature of the assembled data.

This project will offer insights into how infectious diseases, invasive species, and extreme events that alter water availability interact to affect amphibian communities. The resulting findings will provide knowledge and tools for natural resource managers at DoD installations in California to effectively manage populations of threatened and endangered amphibians in an uncertain and increasingly variable future. Specifically, the combination of analyses from a long-term, spatially expansive dataset, which encompassed multiple drought and flood events, with new data from the freshwater systems at DoD installations and adjacent lands, high-resolution information from the sensor arrays, and manipulative experiments will provide valuable insight into the mechanisms through which stressors combine to affect amphibian populations. The novel statistical approaches and site-specific models will be used with scenario planning to provide managers with a strategic decision-making framework to apply information about multiple stressors in applied management. These analytical approaches will further allow for inclusion of potential feedbacks, nonlinearities, and time lags in the impacts of the various stressors. Given the limited resources available for conservation and the vast size of many DoD installations, the research will help target specific areas for cost-effective management strategies.