“A great deal of effort and funds have been directed toward protection and preservation of the NWHI, but the focus has largely been on direct human impacts (e.g., disturbance, hunting, fishing, introduced species). The effectiveness of these measures could be substantially diminished if large portions of these essentially off-limits islands simply slip into the sea.” – Jason Baker, Charles Littnan, and David Johnston
At French Frigate Shoals in the Northwestern Hawaiian Islands, a handful of Galapagos sharks have been taking advantage of the high concentration of seal pups at Trig Island, a result of the subsidence in the late 1990s of nearby Whaleskate Island, once a major pupping site for the endangered Hawaiian monk seal. Since 2000, Galapagos sharks have killed an average of 10 pups a year there.
While no one really knows why Whaleskate disappeared, the May 24 Endangered Species Research article, “Potential effects of sea level rise on the terrestrial habitats of endangered and endemic megafauna in the Northwestern Hawaiian Islands,” by Jason Baker, Charles Littnan, and David Johnston, suggests that monk seals, as well as Laysan finches at Pearl and Hermes Reef, and green sea turtles may lose even more habitat as a result of climate change-induced sea level rise.
Using sea level rise predictions by the Intergovernmental Panel on Climate Change, the authors established a median rise of 48 centimeters (19 inches, roughly) by the year 2100. In addition, they evaluated what effects sea level rise would have at the lowest likely level (9 cm), the median level, and the highest level (88 cm) at both low and spring tides at various sites across the Northwestern archipelago.
Lisianski Island, they found, will be the least affected, losing only 5 percent of its area under the maximum rise scenario.
“In contrast, the islets at FFS (French Frigate Shoals) and PHR (Pearl and Hermes Reef) are projected to lose between 15 and 65 percent of their area under the median sea level rise scenario,” they write. Pearl and Hermes islands would lose 51 percent of its habitat at low tide and 69 percent at spring tide. French Frigate shoals would lose 40 and 57 percent, respectively. What’s more, they write, at spring tide, “Gin and Trig Islands at FFS all but disappear.”
The authors note that in 1963, Whaleskate was a vegetated island of 6.8 hectares, and was the second largest island used by monk seals. Between 1985 and 1996, an average of 35 percent of seals born at FFS were born at Whaleskate. When the island disappeared in the late 1990s and the seals moved to Trig, “pup survival fell dramatically in large part due to nearshore predation on pups by Galapagos sharks Carcharhinus galapagensis, a species previously not known to take monk seals. It has been suggested, though not confirmed, that the crowding of females and pups onto Trig Island has facilitated shark predation. Our scenarios project that Trig Island may shrink an additional 7 to 75 percent from its currently already reduced size. This reduction in habitat of an endangered and declining species can only be expected to exacerbate and already lamentable situation,” they write.
Green sea turtles, which are federally listed as threatened, also nest in large numbers at French Frigate Shoals. Fortunately, the authors found, the most popular nesting site, at East Island, is not expected to lose as much habitat as Trig, Gin and Little Gin islands. Still, they warn that increased nest density at East Island could lead to nest destruction and limit population growth.
As for Laysan finches, an endangered honeycreeper, most live on Laysan Island, which, like Lisianski, is projected to lose five percent of its habitat to sea level rise. However, a secondary population established by the U.S. Fish and Wildlife Service at Pearl and Hermes Reef is in danger of going extinct.
“Considerable habitat for these already small populations would be lost under our median scenario, which could greatly increase extinction risk,” they write. To offset the loss, they suggest that finches be established at other, less vulnerable locations.
— Teresa Dawson
Charles Littnan is a researcher with the National Marine Fisheries Service’s Pacific Islands Fisheries Science Center. Baker is also with the science center, as well as the University of Aberdeen’s School of Biological Sciences. Johnston is a researcher with the Joint Institute for Marine and Atmospheric Research in Honolulu. Their article is available on the web at www.hawaiiconservation.org/archives.html.
Volume 17, Number 1 July 2006