Ongoing collapse of avifauna in temperate oceanic islands close to the mainland in the Anthropocene

1. Oceanic island ecosystems are highly vulnerable to human activity. Furthermore, oceanic islands close to the mainland provide distinct perspectives on natural processes, including overseas dispersal from the mainland. However, the effects of overseas dispersal and human activity on insular community changes have not been empirically demonstrated. 2. To clarify the mechanisms driving changes in insular communities due to human activity, we investigated bird assemblages on ten islands from 2016 to 2021 and compared them to bird assemblages from 1970 to 1973 on the Izu Islands, Japan. 3. The effects of changes in mainland distribution and species traits on the colonisation and disappearance of bird species on the islands were examined using phylogenetic generalised least squares (PGLS) models. Bird community structures on each island were examined based on species richness and functional and phylogenetic structures. A null modelling approach was used to examine functional and phylogenetic cluster/overdispersion structures; clustering implies species elimination by environmental filters and overdispersion implies a limiting similarity. The effects of geographical and topographical characteristics of the islands, weasel introduction, and landscape transformation were examined using generalised linear models (GLMs). 4. Species that expanded their mainland distribution and have large clutches colonised more islands. Raptors disappeared from many islands. Species richness decreased, and functional and phylogenetic structures clustered on almost all islands, which were only statistically associated with geographical and topographical characteristics of the islands. The lack of an association between changes in community structures and landscape transformation or weasel introduction might be due to the spillover effects of weasels introduced on several islands in the archipelago. 5. The mirroring of the distribution changes in species between the mainland and islands indicates that changes in communities on the mainland affect insular communities via overseas dispersal. Furthermore, introduced predators on an island would have spillover impacts on avian communities across the archipelago through competition for food resources and direct predation, which suggests the need for comprehensive conservation strategies across the archipelago to effectively conserve insular biodiversity.