Threatened and extinct island endemic birds of the world: Distribution, threats and functional diversity

Aim The world's islands support disproportionate levels of endemic avian biodiversity despite suffering numerous extinctions. While intensive recent research has focused on island bird conservation or extinction, few global syntheses have considered these factors together from the perspective of morphological trait diversity. Here, we provide a global summary of the status and ecology of extant and extinct island birds, the threats they face and the implications of species loss for island functional diversity. Location Global. Taxon Birds. Methods We provide a review of the literature on threatened and extinct island birds, with a particular focus on global studies that have incorporated functional diversity. Alongside this, we analyse IUCN Red List data in relation to distribution, threats and taxonomy. Using null models and functional hypervolumes, in combination with morphological trait data, we assess the functional diversity represented by threatened and extinct island endemic birds. Results and main conclusions We find that almost half of all island endemic birds extant in 1500 CE are currently either extinct or threatened with extinction, with the majority of threatened extant species having declining population trends. We also found evidence of 66 island endemic subspecies extinctions. The primary threats to extant island endemic birds currently are agriculture, biological resource use, and invasive species. While there is overlap between the hotspots of threatened and extinct island endemics birds, there are some notable differences, including the Philippines and Indonesia, which support a substantial number of threatened species but have no recorded post-1500 CE bird extinctions. Traits associated with threatened island endemic birds are large body mass, flightlessness, aquatic predator, omnivorous and vertivorous trophic niches, marine habitat affinity, and, paradoxically, higher dispersal ability. Critically, we find that threatened endemics (i) occupy distinct areas of beak morphospace, and (ii) represent substantial unique areas of the overall functional space of island endemics. We caution that the loss of threatened species may have severe effects on the ecological functions birds provide on islands.