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Oceanic islands are biologically unique and irreplaceable because they have never been connected to continental mainlands and often host endemic species. Human activities have considerably altered these ecosystems. Predator introduction and landscape transformation have severely impacted these ecosystems, as reported by numerous studies.

In addition, these ecosystems change naturally through biological dispersal, in which species move from the mainland to the island. However, no empirical studies have yet integrated the combined effects of human activity and colonization from the mainland to fully capture changes in animal communities on oceanic islands.

In a new study in the Journal of Animal Ecology, the research team investigated bird communities on 10 Izu Islands (nine inhabited and one uninhabited) using literature reviews and field surveys during two periods: the historical period (1970–1973) and the recent period (2016–2021).

The results showed that bird species that had expanded their distribution on the mainland had colonized the Izu Islands. Meanwhile, bird species richness declined on nearly all islands, resulting in a degradation in bird communities. Raptors have also disappeared from many islands. However, at the individual-island level, no direct relationship was found between changes in community structure and either predator introduction (Japanese weasels) or landscape transformation.

The lack of a detected relationship does not necessarily mean that the weasels had no influence, as previous studies have shown that declines of invertebrates, reptiles, and birds were observed on islands where weasels were introduced.

The findings indicate that the degradation of bird communities on islands where weasels were introduced is likely due to direct predation and reduced food resources. This degradation may have spread to neighboring islands through inter-island bird movements, contributing to a further degradation in bird communities across the archipelago. Effective biodiversity conservation in archipelagos therefore requires targeted conservation measures for individual islands and the entire island network.