Direct evidence of poison-driven widespread population decline in a wild vertebrate

Toxicants such as organochlorine insecticides, lead ammunition, and veterinary drugs have caused severe wildlife poisoning, pushing the populations of several apex species to the edge of extinction. These prime cases epitomize the serious threat that wildlife poisoning poses to biodiversity. Much of the evidence on population effects of wildlife poisoning rests on assessments conducted at an individual level, from which population-level effects are inferred. Contrastingly, we demonstrate a straightforward relationship between poison-induced individual mortality and population changes in the threatened red kite (Milvus milvus). By linking field data of 1,075 poisoned red kites to changes in occupancy and abundance across 274 sites (10 × 10-km squares) over a 20-y time frame, we show a clear relationship between red kite poisoning and the decline of its breeding population in Spain, including local extinctions. Our results further support the species listing as endangered, after a breeding population decline of 31% to 43% in two decades of this once-abundant raptor. Given that poisoning threatens the global populations of more than 2,600 animal species worldwide, a greater understanding of its population-level effects may aid biodiversity conservation through increased regulatory control of chemical substances. Our results illustrate the great potential of long-term and large-scale on-ground monitoring to assist in this task.Hundreds of volunteers and personnel from Spanish autonomous governments (Comunidades Autonomas [CCAA]) participated in the national census of red kites, organized by SEO/Birdlife, with financial support from the Royal Society for the Protection of Birds, the Spanish Ministry of Environment (SME), and CCAA. The database of poisoned animals was compiled by CCAA and SME and coordinated by WWF Spain and SEO/Birdlife through the ANTÍDOTO program. This work was funded by WWF Spain under project UCLM-UCTR170245, and by the BBVA Foundation (“Ayudas Fundación BBVA a Equipos de Investigación Científica 2018”) through the TÓXICO project.Peer reviewe