A new empirical study conducted in collaboration with University of Tsukuba and other institutions revealed that over the past 50 years, bird species that expanded their populations on the mainland have colonized 10 islands in the Izu Islands. However, bird taxonomic, functional, and phylogenetic diversity has declined on nearly all of these islands. On four islands where predators—Japanese weasels—were introduced, the degradation in bird communities was particularly pronounced. This degradation may have spread throughout the Izu Islands owing to reduced bird species populations that move between the islands.

Tsukuba, Japan—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 this study, the research team investigated bird communities on 10 Izu Islands (9 inhabited and 1 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 in 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 interisland 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.

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This work was supported by the 27th Pro-Nature Foundation (Specific Theme Grants-11), Academist (Project-220) and a Grant-in-Aid for JSPS Fellows (JP23KJ1792).

Original Paper

Title of original paper:

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

Journal:

Journal of Animal Ecology

DOI:

10.1111/1365-2656.70070

Correspondence

Assistant Professor IIJIMA Daichi
Institute of Life and Environmental Sciences, University of Tsukuba

ANDO Haruko, Ph.D.
Senior Researcher, Biodiversity Division, National Institute for Environmental Studies

Professor MURAKAMI Masashi
Faculty of Science, Chiba University

Assistant Professor ITO Shun
College of Science, Academic Institute, Shizuoka University

Visiting Researcher FUKUDA Shinpei
Graduate School of Sciences, Toho University

Related Link

Institute of Life and Environmental Sciences