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# of Endemic Species
509: Senegal - Gambia
Major Habitat Type:
tropical and subtropical floodplain rivers and wetland complexes
Michele Thieme, WWF-US, Conservation Science Program, Washington, DC, USA
Christian Lévêque, Centre National de la Recherche Scientifique (CNRS), Paris, France and Papa Samba Diouf, WWF-International, West Africa Regional Office, Dakar, Senegal
Gambia, The; Guinea; Guinea-Bissau; Mali; Mauritania; Senegal
The ecoregion spans parts of southern Mauritania, Senegal, southwestern Mali, Gambia, Guinea-Bissau, and Guinea, and includes the Senegal, Saloum, Casamance, Gébe, and Gambia river basins.
Drainages flowing into:
Main rivers or other water bodies:
The Senegal and Gambia Rivers are the ecoregion’s largest, with drainage areas of about 441,000 km2 and 77,000 km2, respectively (Lévêque 1997).
Rainfall decreases from south to north and from the coast inland. At the southern end it averages about 1,500 mm/year, while the northern portion receives only about 400 mm/year.
During the rainy season (between June and October - duration depending on location), the ecoregion’s major rivers often experience pronounced flooding. For example, historically, the Senegal River flooded 5,000 km2 of land during its peak flood (Welcomme 1979). However, since the construction of two dams (Manantali and Diama dams) and along the river, flooding has been significantly curtailed (Hamerlynck & Duvail 2003). In the dry season, saltwater moves into the deltas of the lowland coastal rivers (including the Saloum, Casamance and Gébe), forcing strictly freshwater fish species to move inland. The penetration of seawater far inland permits the growth of mangrove forests 70-100 km inland in the southern portion of the ecoregion.
Terrestrial vegetation ranges from semi-desert Sahelian grassland and shrubland in the north to progressively moister Guinea savanna in the south. In the wetter, southern portion of the ecoregion, seasonally inundated swamp forests line the rivers. Floodplain vegetation includes perennial grasses and sedges, in addition to reedmace (Typha domingensis) in the main channels.
Historically, the annual inundation of riverine floodplains created extensive feeding and breeding grounds for fish. Floods triggered migrations in a number of fish species, particularly in the Senegal River. Movements include both longitudinal migrations within rivers and lateral migrations onto the floodplain (Lévêque 1997).
Description of endemic fishes:
The rivers of this ecoregion support a moderately rich aquatic fauna, but levels of endemism are low, with only three endemic frogs and one endemic fish species. The desiccation of the basins and relatively recent re-colonization by a Nilo-Sudanian fauna explains the low level of endemism.
Other noteworthy aquatic biotic elements:
This ecoregion is distinguished by rich coastal deltas that support large populations of migrant birds. The Senegal Delta wetlands, the Gambia delta, and Geba-Corubal are prominent feeding grounds for migrant birds with an estimated 3 million Palearctic and Afro-tropical species using the Senegal delta alone (Wetlands International 2002). Djoudj National Park hosts 366 bird species, including garganey (Anas querquedula), shoveler (A. clypeata), pintail (A. acuta), black-tailed godwit (Limosa limosa), greater and lesser flamingo (Phoenicopterus ruber and P. minor), great white pelican (Pelecanus onocrotalus), and avocet (Recurvirostra avosetta). Mangroves provide important habitat for some bird species, and this ecoregion contains the most northern mangroves on the African continent. This ecoregion also forms the northern limit for the West African dwarf crocodile (Osteaoaemus tetraspis) and West African manatee (Trichechus senegalensis).
Justification for delineation:
This ecoregion is defined by the Senegal and Gambia river basins and supports a Nilo-Sudanian freshwater fauna (Roberts 1975; Lévêque et al. 1990, 1992; Lévêque 1997). Uplifting of the earth’s crust during the late Jurassic created the central Fouta Djallon Mountains, which became the source of the Gambia and Bafing (now a tributary to the Senegal) Rivers. Further uplifting of these mountains occurred in the late Miocene (Lévêque 1997). In low-lying areas north of the mountain range, the Niger and Senegal Rivers occasionally became connected during wet periods and Nilo Sudanian fishes are thought to have reached the Gambia from the Senegal by crossing low-lying country in between their lower courses. The Senegal and Gambia Rivers are thought to have been nearly dry during the last interpluvial from about 27,000 to 12,000 years ago (Roberts 1975). Thus, Nilo-Sudanian fish from the Niger River likely re-colonized the Senegal-Gambia catchments during the last pluvial about 12,000 to 8,000 years ago (Lévêque 1997).
Level of taxonomic exploration:
Hamerlynck, O.,Duvail, S. (2003) "The rehabilitation of the delta of the Senegal River in Mauritania". Gland, Switzerland and Cambridge, UK. IUCN.
Lévêque, C. (1997) Biodiversity dynamics and conservation: The freshwater fish of tropical Africa. Cambridge, UK: Cambridge University Press.
Lévêque, C., Paugy, D., et al. (1990) The fresh and brackish water fishes of West Africa, Vol. 1. Paris: ORSTOM-MRAC.
Lévêque, C., Paugy, D., et al. (1992) The fresh and brackish water fishes of West Africa, Vol. 2. Paris: ORSTOM - MRAC.
Roberts, T. R. (1975). "Geographical distribution of African freshwater fishes" Zool. J. Linn. Soc. 57 249-319.
Welcomme, Robin L. (1979)"Fisheries ecology of floodplain rivers" In London, U. K. and New York, NY: Longman.
Wetlands, International (2002) "Ramsar Sites Database: A directory of wetlands of international importance" <http://www.wetlands.org/RDB/Ramsar_Dir/_COUNTRIES.htm>(2003)