Long‐term population trends of Sooty Terns Onychoprion fuscatus: implications for conservation status

Abstract

Seabirds have suffered dramatic declines in population over recent decades. The most abundant seabirds of tropical oceans are Sooty Terns Onychoprion fuscatus and they have an IUCN Red List category of ‘Least Concern’. Ascension Island has the largest colony of Sooty Terns in the Atlantic Ocean and censuses between 1990 and 2013 have shown that its population size is static. In this study we have used historical data and recent censuses to describe the population status of Sooty Terns on Ascension Island over a century. We show that the breeding population contained over 2 million individuals in the 1870s and remained at this level for at least 70 years. However, the population declined from >2 million birds in 1942 to 350,000 birds by 1990. The population trend spanning a period equivalent to three generations of the species (63 years; 1942–2005) showed an approximate 84% decline in population size. Using IUCN criteria this suggests that Sooty Terns on Ascension could be considered ‘Critically Endangered'. We conclude that a re‐evaluation of Sooty Tern conservation status is necessary at the local level and possibly globally. Our study highlights that for long‐lived species historic demographic data should be considered when determining conservation status.

Long‐term dietary shift and population decline of a pelagic seabird—A health check on the tropical Atlantic?

“…Sample sizes of birds are indicated within brackets. In both plots, the shaded area represents the transition between before and after the population collapse of the Ascension Island sooty tern population (Hughes, Martin, Giles et al, ) [Colour figure can be viewed at wileyonlinelibrary.com]…”

“…Bivariate plots of δ 15 N against δ 13 C demonstrating the isotopic niche areas reconstructed from SIA of feathers from sooty terns breeding on Ascension Island in the South Atlantic in decades before (1890s–1940s) and after (1970s–2010s) their population collapse (Hughes, Martin, Giles et al, ). The Standard Ellipse Areas corrected for small sample size (SEA C s) are represented by the solid lines (see Parnell et al, for more details on these metrics of isotopic niche width based on SIA) [Colour figure can be viewed at wileyonlinelibrary.com]…”

“…The study was carried out on Ascension (07°57′S, 14°24′W), a 97 km 2 volcanic island that is isolated in the tropical South Atlantic with its nearest neighbour being the island of St Helena 1,300 km to the southeast. The territory contains several Important Bird Areas (IBAs), including the sooty tern nesting grounds at the “Wideawake Fairs” (IBA SH009) (site description retrieved from http://datazone.birdlife.org), and provides the only breeding location for significant numbers of sooty terns in the central, tropical Atlantic Ocean (Ashmole & Ashmole, ; Hughes, Martin, Giles et al, ). A detailed account of the breeding biology of sooty terns on the island is provided by Ashmole ().…”

“…This mirrors the estimated 69.7% decline in the global seabird population between 1950 and 2010 (inclusive) calculated by Paleczny, Hammill, Karpouzi, and Pauly () based upon data from 3,213 breeding populations of 324 seabird species. Such declines in sooty tern numbers on Ascension Island are unprecedented; earlier estimates of the population in 1877 and 1942 suggested it contained 2.43 and 2.35 million birds, respectively (Hughes, Martin, Giles et al, ).…”

“…However, changes to such ecosystems may have occurred that pre‐date modern monitoring programmes. Here, we use a historical ecology approach to investigate how long‐term changes to marine food webs have influenced the foraging ecology of the world's most abundant tropical seabird, the sooty tern ( Onychoprion fuscatus ) at the Atlantic's largest nesting colony on Ascension Island (Hughes, Martin, Giles, & Reynolds, ). Sooty terns are long‐lived (in some cases >38 years old; Schreiber et al, ), pan‐tropically distributed (Schreiber et al, ), and near‐obligate commensals of tuna and other sub‐surface predators (Au & Pitman, ), making them ideal indicators of the status of tropical marine ecosystems.…”