Dynamics of seabird colonies vulnerable to sea-level rise at French Frigate Shoals, Hawai`i

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 sea-level 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 sea-level rise. We compiled a unique
combination of data sets: historical observations of islands and seabirds, a 30-year 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 sea-level 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), Red-tailed Tropicbird (Phaethon rubricauda),
Masked Booby (Sula dactylatra), Gray-backed 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 sea-level rise. We project 12% of French Frigate Shoals (excluding
La Perouse Pinnacle) will be inundated with +1.0 m sea-level rise or 32% with +2.0 m. Gray-backed Terns that nest along the coastal perimeters of islands and shrub-nesting species that
are habitat limited are especially vulnerable to sea-level 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 low-lying Hawaiian Islands by
2100 and emphasize the need to restore higher elevation seabird colonies.