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Ecoregion Description


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Species Richness


# of Endemic Species


Threats

505: Lower Niger - Benue

Major Habitat Type:

tropical and subtropical floodplain rivers and wetland complexes

Author:

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

Reviewers:

Christian Lévêque, Centre National de la Recherche Scientifique (CNRS), Paris, France

Emmanuel Obot, Nigerian Conservation Foundation, Nigeria

Countries:

Benin; Burkina Faso; Cameroon; Central African Republic; Chad; Mali; Niger; Nigeria

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.

Main rivers or other water bodies:

Within this ecoregion, the Niger River stretches for over 2,000 km through regions as varied as dry, rocky narrows in Mali to rainforest in southern Nigeria (Welcomme 1986b). The Benue River originates in the Adamaoua Massif in northern Cameroon and flows westward for 1,400 km until it meets the Niger River about 450 km above the delta, near the city of Lokoja, Nigeria.

Climate:

The climate of this ecoregion is tropical and warm year-round. Mean temperatures in the center of the ecoregion vary from 25o C in the coolest month to 34o C in the warmest month. The mean temperatures further east in Jos, Nigeria, are 20 o C and 24o C for the coolest and warmest months. In the Benue headwaters of northern Cameroon, temperatures fluctuate more widely (Hughes & Hughes 1992).

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.

Fish Fauna:

The fish fauna of this ecoregion is typical of the Nilo-Sudan bioregion. Researchers believe that this large savanna river system may have acted as a refuge during periods of climatic drying, and during subsequent wet periods was connected to other rivers within this ichthyological province. For example, the Chad basin is still intermittently connected to the Niger via overflow of the Logone River into the Mayo Kebi, which joins the Benue River. 

The Lower Niger-Benue contains a rich fish fauna with many species adapted to seasonal flooding. About 202 fish species, including 17 endemics, live in the rivers and streams of this ecoregion. Dominant fish families are Cyprinidae, Mormyridae, Mochokidae, Citharinidae, Aplocheilidae, and Alestiidae. Freshwater fish species known from the lower Niger River and coastal areas ascend into the lower Benue and middle Niger. Species such as Pantodon buchholzi, Marcusenius abadii, Phago loricatus, Clarias jaensis and C. macromystax, and Arius gigas move upstream (Lévêque et al. 1991).

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).

Other noteworthy aquatic biotic elements:

The West African manatee, which lives in the Lower Niger, travels up tributaries in the wet season and occasionally becomes stranded as the waters recede (Happold 1987)).

Among the 88 frog species in the Lower Niger-Benue, 16 may be endemic to the forests, savanna woodlands, and associated wetlands. A restricted-range savanna species (also found in the Gambia), Phrynobatrachus francisci, is often found on tufts of grass mounds in swamps during the wet season, and hides from the heat in the relatively damp crevices at the bottom of desiccated savanna pools during the dry season (Rödel 2000). 

The Niger River hosts many Palearctic migrants, including high densities of ducks and geese (Anatidae), storks (Ciconiidae), herons (Ardeiidae), and other wading birds (Brouwer et al. 2001a; Wetlands International 2002). For example, the Parc National du "W" in Niger hosts important populations of Ciconia nigra, Sarkidiornis melanotos, Dendrocygna viduata, and Plectropterus gambensis (Brouwer et al. 2001b). The ‘W’ du Benin National Park hosts Ardea goliath, A. cinerea, Ciconia abdimii, Plegadis falcinellus, and Balearica pavonina along the course of the Niger River (Cheke 2001). The Lower Kaduna-middle Niger floodplain also hosts large colonies of Merops malimbicus with over 15,000 birds documented (Ezealor 2001).

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

References/sources:

Brouwer, J., François Codjo, S., et al. (2001)"Niger" In Fishpool, L.D.C.;Evans, M.I. (Ed.). Important bird areas in Africa and associated islands: Priority sites for conservation. (pp. 661-672) Newbury and Cambridge, UK: Pisces Publications and BirdLife International (Birdlife Conservation Series No. 11).

Brouwer, J., Mullié, W. C., et al. (2001). "A synopsis of bird biodiversity in Niger, with special emphasis on waterbirds and wetlands"

Cheke, R. A. (2001)"Benin" In Fishpool, L.D.C.;Evans, M.I. (Ed.). Important bird areas in Africa and associated islands: Priority sites for conservation. (pp. 93-98) Newbury and Cambridge, UK: Pisces Publications and Birdlife International.

Grove, A. T. (1985)"The environmental setting" In Grove, A.T. (Ed.). The Niger and its neighbours: Environmental history and hydrobiology, human use and health hazards of the major West African rivers. Netherlands: A.A. Balkema.

Happold, D. C. D. (1987). "The mammals of Nigeria" Oxford, UK: Clarendon Press.

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

John, D. M., Lévêque, C., et al. (1993)"Western Africa" In Whigham, D.;Dykjova, D.;Hejny, S. (Ed.). Wetlands of the World, 1. (pp. 47-78) Dordrecht, The Netherlands: Kluwer Academic Publishers.

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

Lévêque, C., Paugy, D., et al. (1991). "Annotated checklist of the freshwater fishes of the Nilo-Sudan river basins, in Africa" Rev. Hydrobiol. Trop. 24 131-154.

Lowe-McConnell, R. H. (1985)"The biology of the river systems with particular reference to the fishes" In Grove, A.T. (Ed.). The Niger and its neighbors. (pp. 101-140) The Netherlands: A. A. Balkema.

Lowe-McConnell, R. H. (1987) Ecological studies in tropical fish communities. Cambridge, UK: Cambridge University Press.

Roberts, T. R. (1975). "Geographical distribution of African freshwater fishes" Zool. J. Linn. Soc. 57 249-319.

Welcomme, R. L. (1986)"Fish of the Niger system" In Davies, B.R.;Walker, K.F. (Ed.). The ecology of river systems. (pp. 25-48) Dordrecht, The Netherlands: Dr W. Junk Publishers.

Welcomme, R. L. (1986)"The Niger River system" In Davies, B.R.;Walker, K.F. (Ed.). The ecology of river systems. (pp. 9-23) Dordrecht, The Netherlands: Dr W. Junk Publishers.

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)

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