Comparing opportunistic and strategic removal efforts to manage invasive fish species using a dynamic multi-state occupancy model.
Growing ecological and economic impacts of invasive species have heightened the need for new science to inform management that prevents and counteracts their impacts. A persistent challenge is to identify whether removal programs, which range widely in their approach, are successful. Given the variability in dynamic environmental conditions and complex biotic interactions among species, identifying the effectiveness of removal programs intended to lessen nonnative species impacts is inherently difficult. We were interested in whether strategic (i.e. significant coordination and investment) and opportunistic (i.e. temporally periodic and implemented at smaller spatial scales) removal programs result in measurable, and if so, comparable benefits to native fish conservation. We applied a novel dynamic co-occurrence model to evaluate nonnative fish removal efforts across the Lower Colorado River Basin (USA), while accounting for potential interspecific species interactions and variable hydrologic regimes. We examined the co-occurrence of two native fishes of conservation concern and two other representative fishes of endemic assemblages, each paired with a nonnative species hypothesized to negatively affect the focal native species. Our model results suggest that both strategic and opportunistic removal programs result in lower extinction probabilities for native fishes (1.2 times greater for strategic programs), especially species of conservation concern, when compared to locations and years not subject to removal. Nonnative species presence mediated native species dynamics, most notably by increasing native species extinction probability during drought years regardless of removal program. Removal efforts resulted in modest changes in extinction probabilities for nonnative fishes, but no measurable effect on colonization dynamics. Synthesis and applications. Both strategic and opportunistic nonnative fish removal programs can help meet recovery goals for native fishes, even over large areas and long after nonnative species are established. Quantifying dynamics of native and nonnative fishes in a multi-state dynamic co-occurrence model highlights the collective management value of individual programs at regional scales. Increased evaluation of ongoing nonnative removal programs remains critical to ensuring that management actions are warranted and effective in meeting desired conservation outcomes.