As Aldabra is a raised coral atoll, most of the limestone terrain lies some 4 - 8 metres above sea level. Smoother ‘platin’ rock forms much of the eastern part of Grande Terre and part of Picard, with a thin layer of soil and a vegetation cover of varying height, density and composition. Much of the rest of the atoll has very eroded and pitted limestone (‘champignon’) with only pockets of soil and usually dense scrub vegetation, often dominated by Pemphis acidula. About 12% of plant species are endemic. The giant tortoise is the dominant land vertebrate, along with terrestrial bird species, most of which are endemic at the species or sub-species level. The only native mammals are bats. Of the three native lizards one is endemic. There are several species of land crab, including the coconut or robber crab (Birgus latro) and a wide variety of other terrestrial invertebrates, of which quite a number are endemic.
Fauna conservation objectives
a) Maintain the biodiversity and genetic diversity of all native animal species.
b) Maintain habitats and communities for these species.
c) Protection of giant tortoises
d) If it is found to be necessary, restoration of habitats to a more natural state.
Vegetation conservation objectives
a) Maintain biodiversity and genetic diversity of the vegetation.
b) Maintain habitat diversity and integrity.
c) Monitor for any changes in the vegetation of the atoll.
d) If found necessary, restore vegetation to a more natural state.
Freshwater and brackish water pools are important, unusual and varied habitats found on Aldabra. Fresh water collects in pools as a direct result of rainfall, therefore truly freshwater habitats are mostly ephemeral and only a few pools are permanent sources of fresh water throughout the year. Many larger animals such as tortoises, birds and land crabs rely on the pools for water and often also for food. Most pools are brackish and many are linked with the sea through underground passages. A freshwater lens is sometimes present on the surface of the saline water.
a) Maintain the pool habitats and the communities and biodiversity within them.
b) Note any changes that might be affecting the freshwater supply for Aldabra plants and animals.
Coastal ecosystems (beaches, mangroves, lagoon islets)
Some fifty small sandy beaches are scattered along the coastline of Aldabra, especially on the north, west and south coasts. Behind some of them are dunes up to about 15 metres high. Together these beaches form a very important breeding ground for green turtles (Chelonia mydas). Some hawksbill turtles also nest there, although this species tends to prefer the small beaches inside the lagoon. Along the inner atoll edge, the lagoon is bordered with mangrove forest, which is more extensive along the eastern edges of the lagoon and is a very important breeding site for greater and lesser frigate birds, and also red-footed boobies. Intertidal mudflats are important feeding grounds for wading birds and shore birds, including migrants. The small lagoon islands and islets are mostly clustered at the western and eastern ends of the lagoon, some of them serving as the only rat-free refuge for ground nesting birds.
a) Maintain biodiversity and habitat diversity of coastal ecosystems, including mangrove, beaches, lagoon islets.
b) Protection of turtles and turtle nesting sites.
c) Protection of frigate and booby colonies and their nesting areas.
d) Protection of nesting sites of other marine birds.
Marine ecosystems (lagoon/reefs/open water)
There are several large channels which link Aldabra’s large lagoon with the reefs and ocean outside. Of these Grande Passe is the largest, through which about 60% of lagoon water passes in and out at speeds of up to 6 knots at each change of tide. A wide variety of marine habitats is found within the lagoon and through the channels, extending out to the reef flats and fringing coral reefs surrounding the atoll. Beyond the reef there is a gradual slope down to 400 metres or more. The lagoon is an important sheltered site for numerous fish (including sharks and rays) and other marine organisms. Fish life is diverse, although certain invertebrate groups, such as echinoderms, seem to be under-represented.
a) Maintain habitat diversity, and biodiversity within habitats, including coral reefs, sea grass beds, reef flats and habitats within the lagoon and channels.
b) Protection of the coastline, through protection of the fringing reefs.
c) Protection of breeding grounds for fish, lobsters, octopus, prawns, etc.
d) Protection of populations of foraging immature turtles and their habitat.