Ecoregion Description

511: Northern Upper Guinea

Major Habitat Type:

tropical and subtropical coastal rivers

Author:

Ashley Brown and Michele Thieme, WWF-US, Conservation Science Program, Washington, DC, USA

Reviewers:

Christian Lévêque, Centre National de la Recherche Scientifique (CNRS), France; Emmanuel Williams, Wetlands International, Dakar, Senegal; Samba Diallo, Centre National des Sciences Ha

Countries:

Guinea; Guinea-Bissau; Liberia; Sierra Leone

Boundaries:

The Northern Upper Guinea ecoregion lies on the western side of the Guinean range, extending from the foothills of the Fouta Djalon in Guinea southeast to Sierra Leone’s southern border and encompasses a small portion of Guinea-Bissau and Liberia. This ecoregion is defined by the basins of the Coliba (Tominé and Komba), Kogon, Tinguilinta, Fatala, Konkouré, Kolenté, Kaba and Mongo rivers and is characterized by a rich aquatic fauna with high endemism among fish, molluscs, amphibians, and crabs. Together with Southern Upper Guinea [512], Fouta Djalon [510] and Mount Nimba [513], this ecoregion forms the Upper Guinean bioregion, which has a distinct fish fauna (Lévêque 1997).

Main rivers or other water bodies:

The main rivers are the Coliba (Tominé and Komba), Kogon, Tinguilinta, Fatala, Konkouré, Kolenté, Kaba and Mongo rivers. The Tominé and Komba Rivers join to form the Coliba River. The Coliba (basin size: 17,807 km²) drains the northwest side of the Fouta Djalon. It is 407 km long and flows into the Atlantic Ocean via an estuary shared with the Geba River in Guinea-Bissau. The Kogon River (basin size: 7,288 km²) flows northwest towards the border with Guinea-Bissau (379 km) and then flows along the border in a southwest direction until it flows into the estuary, Rio Komponi adjacent to the Atlantic Ocean. The Tinguilinta River (basin size: 4,858 km²) is 160 km long and flows through the Boké region, meeting the Atlantic Ocean at the Rio Nunez estuary in Kamsar. The Fatala River (basin size: 692 km²), is 205 km long; takes it source in the Fria region, flows through the Boffa region, and reaches the Atlantic ocean at Rio Pongo. The Konkouré River (basin size: 17,046 km²) receives the Kakrima river from the Labe plateau and many other tributaries along its 139 km length. The Kolente river (basin size: 5,170 km²) flows through the Kindia and Forecariah regions. It has a gentle slope throughout most of its 210 km length and it meets the Atlantic ocean in Sierra Leone. The Kaba (basin size 5,427 km²) is 91 km long. It is formed by the Kaba and Mongo Rivers coming from the Mamou region. It flows down to the Atlantic ocean through Sierra Leone (Samoura et al. 1999). The many rivers and streams within this ecoregion form an intricate hydrological network.

Topography:

The relatively short rivers of the ecoregion descend from the Guinean Dorsale and cross the coastal plain adjacent to the Atlantic Ocean. The rivers begin at elevations of around 500 m asl (and as high as 1,946 m asl at Mt. Bintumani in the Loma Mountains) (Hughes & Hughes 1992). Moving west, the gradient decreases and the landscape changes from undulating foothills to a coastal plain where riverine and floodplain lakes are common. Almost all of the coastal rivers have a torrential flow regime due to their steep downward slopes and rocky bottoms (DNE et al. 1999).

Climate:

The climate of the ecoregion is tropical and wet, with rainfall influenced by the moist southwest trade winds (Hughes & Hughes 1992). The ecoregion receives heavy but seasonal precipitation with a mean annual rainfall of 3,000 to 5,000 mm at the coast and 2,000 mm inland (Sayer et al. 1992). The wet season lasts 5-6 months in the middle of the ecoregion near Conakry, with most of the yearly rainfall occurring between May and November. Similarly, in the Fouta Djalon highlands, the rainy season lasts for approximately six months. Even within the rainy season, precipitation is concentrated, with an average of 2,493 mm per year falling between August and September (Hughes & Hughes 1992). The dry season lasts at least five months.

