Assessing responses of reef island seabirds to environmental and anthropogenic drivers

Tropical insular ecosystems face escalating, cumulative impacts of land- and sea-based ecological threats that jeopardize the long-term sustainability of their native communities. This joint vulnerability is epitomized by reef island seabirds, which are exposed to reef decline and coastal erosion driven by climate change, reduction in prey availability, human disturbance, and predation by invasive non-native species. Using site monitoring data acquired from 2000 to 2023, we investigated drivers of species presence and abundance for 15 seabird species (nine Laridae, three Sulidae, two Fregatidae, and one Procellariidae) on 100 coral reef islands around New Caledonia, South Pacific. We established presence and abundance estimates using a compendium of 2805 observations and tested the relative influence of onsite breeding habitat area, characteristics of neighboring marine environments, human visitation, and rodent presence. Responses to ecological drivers varied across species and highlighted distinct functional and taxonomic groups. The presence and abundance of pelagic-feeding species with high site fidelity were predominantly determined by the availability of breeding habitat, whereas the presence of coastal foragers, including most gulls and terns, appeared constrained by resource availability near breeding sites. Human disturbance had a significant negative influence on the presence of brown noddy (Anous stolidus; [-3.41; -1.20]), masked (Sula dactylatra; [-3.85; -1.21]), and brown (Sula leucogaster; [-4.57; -1.46]) and red-footed booby (Sula sula; [-9.58; -1.20]) and a significant positive influence on the abundance of the commensal silver gull (Larus novaehollandiae; [0.03; 0.09]). Our results emphasize the importance of habitat restoration and mitigation of human disturbance for preserving breeding sites of pelagic-feeding species. However, it may be difficult to sustain populations of coastal seabirds whose presence is mostly driven by small-scale prey distribution. Thus, a comprehensive approach that fully considers the functional diversity of target seabird communities is needed.