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).

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).

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.

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.

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.