Freshwater habitats:

Floodplain lakes, which support beds of floating and submerged aquatic macrophytes, also occur within the coastal plain, and these lakes are surrounded by extensive tracks of swamp forests (Hughes & Hughes 1992). Mangroves backed by freshwater swamp forests grow along most of the coast, especially along many of the riverine estuaries. Sediments brought downstream by the rivers constitute the soils of these swamps. Species such as Pterocarpus santalinoides, Napoleonaea vogelii, and Mitragyna stipulosa can be found in the swamp forests behind the mangroves (Sayer et al. 1992). In Guinea, mangrove forests cover almost the entire coastline for about 300 km. In 1965, the area of mangroves was estimated to be about 3,500 km2, but today it is only 2,960 km2 with a destruction rate of 4.5 km2 or 4.2% per year. According to Rouanet (1957), cited in DNE (1999), the Guinean mangroves covered 4,000 km2 in 1957, so these mangroves have been reduced by 50% in 40 years. Seven mangrove species occur here: Rhizophora mangle, Rhizophora racemosa, Avicenia africana, Drepanocarpus erectus, Banisteria leona, Conocarpus erectus, and Laguncularia racemosa (Uschakov 1970; Matthes 1993; Diallo 1995). Other vegetation includes Dalbergia, Sesuvium portulacastrum, and Paspalum vaginatum.

Terrestrial Habitats:

According to Bazzo (2000)(Bazzo 2000), this ecoregion is formed by the raised edge of the Fouta Djalon plateau. The highest elevations have poor soils called “Bowe” with scrub and grass vegetation. This raised edge of the plateau has numerous rocky outcrops. Sandstone soils support relict mesophil forests. These include the species Ceiba pentandra, Daniella oliveri, Parkia biglobosa, Bombax costatum, Khaya senegalensis, Erythrophleum guineense, Milicia excelsa, Terminalia ivorensis, and Afzelia africana. The lower elevations contain more iron and have deeper soils that affect the vegetation cover. Broad-leaved lowland forest covers the upland coastal plain, with seasonally inundated grasslands and riverine forests occurring along rivers, particularly in the southwestern portion of the ecoregion. Many secondary species occur here, such as Hymenocardia acida, Daniella oliveri, Cordyla pinnata, Pterocarpus erinaceus, Prosopia africana, Imperata cylindrica, Andropogon gayanus, and Penisetum sp.

Fish Fauna:

The forested coastal streams and rivers of Upper Guinea support a diverse and endemic aquatic fauna (Lévêque et al. 1989). About 28% of the 160 fish species are endemic. 

Description of endemic fishes:

The endemic fishes are generally small-bodied and adapted to the swift currents and clear waters of the ecoregion (see ecoregion [512] for more detailed descriptions of adaptations). One-quarter of the endemic fish are rivulines, some of which are annuals. During the wet season, these annuals lay their eggs in the soil of temporary floodplain pools that desiccate in the dry season. These eggs hatch with the inundation of floodwaters in the rainy season (Lévêque et al. 1992). Species from the Cyprinodontidae (Aphyosemion and Epiplatys) and Cyprinidae (Barbus) families dominate the endemics of the coastal streams and rivers. There are also several endemic fish from the Mochokidae, Mormyridae, Claroteidae, and Cichlidae families. 

The Konkoure River is one of the richest among the Atlantic basins, supporting 88 fish species. About twenty percent of the Konkoure fish species are not shared with any other Upper Guinean basin. According to Daget (1962) even the faunistic barriers of the rapids that divide up the river could not entirely explain this level of endemism.

Other noteworthy aquatic biotic elements:

Ten endemic frogs, four endemic freshwater crabs, at least two endemic dragonflies (Argiagrion leoninum and Allorhizucha campioni), and five endemic molluscs also live in the waters of the Northern Upper Guinea ecoregion. Several nesting and overwintering birds can be found in the coastal floodplain, especially within the mangroves. Mangrove forests provide a variety of food sources for waterbirds such as fish, insects, and shellfish (Hughes & Hughes 1992). Species such as the great egret (Ardea alba), cattle egret (Bubulcus ibis), green-backed heron (Butorides striatus), purple heron (Ardea purpurea), pygmy goose (Nettapus auritus), and white-faced whistling duck(Dendrocygna viduata) nest in the freshwater and brackish swamps near the mouth of the Nunnez and Koumba Rivers (Wetlands International 2002). At the mouth of Rio Pongo, the yellow-billed stork (Mycteria ibis), wooly-necked stork (Ciconia episcopus), and goliath heron (Ardea goliath) breed. About 40 km north of Conakry on the coast, the Ramsar site Delta du Konkoure hosts large numbers of wintering paleartic migrants (Wetlands International 2002).

