Building genomically-informed demographic models to guide management of invasive hybrids

Abstract

Invasive species present one of the most challenging threats to native biodiversity, particularly when they hybridize with imperiled native taxa. In California, hybridization between the endangered California tiger salamander ("CTS," Ambystoma californiense) and the invasive barred tiger salamander ("BTS," Ambystoma mavortium) is one of the best understood examples of this management challenge. Reclusive life history and cryptic hybridization, often on private land, render eradication programs difficult or impossible. This study evaluates hydroperiod management as a tool to conserve and maintain native CTS populations threatened by hybridization. We adapt a recent, empirically informed Bayesian integral projection model (IPM) for CTS to incorporate new results that link genotype and ecology to fitness, and use this individual-based model to evaluate alternative management scenarios. We found overwhelming support for the importance of hydrology in both native and hybrid populations, where a 10-day increase in hydroperiod can increase population growth rate ( λ $$ \uplambda $$ ) 17% and triple the carrying-capacity (K). We assess hydroperiod management as a strategy to control and contain hybrid introgression, and suggest a three-pronged strategy. First, for native populations not at risk of hybridization, hydroperiod should be increased to >120 days to support robust populations. Second, within the geographic hybrid zone, hydroperiod should be reduced to limit hybrid populations, maintain vernal pool function, and improve the efficiency of adult hybrid removal. Finally, our models indicate that managers should combine hydroperiod management with rapid field-based genotyping and hybrid removal, focusing on ponds where hybrids are rare, typically at the leading edge of the hybrid swarm. Efforts should also prioritize high-intensity surveys and early removal as opposed to long-duration (10+ years), lower effort surveys. This study demonstrates the value of integrating demographic, genetic, and ecological information to evaluate strategies for endangered species management, and may serve as modeling framework for a wide variety of imperiled species.