Human visitation disrupts natural determinants of breeding seabird communities on coral reef islands

Coral Reef Islands (CRIs) support key colonies of tropical seabirds, whose long-term sustainability is under growing threat from climate change and small-scale, island-specific pressures such as invasive species or human disturbance. Unlike most compartments of reef ecosystems, CRI seabirds have been poorly studied from a biogeographical perspective, which questions the relevance of existing conservation policies. Here, we investigate the effects of ecological, geographical and anthropogenic drivers on the diversity and abundance of CRI seabird communities, using survey data acquired over the 2000-2023 period in 100 CRIs of New Caledonia, South-western Pacific. Our results show that the diversity and biomass of breeding communities are mostly constrained by the availability of breeding habitat and by the characteristics of neighboring marine environments. We observe a negative effect of human visitation on both diversity and biomass, indicating that the mitigation of human disturbance combined with habitat maintenance are key for sustaining seabird colonies on CRIs. Our findings highlight the complementary roles of remote and coastal islands, which host distinct species assemblages, and demonstrate that although isolated islands are often seen as optimal conservation targets, they only host part of the diversity of CRI seabirds. This study emphasizes the importance of combining species and site monitoring for assessing the ability of managers to effectively restore the state of seabird communities and contribute to the health of adjacent reef ecosystems. In addition to local policies, we also underline that future management of CRI-dependent seabird communities must integrate multi -scale, long-term assessments of reef decline and coastal erosion.