Aldabra, un atoll éloigné dans le sud-ouest de l'archipel des Seychelles, constitue un écosystème quasiment intact. Pour visiter l'île, plusieurs options se présentent, quoique aucune ne soit peu coûteuse. L'atoll est riche en flore et faune endémiques, bien que des deux espèces d'oiseaux, la Fauvette d'Aldabra Nesillas aldabranus et le Drongo d'Aldabra Dicrurus aldabranus, la première ait probablement disparu. La population du drongo est actuellement estimée à 1,500 individus. L'élévation de plusieurs autres taxons au rang d'espèce pourrait se justifier, comme par exemple dans les cas du Râle d'Aldabra Dryolimnas cuvieri aldabranus, du Foudi de forêt d'Aldabra Fondia eminentissima aldabrana et de l'Ibis sacré Threskiornis aethiopicus abbotti. Huit autres espèces terrestres sont présenter, mais elles ont été peu etudées jusqu'ici. Plusieurs espèces d'oiseaux de mer nichent en nombres important sur l'atoll, le Flamant rose Phoenicopterus nibery niche également, comme prouvé récemment, et un certain nombre d'espèces occasionnelles observées nulle part ailleurs aux Seychelles y ont été notées. Les espèces endémiques et les oiseaux de mer nicheurs sont menacés par des prédateurs, tels que chèvres, chats et rats, le braconnage et l'introduction d'oiseaux exotiques par le personnel de la mine de phosphate de l'île voisine d'Assomption, tels que le Bulbul orphée Pycnonotus jocusus et le Foudi de forêt malgache Foudia eminentissima. Ces espèces pourraient supplanter les taxons endémiques ou, dans le cas du foudi, s'apparier au foudi local et former une population hybride. Toutefois, le gouvernement seychellois adoptant une attitude positive envers la conservation de la nature, I'on pent espérer qu'Aldabra demeurera une île sans trop d'espèces introduites.
Due to its remote location, lack of freshwater, dense scrub, difficult terrain and no small degree of good fortune, Aldabra's ecosystem has survived relatively intact. Turtles, tortoises, some birds and other resources were once heavily exploited. Aldabra's darkest hour came in the mid-1960s when plans were hatched to turn the atoll into a gigantic aircraft carrier. The campaign to save Aldabra, spear-headed by the UK's Royal Society, was probably the first successful, major international conservation campaign. The proponents of the military option were not used to this and attempted to fight back using ridicule. Denis Healey of Britain's Labour Government declared "As I understand, the island of Aldabra is inhabited - like Her Majesty's Opposition Front Bench - by giant turtles, frigate birds and boobies. Nevertheless, it may well provide useful facilities for aircraft'(1). However, Sir Julian Huxley offered a wider vision: 'the animals and plants of Aldabra...can fairly claim to have international value and as such, the owner into whose hand they happen to have fallen, surely has a responsibility to exercise some degree of limitation of his activities in their favour"(1).
The 1967 devaluation of sterling was used as the reason to back down. Magnanimous in defeat, the UK government (then colonial masters of Seychelles), passed the lease of Aldabra to the Royal Society. A scientific research station was established on the atoll. Dubbed 'the land that time forgot' during the media campaign, the Royal Society's work made Aldabra one of the best researched atoll ecosystems in the world. Subsequently, management responsibility passed to the Seychelles Islands Foundation, founded by presidential decree in 1979. The atoll which comprises approximately one-third of Seychelles' total land area, became a nature reserve. In 1982, it became Seychelles' first World Heritage Site.
Aldabra is situated in the extreme south-west of the Seychelles archipelago, 600 km east of Africa, 400 km north-west of Madagascar and 1,100 km south-west of Seychelles main population centre, Mahé in the granitic group of islands. The atoll of Aldabra comprises four main islands - Grande Terre, Malabar, Polymnie and Picard - and a number of smaller lagoon islands and islets. The Aldabra group comprises Aldabra atoll and with the neighbouring islands of similar age and structure - Assumption, Astove and Cosmoledo atolls.
For many years, Royal Society staff on Aldabra needed to employ devious techniques, such as hitching a lift on supertankers from the Middle East, to reach Aldabra. For others the arrival of cruiseships in the western Indian Ocean were an easier, if more expensive, alternative. This is an excellent way to make a short visit to Aldabra provided you chose the right ship. The best cruiseship for expedition travel in the region is Caledonian Star with its team of expert Zodiac drivers and leaders who know the atoll and its wildlife intimately.
For a longer stay, it was formerly necessary to charter a yacht from Mahé for upwards of US$12,000, allowing c6 days at sea for the return journey. Today, there is one live-aboard motor yacht - the Indian Ocean Explorer - based at Aldabra for part of the year, which takes visitors to and from neighbouring Assumption for the flight to Mahé. The price per person in 1998 on a double occupancy basis was US$300 per day plus flight costs. Another live-aboard - the catamaran Aldabra - takes the scenic route between Mahé and Aldabra.
For landlubbers, there is still hope. In 1997, through funding from the World Bank and Seychelles Island Foundation, the old research station, constructed by the Royal Society almost 30 years earlier, was replaced with an attractive log-frame research facility which includes an accommodation block of six twin rooms, each with en suite toilet and shower. This is principally for the use of visiting scientists, but if there is space it is available for private hire. The cost at SR2,000 (cUS$400) per twin or SR1,500 (US$300) per single room per day, which includes guiding services of the resident warden and research officer, is still not cheap, but this is not an average package deal.
