February 2011 was a bad month for seabirds living on the Midway, Kure and Laysan atolls. A winter storm swept across this portion of the North Pacific, with winds exceeding 115 kilometers per hour that whipped water across the low-lying islands. Then the Tohoku tsunami brought another bout of high waves. The winds and water wiped out some 250,000 Laysan albatross and 30,000 black-footed albatross nests, and an unknown number of adult Bonin petrels, great frigatebirds, red-footed boobies and other seabirds drowned, were trapped beneath fallen trees or killed by other means.
Though a seabird’s life is rarely easy, that winter was especially hard. But scientists now warn that it may be just the beginning — sea level rise could result in even greater losses in coming years.
Sea levels are rising because of the one-two punch of melting glaciers and the thermal expansion of the oceans, both caused by global warming. But the biggest problem isn’t the simple rise of water — it’s that sea level rise makes flooding from storms worse.
Michelle Reynolds of the U.S. Geological Survey in Hawaii National Park and colleagues wanted to know how those wave-driven events would affect seabirds nesting on low-lying islands in the North Pacific. The birds nest on 58 islands within U.S. National Marine Monuments, places where the animals have managed to escape the human occupation and mammal invasions that have driven them off other islands in the region. The team concentrated on 13 seabird species found on the three islands of the Midway Atoll. Using computer models, the team simulated wave-driven flooding and inundation on the islands under various levels of sea level rise, from 0.5 to 2 meters, and then looked at how that water would affect the birds that nested on the islands. They report their findings September 23 in PLOS ONE.
Differences in each bird species — nesting at different times of the year, for instance, and in different places on the islands — and variations in flooding, which was worse in the winter when strong storms were common, meant that the effects of sea level rise were not the same for all 13 species. But three — the Laysan albatross, the black-footed albatross and the Bonin petrel — were highly vulnerable, the researchers found. In these three species, all the birds nest at the same time in the winter.
Rising groundwater threatened ground-nesting Laysan albatrosses. Once sea levels reach 2 meters of rise, 15 percent of the nests on the atoll would be inundated, the team calculated. Wave-driven flooding was a big concern for black-footed albatrosses, which nest along the coasts, with more than 70 percent of habitat suitable for nesting predicted to flood. Benin petrels would be vulnerable to both types of flooding. These losses could lead to population declines and threaten the survival of the species, the team warns.
“With rising sea levels and increased perturbations from storm wave overwash, contemporary colonial seabird habitat is likely to become increasingly unsuitable and may function as an ecological trap,” the researchers write. “Decades of reproductive effort may be lost at colonies as low-lying islands continue to attract breeders … despite chronic nest failure.”
The birds tend to stick to the same spots for nesting and are slow to learn that sites are not good places to make a home for their offspring. The team notes that Laysan albatrosses still build nests near a U.S. Navy airfield on Kauai, Hawaii, despite a nearly three-decade effort to reduce bird-aircraft collisions by removing every egg laid.
What can be done? The birds can’t nest on higher elevation islands in the region because those sites are contaminated with introduced predators, such as rats, and other human-caused threats. But if those threats could be removed or controlled, such as by fencing off refuges, then perhaps the birds that are more adventurous and willing to gamble on a new home might be able to establish new nesting sites and give their species a better chance at survival.