review · Biological reviews/Biological reviews of the Cambridge Philosophical Society
While research on island birds has historically concentrated on volcanic islands, coral atolls represent distinct environments shaped by cyclical sea-level changes. A quantitative synthesis across the Indo-Pacific demonstrates that land birds regularly occur on atolls and frequently include endemic species and subspecies. These dynamic habitats act as evolutionary arenas where birds exhibit rapid divergence, find refuge on uplifted atolls, or persist as relicts of populations lost from volcanic islands. Despite possessing low species richness, atoll land birds perform vital ecosystem functions, such as insectivory, pollination, seed predation, and seed dispersal. Although past extinctions and habitat contractions have diminished these ecological contributions, the wider impacts on atoll integrity remain unmeasured. These insights help build a comprehensive understanding of island evolution and offer foundational guidance for regional conservation and restoration initiatives.
Coral atolls have long been overlooked in island evolutionary biology in favour of volcanic islands. By showing that atolls support unique endemic bird populations and vital ecological functions like pollination and pest control, this research reframes atolls as critical conservation targets. Preserving atoll birds is essential not only for preventing extinctions but also for sustaining the broader health and functioning of fragile low-lying island ecosystems.
This work can inform environmental management, specifically guiding biodiversity preservation and habitat restoration programmes led by conservation practitioners, non-governmental organisations, and regional environmental agencies. Because the synthesis focuses on macroecological patterns and evolutionary concepts, it remains early-stage research that offers baseline scientific evidence rather than operational management tools, commercial products, or market-ready services.
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Oceanic islands are global hotspots of avian biodiversity and endemism. Their isolation and distinct environmental conditions have uniquely shaped the evolutionary trajectories of island birds, while, in turn, land bird communities have become critical providers of ecological functions to island ecosystems. However, these processes and dynamics have been studied almost exclusively on large volcanic islands. Coral atolls - the other principal type of oceanic island - remain largely absent from ecological research on island bird diversity, evolution, and functioning. This omission constitutes a major gap because atoll islands are systems with distinct geo-evolutionary histories and dynamics. Recurrent cycles of emergence and submergence of atolls with glacial sea-level fluctuations have created unique natural experiments that offer novel insights into island assembly and evolution, different to those on volcanic islands. Here, we present the first synthesis of land bird assemblages on atoll islands across the Indo-Pacific. Our quantitative review of land bird records across Indo-Pacific atolls reveals that land birds are widely occurring on atolls throughout this region, often represented by endemic species or subspecies. Building on this macroecological data, we then synthesise the evolutionary pathways of land birds in the context of the atolls' geo-evolutionary dynamics, including refugial persistence on uplifted atolls, rapid divergence following recent colonisation, and relictual persistence of bird lineages extirpated from volcanic islands. Together, this synthesis suggests that atoll islands act as unique evolutionary arenas in which avian colonisation and diversification are repeatedly 'replayed' across successive emergence-submergence cycles. In addition to the unique conditions that atolls provide for land birds, we also review the ecological functions that land birds provide to atoll island ecosystems. Despite typically low species richness, atoll land bird assemblages provide ecological functions including pollination, seed dispersal, seed predation, and insectivory. Extinctions and range contractions have eroded much of this functional spectrum from many atolls but the consequences for atoll ecosystem integrity remain virtually unstudied. By synthesising land bird diversity, evolutionary history, and functional ecology on atolls, we highlight a reciprocal relationship: atolls provide unique evolutionary arenas for land birds, while land birds support ecological functioning on atoll island ecosystems. Together, the generated insights are relevant for domain-specific knowledge of atoll ecosystems, for a more holistic theoretical understanding of island evolution processes in dynamic and young island systems, as well as for conservation and restoration actions aimed at safeguarding both Indo-Pacific avian diversity and the integrity of atoll ecosystems.
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DOI: 10.1002/brv.70206
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