Act for the best, prepare for the worst—Defining strategic mesopredator triggers that safeguard apex predator eradication benefits

Eradicating invasive species is critical for supporting threatened species recovery. In multi-invaded ecosystems, it may only be feasible to remove one or some of the several invasive species that are present. However, this may not deliver the desired benefits, and shifting the complement of threats in some cases can have unexpected adverse consequences. A priori evaluation of potential outcomes, and identifying critical thresholds driven by other invasives, can inform decision-making and decision triggers for adaptive management responses to ensure benefits are realised.  We use population viability analyses informed by field data to evaluate the potential outcomes of a single species removal from a multi-invaded ecosystem, Christmas Island. We assess whether (1) removing an invasive apex predator (cats; Felis catus) in isolation would allow a threatened seabird species (Indian Ocean red-tailed tropicbird; Phaethon rubricaudra westralis) to recover; and based on evidence that this bird species is susceptible to rat predation (2) find the nest predation threshold for a remaining invasive mesopredator (Rattus rattus) that should trigger complementary rat control to ensure seabird population protection. The analyses indicate that with cat eradication, the tropicbird population is expected to recover quickly and benefit long term. The current empirical rates of nest failure due to rat predation are insufficient to cause tropicbird population decline. However, if nest failure from rat predation should exceed 40%, the tropicbird population is predicted to decline by 1% (SD ± 0.25) per year. If rat predation caused 70% or more nests to fail, the tropicbird population is predicted to decline to extinction in 17 years (~1–2 generations). Synthesis and applications: Our results illustrate the value of PVAs as a tool for understanding potential invasive species management outcomes in multi-invaded ecosystems. Defining thresholds of ecologically significant rat impact informs the setting of decision triggers, which in combination with targeted monitoring, can help managers prepare to adapt to deficient or undesirable outcomes in a timely and appropriate way.