The impact of brodifacoum on non-target wildlife: gaps in knowledge

Anticoagulant poisons, especially the second-generation anticoagulant brodifacoum, are used worldwide to eradicate pest mammals from high priority nature sites. However, the potency and persistence of brodifacoum may present threats to non-target species. In New Zealand, most ecosystems lack native terrestrial mammals; instead, birds, reptiles and invertebrates fulfil key ecosystem roles. Introduced mammals represent the biggest threat to persistence of native species. Therefore, in addition to use in eradications, brodifacoum is often continuously supplied in ecosystems for pest mammal control and detection of mammalian reinvasions, creating a potential long-term risk of poisoning to non-target species. We reviewed literature concerning brodifacoum effects on non-target native fauna in New Zealand as a framework for discussing current research requirements. Birds and their invertebrate prey have, to date, been the focal taxa of such empirical studies (26 species and 11 orders studied, respectively). Brodifacoum is linked to both mortality and sub-lethal contamination in native birds, and the toxicant is consumed by a range of native invertebrates. Reptiles, amphibians, bats and aquatic invertebrates are considered at low risk of anticoagulant poisoning and are not routinely included in risk assessments. However, recent field evidence demonstrates that native geckos consume brodifacoum bait. Reptiles are often abundant on mammal-free offshore islands where brodifacoum is used persistently as a simultaneous rodent detection and killing strategy. Ectothermic vertebrates, though at low risk of toxicosis themselves, may act as vectors of brodifacoum and create a risk of secondary poisoning to native birds. The effectiveness of using poison bait to protect mammal-free ecosystems is uncertain, due to the abundance of alternative food supplies available to an invading rodent. However, where sustained brodifacoum use is deemed appropriate, the role of reptiles as consumers and vectors of anticoagulant poison should be a research priority.