How Many Deaths Are Too Many? Assessing the Impact of Additional Mortality on a Critically Endangered Bird

ABSTRACT Small, threatened populations can be vulnerable to changes in vital demographic rates such as mortality. Managers must intervene if mortality outpaces reproduction. Background mortality (e.g., from disease, predation, or inbreeding) may sometimes be offset by conservation actions like supplementation of in situ populations with captive‐bred individuals. However, if emerging threats impose additional mortality above background levels, managers must reassess the intensity and efficacy of their actions. We consider whether additional releases of captive‐bred Critically Endangered orange‐bellied parrots ( Neophema chrysogaster ) can compensate for emerging threats that escalate already high background mortality rates. Using population models, we evaluated four management scenarios: do nothing; status quo; status quo with additional mortality; and a higher effort supplementation with additional mortality scenario. We designed scenarios around observed supplementation rates since 2013, and we estimated additional mortality rates from realistic scenarios. We found that supplementation is preventing the collapse of orange‐bellied parrot numbers, and these efforts support a growing population. Additional mortality (where ≤ 20 parrots were killed each year after background mortality was applied) always produced smaller, declining populations. Even with an unrealistic 1.5× increase in captive breeding output, population sizes were lower than the status quo. The success of orange‐bellied parrot conservation is precariously balanced between the persistent 80% mortality of in situ juveniles each year—an issue that cannot currently be mitigated due to knowledge gaps—and ongoing supplementation with enough captive‐bred juveniles to maintain population stability. We highlight the species' fragility and demonstrate the value of simulations in conservation management where new threats emerge for long‐running conservation programs.