Boundaries

About 1,500 km from the east coast of Africa lay the Seychelles, unique Indian Ocean islands with steep mountains and a primitive fauna that includes several endemic aquatic species. Comprised of 115 islands, the archipelago covers an area of 455 km² and forms the Republic of Seychelles (FAO Inland Water Resources and Aquaculture Service Fishery Resources Division 1999).  

Forty-one of the 115 islands in the Seychelles archipelago are granitic, and the remainder is coralline. The granite islands are the oldest oceanic islands in the world, a continental formation that was once part of Gondwanaland about 65 million years ago (Rocamora & Skerrett 2001). The islands are clustered in a small area, all situated within ninety miles of Mahé (Library of Congress 1994). The granitic islands have a total area of 241 km², whereas the coralline islands cover an area of 214 km² (CIA 2001). Some of the more prominent coralline islands include the Aldabra group, the Farquhar group, the Amirantes group, and the geographically isolated Bird and Denis islands.

Topography

The granitic islands are striking in their topography, with mountains rising steeply from the ocean to heights up to 905 m asl (Calström 1995). Some of the granitic islands have narrow coastal plains, and some are fringed by extensive coral reefs (Library of Congress 1994). Mahé, which is only 27 km long and about 11 km wide, is the highest of the islands, with a mountain ridge running along its length (Statistics and Database Administration Section MISD 2000).

The coralline islands average only a few meters above sea level, and were formed by the buildup of coral reefs over an 82 million year old granitic basement which is found less than 1 km below the surface (Rocamora & Skerrett 2001). Aldabra Atoll, with an area of about 130 km², is the world’s largest atoll (Stattersfield et al. 1998; UNEP 1998). The atoll, located in the extreme southwest of the Seychelles archipelago, is comprised of four main islands—Malabar (or Middle Island), Grand Terre (South Island), Polymnie, and Picard (West Island)—as well as a number of smaller islands. These islands enclose a large, shallow lagoon (about 300 km²) that is bordered by mangroves (Fosberg & Renvoize 1980). Aldabra has a greater elevation than any of the other Coralline Seychelles, at about 4 to 8 m above mean low-tide level with some dunes at 10-30 m (Skerrett 1999). The coralline islands have been submerged for several intervals in their history, the last interval being approximately 125,000 years ago (Rocamora & Skerrett 2001).

Freshwater habitats

Freshwater habitats of the Aldabra group include freshwater ponds, abundant ephemeral pools, and crevices with freshwater and subterranean caverns with small reservoirs of water (Fosberg & Renvoize 1980). Surface freshwater habitats are most common during the rainy season. Lagoon systems are also present on Cosmoledo and Astove atolls, which are smaller than Aldabra but of regional significance for marine fauna and birds. These atolls lack freshwater systems. A single permanent brackish water pool is present on Assumption (as a result of mining activities).

Terrestrial habitats

The native vegetation of the granitic islands has many affinities with that of mainland Africa, Madagascar, and the Mascarenes, with palm forest comprising the primary vegetation type. Common upland tree species include Phoenicophorium borsigianum, Paraserianthes falcataria, Pterocarpus indicus (a locally common introduction), Adenanthera pavonina, and native but cultivated coconut palms (Cocos nucifera). Six palm species are endemic to the Seychelles, including the imperiled, monotypic coco-de-mer palm (Lodoicea maldivica), which is restricted to the islands of Praslin and Curieuse (and a small introduced population on Silhouette). At altitudes above 600 m, dense cloud forests occur (Henkel & Schmidt 2000). Specialist cloud forest plants are closely related to south-east Asian species, reflecting the ancient bioegraphical connections. In river valleys and marshes throughout the islands, various species of palms and screwpine (Pandanus spp.) were naturally abundant in historical times (Calström 1995), small areas of natural riverine vegetation can be found on some islands, most significantly on Silhouette.

The vegetation of the coralline islands is xeric and includes dense thickets of the salt-tolerant Pemphis acidula on the rougher limestone and a mixed thicket of low trees, shrubs, herbs, and grasses on the higher, more consolidated rock (Stattersfield et al. 1998). Important families include Sapindaceae, Moraceae, and Tilliaceae. Mangroves line the lagoon on Aldabra Atoll and are an important habitat for various birds. About 20% of the flowering plants on the islands of the Aldabra group are endemic (Fosberg & Renvoize 1980; Mittermeier et al. 1999; Skerrett 1999).

Description of endemic fishes

The golden panchax (Pachypanchax playfairii) is the only endemic species in the ecoregion.

Justification for delineation

It is recognized that the coralline islands differ from the granitic islands because the biogeographic affinities of the coralline islands lie much more with Madagascar and Africa than do those of the granitics (Rocamora & Skerrett 2001). However, at a global scale their geographic proximity warranted grouping all of the islands as a single ecoregion.

Level of taxonomic exploration

Fair (coralline islands) / Good (granitic islands)