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Ecoregion Description


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Species Richness


# of Endemic Species


Threats

585: Seychelles

Major Habitat Type:

Oceanic Islands

Author:

Robin Abell and Ashley Brown, Conservation Science Program, World Wildlife Fund-US, Washington, DC, USA

Reviewers:

Justin Gerlach, The Nature Protection Trust of Seychelles, Mahé, Seychelles, and University of Cambridge, Cambridge, UK

Countries:

Seychelles

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.

Drainages flowing into:

Indian Ocean

Main rivers or other water bodies:

The granitic islands have many small, steep watercourses, but many are ephemeral.

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).

Climate:

The climate of the ecoregion is tropical. Average daily temperatures range from 22o C to 32 o C (Georges 1998; (FAO Inland Water Resources and Aquaculture Service Fishery Resources Division 1999; Henkel & Schmidt 2000). Two wind systems, the southeast tradewinds and the northwest monsoon, influence the seasonality of the islands (Rocamora & Skerrett 2001). From November to April, in conjunction with the northwest monsoon, the ecoregion experiences short, heavy rainshowers, high humidity, and higher temperatures. The remainder of the year constitutes the dry season, with lower temperatures and humidity and a steady breeze from the southeast (Calström 1995; CIA 2001). In the granitic islands the highest rainfall typically occurs on the central highlands of Mahé, which receives up to 3,500 mm/year. Precipitation is markedly lower elsewhere, with an average annual value of 2,370 mm for the entire island of Mahé, 1,990 mm/year on Praslin, and 1,290 mm/year for most other islands (Calström 1995). Average annual precipitation on the coralline islands is about 1,290 mm/year (FAO Inland Water Resources and Aquaculture Service Fishery Resources Division 1999).

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).

Fish Fauna:

The freshwater fauna is extremely depauperate due to the paucity of permanent freshwater habitats. Some of the species that inhabit this ecoregion include the speckled goby (Redigobius bikolanus), Malabar glassy perchlet (Ambassis dussumieri), and Indonesian shortfin eel (Anguilla bicolor).

Description of endemic fishes:

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

Other noteworthy aquatic biotic elements:

The coralline islands host a recently evolved biota that includes a large number of endemic plants. The permanent freshwater fauna is restricted to invertebrates. Five species of freshwater Heteroptera have been recorded (Micronecta praetermissa, Anisops vitrea, Mesovelia vittigera, Limnogonus cereiventris and Microvelia diluta diluta) (Polhemus 1993). There are a small number of dragonflies but no caddisflies (Gerlach, pers. comm.). Landlocked pools support algae and cyanobacteria (e.g., Phormidium, Lyngbya, Pleurocapsa), and other aquatic organisms that feed upon these food sources (Braithwaite et al. 1989).

Within the granitic islands the only endemic frog is Tachycnemis seychellensis, considered vulnerable by the IUCN (2002).The native Mascarene frog Ptychadena mascareniensis is common on the larger islands. Seven species of caecilian occur on several islands with the greatest species diversity on Mahé, Praslin and Silhouette islands; little is known of their conservation status. The vulnerable Seychelles mud turtle (Pelusios seychellensis) is also endemic to this ecoregion, though its taxonomic status is uncertain and it may be extinct (Gerlach & Canning 2001; IUCN 2002). There are two endemic subspecies of terrapin; P. subniger parietalis and P. castanoides intergularis, both recently proposed as Critically Endangered (Gerlach & Canning 2001; Gerlach 2002). There is also one endangered freshwater snail known only from small mountain streams above 250 m asl on Silhouette and Mahé islands and one endemic freshwater crab, Seychellum alluaudi (Gerlach 1997). Other crustacea include several widely distributed Indo-Pacific shrimps and crayfish. Ostracods and copepods have only recently been recorded in the islands; the only identified species to date is widespread in Eruope, Africa and Asia (Wouters 2002). Other more widely distributed freshwater fauna include four species of mollusc from the family Nertididae, the cosmopolitan pond snail Melanoides tuberculatus and, formerly, the estuarine crocodile (Crocodylus porosus) (extinct since the early 1800s) (Gerlach & Canning 1994).

The granitic islands contain a rich dragonfly (Odonata) fauna, with 22 species and 7 endemics. Endemics include Allolestes machlachlani, Leptocnemis cyanops, and Teniobasis alluaudi. Endemic species live in streams of higher elevation forests and are capable of tolerating temporary drying of streams. More widespread species that live on the islands tend to inhabit, are not as dependent on forest cover, and are generally less tolerant of ephemeral aquatic habitats (Samways in prep.). Seven species of caddisfly have been identified, all endemic and including endemic genera of considerable biogeographical and ecological interest. Mayflies are represented by two endemic genera and a recently discovered unidentified species. There are also several species of diving beetles and aquatic bugs. Thirty percent of the aquatic insects are considered endemic, most are restricted to the least disturbed habitats (mountain streams and isolated pools). The aquatic beetle family Gyriniidae has only recently been collected in the islands and is represented by a widespread African species. Although not aquatic, several species of carabid ground beetle are closely associated with marshy habitats, as are unidentified pygmy mole crickets and some spiders (especially the lycosid Trochosa urbana).

