Summary

Description

Globally, seabirds are vulnerable to anthropogenic threats both at sea and on land. Seabirds typically nest colonially and show strong site fidelity; therefore, conservation strategies could benefit from an understanding of the population dynamics and vulnerability of breeding colonies to climate change. More than 350 atolls exist across the Pacific Ocean; while they provide nesting habitat for many seabirds, they are also vulnerable to sealevel rise. We used French Frigate Shoals, the largest atoll in the Hawaiian Archipelago, as a case study to explore seabird colony dynamics and the potential consequences of sealevel rise. We compiled a unique combination of data sets: historical observations of islands and seabirds, a 30year time series of population abundance, LiDAR light detection and ranging derived elevations, and satellite imagery. To model population dynamics for ten species at Tern Island from 1980 to 2009, we used the Gompertz model with parameters for the population growth rate, density dependence, process variation, and observation error. We used a Bayesian approach to estimate the parameters. All species increased in a pattern that provided evidence of density dependence. Density dependence may exacerbate the consequences of sealevel rise on seabirds because species that are already near the carrying capacity of the nesting habitat will be limited more than species that still have space for population growth. Laysan Albatross Phoebastria immutabilis, Great Frigatebird Fregata minor, Redtailed Tropicbird Phaethon rubricauda, Masked Booby Sula dactylatra, Graybacked Tern Onychoprion lunatus, and White Tern Gygis alba are likely already at carrying capacity at Tern Island and therefore are most likely to be negatively impacted by sealevel rise. We project 12 of French Frigate Shoals excluding La Perouse Pinnacle will be inundated with 1.0 m sealevel rise or 32 with 2.0 m. Graybacked Terns that nest along the coastal perimeters of islands and shrubnesting species that are habitat limited are especially vulnerable to sealevel rise. However, at Tern Island, seawalls and habitat creation may mitigate projected seabird population declines due to habitat loss. We predict substantial losses in seabird nesting habitat across the lowlying Hawaiian Islands by 2100 and emphasize the need to restore higher elevation seabird colonies.