Ducie is a low oceanic atoll of exceptional remoteness at the extreme south-east of Polynesia. It is the southernmost atoll in the world. The fringing reef is oval, roughly 3 km along the east-west axis and 2.5 km along the north-south axis. The single passage at the south-west corner faces south-west and experiences the full impact of the prevailing heavy swells. It is not navigable. The principal motu, called Acadia Island, has an area of 55 ha and stretches for about 3.5 km along the northern reef. Three other small motus are named Edwards, Pandora and Westward; all can be reached by walking from Acadia over the reef at low tide. Their combined land area is about 5 ha.The central lagoon, up to 12 m deep, is characterised by a well-preserved death assemblage of a formerly prolific coral fauna, encrusted by a much sparser live coral assemblage. Presumably the formerly abundant corals have been killed by influxes of cold water at this island which is towards the southern limit of coral growth.Ducie's remoteness is evident in the depauperate flora. Acadia is covered in a monospecific forest of Argusia argentea. A second woody species, Pemphis acidula, was recorded in 1991, and there are records from the 1922 Whitney Expedition of a grass and a vine. No other vascular plants are known. Ducie receives visits once or twice a year from cruise ships which land their passengers on the north shore of Acadia. It is quite possible that there are other visits which are unrecorded, but probably infrequent.
Over 90 percent of the world's Murphy's Petrels nest on Ducie, making the atoll of supreme importance for this species. It is also important for two other surface-nesting Pterodroma petrels, the Herald and Kermadec. All three species may benefit from the 1997 eradication of Pacific rats, Rattus exulans. It is also possible that Henderson Petrels may begin to nest again on Ducie. However, Ducie's extreme remoteness probably precludes any monitoring of population recoveries.The fourth seabird species contributing to Ducie's status as an IBA is the Christmas Shearwater. Because of the species' sub-annual breeding cycle, the current population estimate, a minimum of 3000 pairs, is not especially reliable, but it makes Ducie one of the species' largest colonies, holding around five percent of the known world breeding population. Phoenix Petrels, considered globally Vulnerable, apparently disappeared from Ducie between the Whitney visit in 1922 and the 1991/1992 Expedition. The populations of Fairy Tern (5000 pairs) and Red-tailed Tropicbird (500-1000) are substantial. Other seabird species breed in lesser numbers. There are no landbirds on Ducie.
Non-bird biodiversity: For all taxonomic groups hitherto investigated, Ducie has proved to be poor in species. This is a consequence of its isolation at the extreme south-east of Polynesia, the source of nearly all taxa. Somewhat perversely, the very paucity of species is what makes the atoll so interesting to students of biogeography.
Site-account by M. de L. Brooke.
BirdLife International (2021) Important Bird Areas factsheet: Ducie Island. Downloaded from http://www.birdlife.org on 05/12/2021.