Climate oscillation and alien species invasion influences oceanic seabird distribution

Aim: We aim to document the extent to which climate oscillation and rat infestation on islands affect the distribution of seabirds at sea. Location: The Chagos Archipelago, British Indian Ocean Territory, Central Indian Ocean Methods: At sea observations of seabirds (n = 425) were collected from 2012 to 2017 during the breeding season. We used generalized additive models to identify relationships between dominant seabird families (Laridae, Sulidae, and Procellariidae), geomorphology, oceanographic variability, and climate oscillation. We built boosted regression trees to quantify the effects of proximity to both rat-free and rat-infested islands on seabird distribution, identifying breaking point thresholds in distribution. Results: We identified oceanic hotspots and common geomorphic and oceanographic drivers for all seabird families. We documented positive relationships between Sulidae and Procellariidae abundance and the Indian Ocean Dipole, as represented by the Dipole Mode Index. The abundance of Laridae and Sulidae declined abruptly with greater distance to island. Both families aggregated more densely (1.08 and 1.25 times higher respectively) and in greater proximity (distribution thresholds at 16 and 44 km closer to islands, respectively) next to rat-free island compared with to rat-infested islands. In contrast, Procellariidae increased in abundance with greater distance to islands, plateauing at 83 km and were not significantly influenced by rat presence on nearby islands. We identified areas of increased abundance at sea under a scenario where rats are eradicated from infested islands with subsequent seabird recolonization. Main conclusions: Climate oscillations may cause shifts in seabird distribution, possibly through changes in regional productivity and prey distribution. Invasive species eradications and subsequent island recolonization can lead to predictable distribution gains and increased competition. Our analysis predicting range extension after successful eradications enables anticipatory threat-mitigation in these areas, minimising competition between colonies and thereby maximising the risk of success and the conservation impact of eradication programmes.