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.
Highlights
Determining the conservation status of species according to a formal set of criteria is widely regarded as essential for conservation action
Our findings show that over the recent three generation time period (1942–2005) the Ascension Island population has declined by 84%
This disparity between the current global conservation status and our regional findings is of concern because even avian species with large populations are not immune from extinction; consider, for example, the cases of Passenger Pigeons Ectopistes migratorius (Halliday 2003) and Carolina Parakeets Conuropsis carolinensis (Snyder 2004)
Summary
Determining the conservation status of species according to a formal set of criteria is widely regarded as essential for conservation action. Over recent decades the Red List of Threatened Species produced by the International Union for Conservation of Nature (hereafter referred to as the ‘IUCN Red List’) has been adopted internationally as the main framework within which the conservation status of animal and plant taxa are assessed (see review by Rodrigues et al 2006). A major criterion among birds is based upon population trend over 10 years, or three generations, whichever is longer. Compared with many other avian taxa, seabirds present challenges when assessing their conservation status because they are typically long-lived, migratory and widely distributed, and individuals may not breed annually. Sources of data for establishing population trends Seabirds are the subject of many long-term monitoring programmes which have revealed that, compared with other taxa, seabird populations have declined rapidly over recent decades (Croxall et al 2012), with the combined global population size of all seabirds having declined by nearly 70% between 1950 and 2010 (Paleczny et al 2015).
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