Aldabra, the world's largest atoll, is one of the coralline islands of the Seychelles, lying some 1,000 km to the south-west of the granitic Seychelles (EBA 100). Within the same remote archipelago, but not included in the EBA, are three other islands: Astove, Cosmoledo and Assumption. Aldabra comprises four main islands-Malabar (or Middle Island), Grand Terre (South Island), Picard (West Island) and Polymnie-enclosing a large central lagoon bordered by mangroves. Mixed scrub (a particularly complex association) grows on the higher and more consolidated rock which runs round the north, west and south ocean coast edges, as well as covering much of a large area of flat rock at the east end of the atoll; between the mangroves and mixed scrub, there is a band (of varying width) of scrub which is heavily dominated by the salt-tolerant shrub Pemphis acidula (Prys-Jones and Diamond 1984).

This EBA has been identified on the basis of its one endemic extant and one endemic extinct species; two additional (extant) restricted-range species also occur there. Nesillas aldabrana was only known from a 10-ha strip of dense mixed scrub on the north-west coast of Malabar (an area free from goats), where no more than seven birds have ever been seen, and since 1977 there have been only two records, both of males; in 1986 the species could not be found, despite intensive searches, and it is now feared extinct (Roberts 1987). Dicrurus aldabranus is widespread and is considered (based on territory sizes and habitat distribution) to number roughly 1,500 birds (Collar and Stuart 1985).

In addition to the restricted-range species, there are ten endemic subspecies including: Aldabra Sacred Ibis Threskiornis aethiopica abbotti, once very scarce due to human exploitation and disturbance but now widespread; and Aldabra White-throated Rail Dryolimnas cuvieri aldabranus, now only on two of the main islands (Malabar and Polymnie) and one of the few small islets (this race differs from the nominate form in Madagascar by being flightless, and is thus the only remaining flightless bird on western Indian Ocean islands) (Collar 1982, Collar 1993, Hambler et al. 1993). The ecology of all landbirds on Aldabra is discussed in Prys-Jones and Diamond (1984).

Unlike neighbouring islands, Aldabra has escaped extensive interference by man and is now one of the few elevated limestone atolls in the world to remain relatively undisturbed (see also Henderson Island in the south-east Pacific Ocean, EBA 215). Nevertheless, coconut palms and Casuarina trees, probably introductions, have become established groves in a few places, and substantial amounts of mangrove wood were once exported to Mahé, and its marine resources (in particular fish, oysters and shells, but also seabirds) were harvested for sale on Mahé and overseas.

The past introduction of cats, goats and rats poses a significant threat to the bird populations, and it is likely that rats were the main cause of the (presumed) extinction of Nesillas aldabrana. There is also a very real risk that an array of exotic bird species (including Red-whiskered Bulbul Pycnonotus jocosus, a nest predator of small passerines) introduced to neighbouring Assumption Island in the 1970s, will reach Aldabra, just 27 km away (Roberts 1988).

In 1982 Aldabra was decreed a World Heritage Site in recognition of its unique unspoilt environment. The island is managed by the Seychelles Island Foundation which has recently been the beneficiary of a Global Environment Facility grant to fund conservation/research officers and to refurbish the infrastructure on the island including the research station, as well as funding a goat-eradication program which is now virtually complete (although, given the size of Aldabra, there is some doubt about whether complete elimination is possible: A. Skerrett in litt. 1996). Tourists may visit Aldabra for the day, arriving by boat, and will be able to stay overnight in small numbers in the near future. If this tourism is not managed adequately, Aldabra's fragile ecosystems and wildlife could be locally threatened (G. Rocamora in litt. 1996, N. Jivan Shah in litt. 1996).

Stattersfield, A. J., Crosby, M. J., Long, A. J. and Wege, D. C. (1998) Endemic Bird Areas of the World. Priorities for biodiversity conservation. BirdLife Conservation Series 7. Cambridge, UK: BirdLife International.

Recommended citation
BirdLife International (2024) Endemic Bird Area factsheet: Aldabra. Downloaded from https://datazone.birdlife.org/eba/factsheet/107 on 27/12/2024.