Impacts of Invasive Species on Tropical Seabird Populations on Tetiaroa Atoll, French Polynesia

Abstract

Seabird populations provide crucial ecological services to their environments, yet their populations are declining globally due to invasive species, climate change, habitat loss and degradation, over-fishing, and pollution. Although they are keystone species and bio-indicators of coral island health, tropical seabirds remain understudied. Their reproductive success is influenced by an array of biotic and abiotic factors, particularly in coral island (atoll) ecosystems where low-lying nesting habitats are vulnerable to anthropogenic and environmental disturbances. In Chapter 1, we modeled daily nest survival of Brown Boobies (Sula leucogaster) on Tetiaroa Atoll, Society Islands, French Polynesia, using a Bayesian framework. We analyzed data from 903 nests monitored between 2021 and 2024 across three motus (islets) that differed in exposure to invasive rats before and during a rat-eradication effort. Our daily nest survival model included motu-specific intercepts, a before-after-control-impact design to assess effects of rat suppression, and covariates representing ocean swell height, El Niño-Southern Oscillation conditions, colonial nest density, and an avian disease outbreak event. We found no evidence that rat presence negatively affected daily nest survival, which may indicate that rats are less of a threat to Brown Booby reproduction when compared to other seabird species. Higher ocean swells were associated with lower daily nest survival probability, while positive Southern Oscillation Index values, indicative of La Niña conditions, were associated with higher daily nest survival probabilities. These findings suggest that oceanic variability, rather than rat presence, was the dominant driver of nest success during the study period, suggesting the importance of accounting for risks posed by a changing climate and rising sea levels when developing seabird conservation strategies on atolls. In Chapter 2, we investigated terrestrial habitat associations of four seabird species on Tetiaroa, the Red-footed Booby (Sula sula), Brown Noddy (Anous stolidus), Black Noddy (Anous minutus), and White Tern (Gygis alba). We conducted 824 avian point counts with multiple observers at 110 sites between 2022 and 2024. A variety of vegetation variables were measured at each site, including the abundance of mature native broadleaf trees and previously cultivated coconut palm (Cocos nucifera), which is considered invasive and may be harmful to atoll ecosystem functioning. We fit species-specific N-mixture models to the avian point count data, using a zero-inflation Poisson distribution to model abundance, and habitat variables in both the zero-inflated and abundance components. Estimated Red-footed Booby abundance was higher in areas closer to the shoreline, with greater amounts of the coastal shrub, Heliotropium arboretum, and native broadleaf trees including Pisonia grandis, and in areas with more open space in the herbaceous layer. Abundance was lower with greater amounts of invasive Cocos nucifera trees. We found Brown Noddy estimated abundance was lower in areas with greater amounts of Pandanus tectorius trees. Estimates of Black Noddy abundance and probability of White Tern presence were higher with greater amounts of native broadleaf trees. Our findings provide information on habitat associations of these seabird species, which can be used to inform terrestrial restoration projects on Tetiaroa and other atolls.