There are two endemic subspecies of waterbirds in the granitic Seychelles, Butorides striatus degens and Bubulcus ibis sechellarum. There are also eight endemic and threatened landbirds, as well as globally and regionally important seabird congregations (Rocamora & Skerrett 2001). The only landbird associated with wetland habitats is the Seychelles black paradise flycatcher (Terpsiphone corvina) which currently breeds only on La Digue island. Though habitat loss has led to the decline in the range of this species, secure marsh areas are available on Silhouette and Curieuse to reverse the historic decline. The yellow bittern (Ixobrychus sinensis) breeds in the larger marshes; this highly threatened population is the only one of this Asian species in the African region (Gerlach & Skerrett 2002).

Numerous bird species inhabit or visit the coralline islands, though only a portion is closely associated with aquatic habitats. Among Aldabra’s endemic (though one extinct) species and ten endemic sub-species are the Aldabra sacred ibis (Threskiornis aethiopicus abbotti), which inhabits tidal pools and coastal lagoons and the Aldabran white-throated rail (Dryolimnas cuvieri aldabranus) (Seabrook 1990; Stattersfield et al. 1998). A small population of greater flamingo (Phoenicopterus ruber) breeds on the island. Several other waterbirds, including Ardeola idea, Egretta dimorpha, Ardea cinerea, and Butorides striatus crawfordii are found in the coralline Seychelles.The coralline islands also support seabird colonies of global and regional importance including several species of shearwaters, frigatebirds, terns, tropicbirds, and boobies (Rocamora & Skerrett 2001).

The Critically Endangered Seychelles sheath-tailed bat (Coleura seychellensis) is recorded from four islands and records are often associated with marshy habitats (although not exclusively) (IUCN 2002). Only one surviving roost site is known, which contains up to 32 bats.

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)

References/sources:

Braithwaite, C. J. R., Casanova, J., et al. (1989). "Recent stromatolites in landlocked pools on Aldabra, western Indian Ocean" Palaeogeography Palaeoclimatology Palaeoecology 69(3-4) 145-166.

Calström, A. (1995) "Seychelles: Country report to the FAO International Technical Conference on Plant Resources" <http://www.fao.org/WAICENT/FAOINFO/AGRICULT/AGP/AGPS/Pgrfa/pdf/seychell.pdf>(2001)

Cia (2001) "The world factbook 2001" <http://www.cia.gov/cia/publications/factbook/index.html>(2001)

Food and Agriculture, Organization (1999) "State of the World fisheries and Aquaculture 1998". Rome, Italy. FAO.

Fosberg, F. R.,Renvoize, S. A. (1980). "The flora of Aldabra and neighboring islands" London, UK: Her Majesty's Stationery Office.

Georges, E. (1998) The Indian Ocean: Madagascar, Réunion, Mauritius, The Seychelles. Cambridge, UK: Evergreen (Benedikt Taschen Verlag GmbH), First Edition Translations Ltd..

Gerlach, J. (2002). "Seychelles Terrapin action plan" Phelsuma 10B 1-16.

Gerlach, J. (1997) Seychelles Red Data Book - 1997. Mahé, Seychelles: The Nature Protection Trust of Seychelles.

Gerlach, J.,Canning, K. L. (1994). "On the crocodiles of the western Indian Ocean" Phelsuma 2 54-58.

Gerlach, J.,Skerrett, A. (2002). "The distribution, ecology and status of the yellow bittern Ixobrychus sinensis in Seychelles" Journal of African Ecology 40 194-196.

Henkel, Friedrich-Wilhelm,Schmidt, Wolfgang (2000) Amphibians & Reptiles of Madagascar, the Mascarenes, the Seychelles & the Comoros Islands. (pp. 324) Malabar, Florida, USA: Krieger Publishing Company.

Library of, Congress (1994) "Country studies: Seychelles" <http://lcweb2.loc.gov/frd/cs/sctoc.html#sc0000.>(2001)

Mittermeier, R. A., Myers, N., et al. (1999). "Hotspots: earth’s biologically richest and most endangered terrestrial ecoregions" Mexico: CEMEX.

Polhemus, Dan A. (1993). "The Heteroptera of Aldabra Atoll and nearby islands, western Indian Ocean, Part 2: Freshwater Heteroptera (Insecta): Corixidae, Notonectidae, Veliidae, Gerridae and Mesoveliidae" Atoll Research Bulletin 381 1-9.

Rocamora, G.,Skerrett, A. (2001)"Seychelles" In Fishpool, L.D.C.;Evans, M.I. (Ed.). Important bird areas in Africa and associated islands: Priority sites for conservation. (pp. 751-768) Newbury and Cambridge, UK: Pisces Publications and BirdLife International (Birdlife Conservation Series No. 11).

Skerrett, A. (1999) "Bulletin of the African Bird Club, Volume 6.1" <http://www.africanbirdclub.org/feature/aldabra.html>(2001)

Statistics and Database Administration Section, Misd (2000) "Seychelles In Figures" <http://www.seychelles.net/misdstat/__Geography_Climate_History_An
/__geography_climate_history_an.htm>(2001)

Stattersfield, A. J., Crosby, M. J., et al. (1998). "Endemic bird areas of the world: Priorities for biodiversity conservation" Cambridge, UK: BirdLife International.

Unep (1998) "Island directory" <http://www.unep.ch/islands/CMP.htm>(2002)

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