Boundaries

Biannual floods course through the two large rivers, the Niger and Benue, in this largely savanna ecoregion of West Africa. The Lower Niger-Benue ecoregion extends from below Tombouctou (at the northwest end of the Inner Inner Niger Delta [508]) along the course of the Niger River and its tributaries downstream until it reaches the Niger Delta [506]. The Niger-Benue system flows through Niger, Mali, Burkina Faso, Guinea, Côte d’Ivoire, Benin, and Cameroon.

Freshwater habitats

No floodplains exist along the 460-km stretch of the Niger between Ayourou and the Mekrou River confluence. Below this confluence and before the Kainji Dam, there are numerous floodplains with a combined inundated area of 5,673 km² (2,062 km² during the dry season) (Welcomme 1986b). The diminished extent of floodplains in this ecoregion compared to the Inner Niger Delta [508] is the result of flood attenuation upstream. Swamps border the rivers in places and can be found in the old riverbeds that capture water from groundwater sources and rainfall (John et al. 1993). Floodplain vegetation includes grasses such as Echinochloa pyramidalis and Oryza barthii. Gallery forest, including Cola laurifolia and Khaya senegalensis, lines the Niger as it flows through Benin. In the upper portion of the Benue River, about 1000 km upstream of where the Benue joins the Niger, seasonal floods inundate the floodplains (Hughes & Hughes 1992). 

The flood regime of the Niger downstream of Niamey, in the middle of the ecoregion, is bi-modal. Floods originating in the headwaters of the Niger reach Nigeria in January-February and constitute the "black flood" (Hughes & Hughes 1992). Much of the silt and salt that the river carries out of the highlands is deposited in the inner delta floodplain or absorbed by aquatic vegetation, resulting in water that is relatively clear (John et al. 1993). The "black flood" refers to the dark water color derived from the influx of solute-rich water from tributaries downstream of the inner delta (Welcomme 1986b; John et al. 1993). Flooding from overland runoff of precipitation, mainly in the catchments of the Malendo and Sokoto rivers, constitutes the larger "white flood" in August-October (Hughes & Hughes 1992). These rivers are laden with kaolinitic colloids, giving the waters of the flood a milky color (John et al. 1993). The Benue flood, originating in the Adamaoua Mountains and reaching peak flow rates in August-September, reaches the Niger in October (Welcomme 1986b; Hughes & Hughes 1992).

Terrestrial habitats

As the lower Niger flows towards the delta, rainfall increases, influencing the surrounding vegetation and the hydrology of the river. Between Tombouctou and Gao, located about 500 km downstream, the Niger River runs through a xeric region with a mean annual rainfall of under 400 mm per year. The mean annual rainfall increases to 800 mm/year in the middle of the ecoregion, about 700 km downriver of Gao, and gradually becomes 1,400 mm per year at approximately 700 km above the delta (Grove 1985). As the river flows towards the southeast, the vegetation becomes lush and the river enters coastal rainforest near Onitsha, Nigeria (less than 100 km above the delta). The upland vegetation along the Benue River consists of tropical savanna and moist savanna-woodland.

Description of endemic fishes

Two endemic fish genera, Dagetichthys and Dasyatis, are present in the Niger-Benue ecoregion. The endemic Dagetichthys lakdoensis, a benthic, freshwater sole or flatfish, is the only freshwater representative of this family in Africa and lives primarily in the upper Benue (Welcomme 1986a). The freshwater stingray, Dasyatis garouaensis, only inhabits the waters of the Lower Niger Benue and those of the Niger Delta (Welcomme 1986a; Lévêque et al. 1991).

Other noteworthy fishes

Several fish species may undertake long-range migrations in this large savanna river. Most longitudinal migrations occur during the wet season when fish travel to spawning grounds. Anecdotal evidence exists of fish traveling up to 640 km upstream towards the inner delta (Welcomme 1986a). One study found Brycinus leuciscus to travel at rates up to 9 km per day as floods receded, for a total distance of 400 km (Welcomme 1979). Several marine fish species, such as Trachinotus goreensis, Mugil cephalus, Pomadasys jubelini, and Cynoglossus senegalensis, migrate upstream as far as 500 km above the delta and Eleotris spp. have been documented near Lokoja (300 km upstream from delta) (Welcomme 1986a). 

Many fish of the Niger River have special adaptations for surviving the anoxic conditions that often occur during the dry season. Lungfish (Protopterus) possess lungs for aerial respiration and can also aestivate in a mucous cocoon in dry conditions, while other species are capable of burrowing in the mud or producing drought-resistant eggs (Lowe-McConnell 1985). A representative of the Polypterus genus has lung-like modifications of the air bladder that allow it to breathe surface oxygen (Welcomme 1986a). Many species in the genera Epiplatys, Aphyosemion, and Aplocheilichthys have dorsally-oriented mouths and a flattened head, which facilitates breathing from the surface film. Other species have developed arborescent (branching) respiratory organs (Heterobranchus bidorsalis), supra-branchial organs (Ctenopoma kingsleyae), or vascularized intestines (Gymnarchus niloticus), for capturing scarce oxygen (Lévêque 1997).

Justification for delineation

This ecoregion comprises the Benue River basin and the lower and middle portions of the Niger River basin below the Inner Niger Delta [508] and above the Niger Delta [506]. In contrast to conditions in the two deltas, the river is characterized by broad channels with numerous sandbanks and floodplains are limited in this ecoregion. The river flows largely through savanna vegetation and the species assemblages are typical of the Nilo-Sudanian bioregion (Roberts 1975).

Level of taxonomic exploration

Good