Recovery under the Endangered Species Act (ESA) requires that threats have been addressed at the spatial extent of a species' entire geographic range and the temporal extent of the “foreseeable future.” This means that long-standing and chronic threats must be addressed sufficiently so these same threats would not be expected to recur. For some species, threats may be so persistent that a species becomes conservation-reliant, requiring long-term commitments for continued management after delisting. The federally endangered Least Bell’s Vireo (LBVI, Vireo bellii pusillus) nests in narrow riparian areas surrounded by the heavily urbanized matrix of Southern California. Several of the largest remaining patches of LBVI nesting habitat occur on U.S. Department of Defense (DoD) military training areas and in U.S. Army Corps of Engineers (USACE) flood control basins. Expenses for ESA compliance related to LBVI are among the highest of all listed bird species, with widespread operational and financial impacts on federal action agencies and military installations. A complex mixture of stressors threatens LBVI populations. One challenge in developing management strategies for recovery is understanding how different stressors may interact, particularly with emerging threats like new invasive plant and insect species or major changes in disturbance regimes due to climate change.

The project team will develop a spatially explicit agent-based model, called Recovering Endangered Species with Emerging Threats (RESET), to understand the effects of individual and interacting threats and stressors on LBVI populations and to develop alternative management strategies to address them. RESET will be developed to address the specific case of LBVI; however, it will also be designed as a flexible framework to address problems with other listed species along arid and semiarid streams in the western United States. The model will include inputs and submodels for stressors related to groundwater extraction, loss of streamflow inputs due to water recycling programs, changing flood and fire regimes, nest parasitism by Molotrhus ater, and negative impacts to riparian vegetation from invasive non-native plants and insects. The project team will take a strong inference approach to the design of simulation experiments to isolate the effects of individual stressors and to understand how multiple, interacting stressors may affect the distribution and quality of LBVI nesting habitat, reproductive performance, and metapopulation dynamics. Based on this information, the project team will use simulation experiments to compare a suite of alternative management strategies to understand how specific management actions directed at one or more of these stressors may improve LBVI nesting habitat conditions, alleviate key threats, and provide the least cost path to recovery.

RESET will provide DoD, USACE, and other action agencies, with a novel tool for predicting the effects of specific stressors, and stressor management strategies, on LBVI populations locally, regionally, or at the scale of the listed population. Model analyses will directly support future management decisions, and allocation of resources, on both military installations and civil works projects where compliance with existing and future Biological Opinions incur significant expenditures. Specifically, the model will be used to create an ESA Section 7(a)(1) conservation plan for USACE that should improve operational flexibility while improving outcomes for LBVI. Model analyses could also be used to meet other objectives of recent U.S. Fish and Wildlife Service (USFWS) guidance for LBVI recovery (USFWS 2020a) like the development of a cost-effective range-wide cowbird management or regional strategies for invasive exotic plant removal. Importantly, outcomes from management-related simulation experiments will be expressed as distributions of possible outcomes given natural variation that typify riparian systems and uncertainty related to interacting stressors and emerging threats.