The ecoregion also supports a variety of large aquatic reptiles and mammals. All three species of African crocodiles—the slender-snouted (Crocodylus cataphractus), Nile (Crocodylus niloticus), and dwarf (Osteolaemus tetraspis) crocodile—have historically inhabited the riverine floodplains and swamps of this ecoregion. The vulnerable West African manatee (Trichechus senegalensis), the common hippopotamus (Hippopotamus amphibius), and the vulnerable pygmy hippopotamus (Hexaprotodon liberiensis), also live in the ecoregion (IUCN 2002). Important areas for the pygmy hippopotamus include the Moa River around Tiwai Island and the Mahoi River in the Gola Forest (Sayer et al. 1992).

Justification for delineation:

The high level of endemism among fishes in this ecoregion is postulated to be the result of isolation over time (Lévêque 1997). The Guinean range is an impassable barrier to the dispersal of fish from the Upper Guinean streams to the basins to the northeast, such as the Niger. Rapids and waterfalls within individual basins have likely served as additional barriers. Different habitats potentially contributed to the divergence as well; for example, forested streams characterize the Guinean region, whereas savanna streams predominate in the Nilo-Sudan ichthyofaunal province periods (Hugueny & Lévêque 1994). Another hypothesis for the high endemism in the bioregion is that the rivers and streams of this forested area acted as a “refuge” during dry climatic periods (Hugueny & Lévêque 1994).

Level of taxonomic exploration:

Good. The fish fauna of this ecoregion is fairly well investigated, although new species are discovered occasionally. The comprehensive volumes of The Fresh and Brackish Water Fishes of West Africa (Lévêque et al. 1990, 1992) cover this ecoregion. In contrast, the aquatic invertebrates are poorly known.

References/sources:

Bazzo, D. (2000) "Atlas Infogéographique de la Guinée Maritime”, Ministère de l’Agriculture et de l’Elevage et Ministère de la Pêche et l’Aquaculture". Conakry, Guinea. CNRS-IRD-CNSHB.

Daget, J. (1962). "Les poissons du Fouta Djalon et de la basse Guinée" Mém. Inst. fr. Afr. noire 65 1-210.

Diallo, A. (1995) "Rapport sur les plantes flottantes de Guinée". Conakry, Guinea. Centre d'Étude et de Recherche en Environement, Université de Conakry.

Dne, Pnud, et al. (1999) "Evaluation de la diversité biologique de la Guinée: Vision, buts, et objectifs de la strategie nationale pour sa conservation et son utilisation durable. PROJET / GUI/97/G32/A/1A/99 SNPA-DB". Conakry, Guinea. Ministère Mines Géologie et Environnement.

Hughes, R. H.,Hughes, J. S. (1992). "A directory of African wetlands" Gland, Switzerland, Nairobi, Kenya, and Cambridge, UK: IUCN, UNEP, and WCMC.

Hugueny, B.,Lévêque, C. (1994). "Freshwater fish zoogeography in West Africa: Faunal similarities between river basins" Environmental Biology of Fishes 39 365-380.

Lévêque, C. (1997) Biodiversity dynamics and conservation: The freshwater fish of tropical Africa. Cambridge, UK: Cambridge University Press.

Lévêque, C., Paugy, D., et al. (1992) The fresh and brackish water fishes of West Africa, Vol. 2. Paris: ORSTOM - MRAC.

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. (1989). "Inventaire taxonomique et distribution des poissons d’eau douce des bassins cotiers de Guinee et de Ginee Bissaau" Revue d’Hydrobiologie tropicale 22 107-27.

Matthes, H. (1993) "Rapport préliminaire de la mission d'évaluation de la pêche continentale et de l'aquaculture en République de Guinée". FAO/SEP.

Samoura, A. B., Diallo, S. T., et al. (1999) "Analyse de la biodiversité des écosystemes des eaux continentales, DNE/projet Gui/97/G32/A/1G/99. Stratégie Plan D'action Diversité Biologique". Ministere Mines, Géologie et Environnement.

Sayer, J. A., Harcourt, C. S., et al. (1992) The conservation atlas of tropical forests: Africa. London, UK: IUCN.

Uschakov, P. V. (1970). "Observation sur la repartition de la faune benthique du littoral guinéen" Cahier de biologie marine XI 435-457.

Wetlands, International (2002) "Ramsar Sites Database: A directory of wetlands of international importance" <http://www.wetlands.org/RDB/Ramsar_Dir/_COUNTRIES.htm>(2003)