Evaluating use of extant species response to invasive predator control as an indicator of functional eradication to support species reintroductions

Abstract

Invasive predators on islands are major contributors to global biodiversity loss. In inhabited or large islands, total eradication of invasive predators can be prohibitively difficult. Predator suppression to mitigate threats to native species is an alternative approach, which can be conceptually framed as functional eradication. For species that have been extirpated by invasive predators, acquiring direct knowledge of predator–prey relationships to evaluate whether threat mitigation supports reintroduction can be challenging. We thus evaluated the hypothesis that increases in extant species counts indicated predator threat was mitigated sufficiently to support survival after reintroduction of lost species. We suppressed an enclosed population of invasive brown treesnakes ( Boiga irregularis ) in Guam, a predator responsible for widespread vertebrate declines and losses after their introduction. We also counted extant lizards during snake removal. We selected three lizard species that were not of conservation concern but were locally extirpated after snake introductions. We translocated the three species and monitored counts and survival after translocations in the suppressed and a paired, unsuppressed site. In the suppression site, we found that snake density was a predictor of lizard counts, with lizards increasing most dramatically when the density of snakes ≥950 mm snout‐vent length reached densities of less than 3 snakes per ha. We found that lizards released in the suppression area after extant lizard responses had significantly greater survival following release, with brown treesnakes being the primary cause of death post‐release. In the suppressed area, we continued to detect two of the three species 3 years after the initial release in the suppressed area, although they rapidly became undetectable in the untreated area. We suggest that using responses of taxonomically similar, extant species to invasive predator control may indicate threat mitigation, or successful functional eradication, such that a subsequent reintroduction of extirpated native species may succeed.