Effects of long-term isolation on genetic variation and within-island population genetic structure in Christmas Island (Indian Ocean) seabirds

Abstract

Seabirds endemic to remote oceanic islands face several unique threats to their population viability. Christmas Island is a small oceanic island with remarkable seabird biodiversity, including two endangered endemics: Christmas Island frigatebirds (Fregata andrewsi) and Abbott’s boobies (Papasula abbotti). Christmas Island seabirds are currently threatened by habitat destruction at breeding sites due to phosphate mining and by the adverse effects of the invasive yellow crazy ant (Anoplolepis gracilipes); however, conservation initiatives are limited by a paucity of genetic data on the breeding seabirds. To address this shortcoming, we collected genetic samples from five breeding seabird species, including the two endemics. We surveyed mitochondrial and nuclear genetic diversity, estimated effective population sizes, and tested whether species had undergone recent or long-term effective population size decline. Effective population sizes for Christmas Island frigatebirds and Abbott’s boobies were both low (approximately 5,000 and 2,100 individuals, respectively). Moreover, four out of the five species appeared to have undergone long-term population decline, suggesting that mitigation of the threats to population viability of Christmas Island seabirds is critical. We also detected cryptic population genetic structure within Abbott’s booby, perhaps due to in situ divergence on Christmas Island or due to the influx of individuals from an extirpated colony. Within-island differentiation may be common among endemic seabird species due to high levels of natal philopatry, and we suggest that careful consideration of population genetic structure on small geographic scales is crucial when designing conservation initiatives.