The least-disturbed large island in the Indian Ocean, Aldabra is of outstanding scientific interest. It is the only place in the world where a reptile is the dominant herbivore; some 150,000 giant tortoises (more than on the Galápagos Islands) feed on tie grasses and shrubbery, including plants that have evolved to take advantage of tortoise grazing patterns. The tortoises are the last survivors of a life form once found on many Indian Ocean islands; slow-moving and vulnerable, the giant land tortoises on all other Indian Ocean islands have been driven to extinction by human exploitation, leaving Aldabra as their only remaining stronghold. The island's isolation has allowed the evolution of a distinct fauna, with two endemic birds (Aldabra arush warbler and Aldabra drongo) and another 11 birds that have distinct subspecies (showing evolution in action); among the most interesting is the Aldabran white-throated rail, the last representative of the western Indian Ocean flightless birds - all others have gone the way of the dodo.
Aldabra is a classic coral atoll which has been built up from the seabed. It consists of four main islands of coral limestone separated by narrow passes and enclosing a large shallow lagoon. Most of the land surface consists of ancient coral reef (about 125,000 years old) now raised above sea level, the rest being even older reef limestones. The lagoon contains many smaller islands and the entire atoll is surrounded by an outer reef. Geomorphological processes have produced a varied topography, generally rugged, which supports a variety of habitats with a relatively rich biota for an oceanic island, and a high degree of endemism. Over much of the surface of the islands, weathering has led to dissection of the limestones into holes and pits, although at the eastern end the surface is more continuous on upraised lagoonal sediments. Along the coast are undercut limestone cliffs, with a perched beach and sand dunes on the southern (windward) coast. Marine habitats range from coral reefs to mangrove mudflats with minimal human impact. Tidal range is more than 3 m, which can lead to strong channel currents.
The terrestrial flora is exceptionally rich for a small coral island, with 273 species of flowering plant and fern. Much of the land is covered with dense Pemohis acidula thicket and other shrubs. There are 19 endemic species, a further 22 species are shared only with neighbouring islands. Many of these plants are considered to be threatened. Mangroves surround the lagoon, and inshore waters also support seagrass meadows.
This island group is one of the few areas of the world where reptiles dominate the terrestrial fauna, with the largest world population of giant tortoise, which appears to be self-sustaining. Green turtle breed here, with approximately 1,000 females laying annually. There are 13 species of terrestrial bird including the last representative of the western Indian Ocean flightless birds - Aldabran rail with two endemic Aldabran forms. The Aldabra warbler has not been seen for several years and might be naturally extinct. Previously restricted to 10 ha of coastal tall scrub, this was considered possibly the most endangered bird in the world, as only five birds have been seen since its discovery in 1968. Aldabran drongo and some endemic subspecies are also found. There is a population of about 8,000 birds of this flightless race, which does not seem seriously threatened by the feral cats. The islands are important breeding grounds for thousands of seabirds, including several thousand each of red-tailed tropic bird and white-tailed tropic bird, hundreds of masked booby, several thousand red-footed booby, some Abbott's booby, and thousands each of greater frigate bird and lesser frigate bird. There are also thousands of nesting terns. The only endemic mammal is a flying fox. So far about 1,000 species of insect have been recorded, many' of them new and endemic forms.
There is no permanent settlement. The resident population is composed of Foundation employees and visiting scientists.