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Aldabra is the world’s largest raised coralline atoll. It is situated 640 km east of the African continent, 420 km north-west of Madagascar, and 1,100 km south-west of Mahé. It comprises four main islands circling a central lagoon, Grande Terre, Malabar, Picard and Polymnie. There are many smaller islands and islets, some of which are rat-free. The land area comprises 15,380 ha, the lagoon 22,400 ha, and the IBA also includes a coastal marine area of 10,700 ha. The terrain is characterized by irregular limestone formations called ‘champignon’. The vegetation is dense and shrubby, adapted to relatively low rainfall and strong winds, and dominated in large areas by Pemphis acidula. Around 15% of the land area is more open, and covered in low grasses known as ‘tortoise turf’. The inner rim of the lagoon is lined with mangroves. The flora is exceptionally rich in endemics, with 19 species restricted to Aldabra and 22 others found also only on other islands of the Aldabra group. There is also a pristine marine ecosystem, with an intact fringing reef and an abundant and diverse fish and invertebrate fauna. A small permanent human population is based on Picard, engaged in wardening the reserve and scientific research. There are several huts on the other main islands for the overnight accommodation of scientists and fieldworkers. In the past, the islands were exploited for mangrove wood, turtles and giant land tortoises.
See Box and for key species. The site has, unlike almost every other tropical island of comparable size in the Indian Ocean, no introduced avifauna. There is one extant endemic species, Dicrurus aldabranus (500 pairs), although the status of several other taxa, currently classed as subspecies, is unclear. Another endemic species, Nesillas aldabrana, only discovered in 1967, has not been seen since 1983, and is thought now extinct. Endemic subspecies include Dryolimnas cuvieri aldabranus (1,500–2,700 pairs), the last surviving flightless bird in the Indian Ocean. Another endemic form, Threskiornis aethiopica abbotti (100–250 pairs), is most closely related to the subspecies bernieri of Madagascar which, with it, may form a species distinct from populations on the African mainland. The other endemic subspecies of landbird include Alectroenas sganzini minor (1,000–2,500 pairs), Streptopelia picturata coppingeri (2,000–5,000 pairs), Centropus toulou insularis (400–800 pairs), Caprimulgus madagascariensis aldabrensis (500–1,000 pairs), Hypsipetes madagascariensis rostratus (4,000–8,000 pairs), Nectarinia sovimanga aldabrensis, Zosterops maderaspatana aldabrensis (4,000–10,000 pairs) and Foudia eminentissima aldabranus (1,000–3,000 pairs)—all population estimates are only very approximate. The site is also important for several breeding seabirds, including the world’s second-largest colony of frigatebirds (both Fregata minor and Fregata ariel) as well as, in addition to those listed below, Sterna bergii (60–100 pairs) and the world’s only oceanic breeding colony of Sterna caspia (6–10 pairs). Other breeding waterbirds are Butorides striatus, Ardea cinerea, Anous stolidus and Gygis alba. Aldabra is the only atoll in the world (and the only oceanic site other than Galapagos) where Phoenicopterus ruber (25–50 birds) breeds; evidence of nesting was only discovered in 1995. The lagoon forms an important feeding area for both resident seabirds and shorebirds, and for migrants, notably Dromas ardeola, Arenaria interpres and Sterna saundersi (up to 800).
Non-bird biodiversity: Aldabra has the world’s largest population of the giant tortoise Dipsochelys dussumieri, estimated to number c.100,000 individuals in 1997. It is also of global importance for the sea-turtle Chelonia mydas (EN), which comes ashore to nest throughout the year; surveys between 1981–1985 indicated 10,000–18,000 nesting emergences annually; smaller numbers of Eretmochelys imbricata (CR) also breed. There are four species of lizard, including one endemic race Phelsuma abbotti abbotti. There are also four species of bat, including one endemic race of fruit bat, Pteropus seychellensis aldabrensis. A healthy population of the coconut crab Birgus latro (DD) exists throughout the atoll.
BirdLife International (2021) Important Bird Areas factsheet: Aldabra Special Reserve. Downloaded from http://www.birdlife.org on 13/04/2021.