Endemics are rarely found on remote coral atolls due to their relatively short geological history. Just 4,000 years ago, sea-levels in the western Indian Ocean were several metres higher than at present. At this time, there was just one point of land between Seychelles granitic islands - 1,000 km to the north-eastand the Aldabra group; the raised coral platform island of St. Pierre. Subsequently, a change in ocean currents led to a localised fall in sea-levels and the emergence of three coral island groups: the Amirantes, the Alphonse group and the Farquhar group.
Given their short history, it is unsurprising that none of the Amirantes, Alphonse group or Farquhar group have any endemic birds. A race of Madagascar Turtle Dove Streptopelia picturata saturata (synonym aldabrana, a misnomer) once occupied some of the Amirantes and a blue pigeon (possibly a race of Comoro Blue Pigeon Alectroenas sganzini) was found in the Farquhar group (29), but both are now extinct. Today, the only landbirds are introduced species.
Aldabra is the world's largest raised coral atoll. Whereas most coral islands, including those of Amirantes, Alphonse and Farquhar do not rise above 3 m, Aldabra is higher, with two platforms at 4 m and 8 m. It has undergone several emergences and submergences during its history, but due to its height Aldabra's last submergence was probably c125,000 years ago.
Given the antiquity of Aldabra, it is perhaps unsurprising that levels of endemism among its flora and fauna are high. Almost all of the landbird species belong to an endemic taxon. Of 176 flowering plants, c40 (22%), are confined to the Aldabra group. By contrast, the coral cay of Bird Island, close to the granitic group, has just 30 flowering plants in total, none of which are endemic (2). Endemic insects too, are well represented: c38% of the estimated 1,000 species. Aldabra's most famous species is the endemic Aldabra Giant Tortoise. The estimated population in 1997 was 100,000 (11), perhaps 90% of the world population of all giant tortoise species.
Sadly, one endemic bird species - Aldabra Warbler Nesillas aldabranus - is probably now extinct. Undiscovered until 1967, except for one record of a bird heard singing, it was only ever known from a 10 ha coastal strip (2 km x 50 m) at the west end of Malabar. A 1974-75 study located five birds: three males and two females (18) and the last sighting, of a single male, was in 1983 (13). At this time it was described as '...almost certainly the rarest, most restricted and most highly threatened species of bird in the world' (5), but by 1994 it was considered extinct (6). It is possible the species might still survive in little-visited and impenetrable south-west Grande Terre, but the chances of this appear increasingly remote. Extinction may have been natural, but rats are also prime suspects given the high level of predation experienced by other species (22).
Aldabra Drongo Dicrurus aldabranus has the distinction of being the sole undisputed endemic still certainly extant. It is relatively common throughout the atoll with an estimated population of 1,500. Related to the Crested Drongo Dicrurus forficatus of Madagascar (3) (the original source of all Aldabra's landbirds), it is accorded species status on the basis of differences in plumage and call. Aldabra awaits a full genetic study of its avifauna. Arguments exist for raising other taxa, currently considered races, to species level. One such is White-throated Rail Dryolimnas cuvieri aldabranus, which has been considered specifically distinct, as Aldabra Rail Dryolimnas aldabranus (25). The Aldabra Rail's differences to Madagascar stock clearly go beyond habitat. It has evolved in geographic isolation and its wings are defunct, giving it the dubious distinction of being the only surviving flightless bird of the Indian Ocean region. Morphologically it is not obviously distinct, but specific status would give this remarkable taxon a higher conservation profile.
A second candidate is Forest Fody Foudia eminentissima aldabrana, which was treated as Aldabra Fody Foudia aldabrana by Sinclair & Langrand (25). Fodies are confined to the western Indian Ocean. The genus comprises 5-6 sexually dimorphic species which vary considerably in plumage and in the size and the shape of the bill (according to diet) (12). Aldabra Fody is particularly distinctive, males being the most attractive of all fodies, possessing a bright scarlet head, neck and upper breast, which is sharply demarcated from the sulphur yellow belly. Its huge bill is remarkable, even relative to its large body size. Forest Fody F. eminentissima of Madagascar is smaller, has a proportionately smaller bill, a grey belly and flanks, and is thus considerably less striking than Aldabra Fody. The taxon confined to Aldabra has a catholic diet, including insects, seeds and nectar (its tongue exhibits a degree of adaptation to a nectar diet). Its large bill is probably the result of adaptive radiation to exploit all available food sources on Aldabra in competition with other landbirds.
Sacred Ibis Threskiornis aethiopicus is currently considered to comprise three subspecies: nominate aethiopicus in Africa, bernieri in Madagascar and abbotti in Aldabra. The ibis of Aldabra and Madagascar are very similar. Both are smaller than the nominate race, the bills are more slender and the tips to the primaries and secondaries are white; these are black in the nominate race (14). The iris of abbotti is blue and that of bernieri white, both considerably paler than the brown iris of the nominate race. It is possible that Madagascan and Aldabran birds merit specific status. If so, the name bernieri has priority and Madagascar Sacred Ibis T. bernieri has two races: nominate bernieri in Madagascar and abbotti on Aldabra. This would have immediate conservation implications. Madagascar Sacred Ibis is probably one of the most threatened species in the Afrotropics. Indeed, Aldabra with 100-250 pairs may prove a vital stronghold.