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# of Endemic Species
508: Inner Niger Delta
Major Habitat Type:
tropical and subtropical floodplain rivers and wetland complexes
Ashley Brown and Miranda Mockrin, WWF-US, Conservation Science Program, Washington, DC, USA
Bakary Kone, Wetlands International, Projet Mali PIN, Mopti, Mali; Christian Lévêque, Centre National de la Recherche Scientifique (CNRS), Paris, France; and Eddy Wymenga, Altenburg &a
The Inner Niger Delta is located in central Mali in the semi-arid Sahelian zone, just south of the Sahara Desert, roughly situated between Djenné in the south and Tombouctou in the north. Boundaries of the ecoregion follow the general extent of the Delta floodplains and associated grasslands. Dune ridges on the Sahara’s edge funnel the waters of the Inner Delta north and east through Mali.
Drainages flowing into:
Part of the mainstem Niger River. The Niger and Bani Rivers and a few small streams flow into this ecoregion and the Niger River flows out of it.
Main rivers or other water bodies:
The Niger River is the longest river in West Africa and the third longest in Africa. Rising in the Fouta Djalon highlands of Guinea, the river extends for 4,100 km before flowing into the Atlantic Ocean on the Nigerian Coast. The Bani River is 1,100 km in length with sources in Côte d’Ivoire and Burkina Faso. Flows within the delta depend completely on river discharge from the Niger and Bani headwaters, and a few smaller and temporal streams that flow down from the Dogonland Plateau. The surge of water that reaches the delta from these two rivers dissipates as it continues downstream, with about half of the Niger’s water volume lost to evaporation(Quensière 1994; Zwarts & Diallo 2002).
Several natural and man-made lakes border the delta. Lake Debo is a shallow lake that expands and contracts as the river level rises or declines. Lake Horo is separated from the river system by a dam and a sluice gate, opened in November to allow floodwaters to enter (Wetlands International 2002). Several ephemeral lakes dot the landscape of the western and eastern periphery of the delta. Nowadays the lakes situated on the east side of the delta are mostly dry, with the exception of Lake Aougoundou and Lake Korarou, the latter being fed by rainwater from the Dogonland Plateau.
The Inner Niger Delta lies in a depression that formed the bed of a large lake during the Quaternary period (Welcomme 1986b). The delta extends for 425 km with an average width of 80 km, tapering into a braided river near Tombouctou where the Niger curves to the east. The floodplain drops only 8 m over its course (Hughes & Hughes 1992) and its topography is a complex mix of submerged lower areas and higher, unflooded areas known as tougérés. A vast network of river channels (mayo’s) with levées separated by low, clay-based floodplains forms networks across the delta. As waters flow through the delta, they pass over Pleistocene and recent alluvium overlying Paleozoic sandstone (Hughes & Hughes 1992). The upper margin of the delta is delimited by the 280 m contour and it is surrounded by sandstone massifs. To the north, huge dunes of the Erg Ouagadou block a former westward course of the river.
Located in the Sahel just south of the Sahara desert, the Inner Niger Delta covers the transition between a humid Guinean climate in the south and a dry climate at the edge of the Sahara. In the south, the rainy season begins in July and lasts through October, with a mean annual precipitation of 750 mm. In the north, the rainy season begins in July and lasts through September, with a mean annual precipitation of 250 mm (Dumont 1987). Temperatures are strongly seasonal with an average maximum in May at Tombouctou (43°C) and Mopti (40°C), while the coldest months are December/January when temperatures drop to an average minimum of 3-6°C in the north of the delta.
Vegetation defines the different habitat types of the delta. Three main plant associations have been identified: submerged and floating plants in shallow or stagnant water, partially submerged and marginal vegetation dominated by grasses, and plants that grow on seasonally exposed sandy soils (Hiernaux 1982; John et al. 1993). Along the rivers, a typical scrub of Mimosa pigra and Salix chevalieri is found, often together with a vegetation of Vetiveria nigritana. Partly floating, long-stemmed grasses – Echinochloa stagnina, E. pyramidalis, Oryza barthii and Andropogon gayanus – dominate in the floodplains. Permanent pools are richer and host the submerged macrophytes, Ceratophyllum spp. and Utricularia spp., in addition to floating Nymphaea spp. The lakes, in particular Lake Debo and Walado Debo in the central part of the delta, are surrounded by Echinochloa spp. and Vossia cuspidata (Dumont 1987). Flooded forests of Acacia kirkii are also characteristic, but increasingly rare due to overharvesting. Though these forests are dominated by A. kirkii, Ziziphus mauritiana may also occur. Algal blooms are common on the lakes and can reduce the water transparency.
Grasses such as Acroceras amplectens, Echinochloa pyramidalis, E. stagnina and Eragrostis atroviriens dominate the low-lying floodplain in the southern half of the delta. Large areas of the flooded delta are occupied by wild rice (Oryza longistaminata) and a characteristic vegetation of E. stagnina, known as bourgou-fields. Bourgou is used as feed for domestic animals and, thus, is often planted by local human populations. Other typical species of the flooded pastures that occur higher in the inundation zone are Vetivera nigritiana and Vossia cuspidata. Along the heavily grazed outer fringes, Andropogon gayanus, Cynodon dactylon and Hyparrhenia dissoluta dominate.
Each year during the rainy season, floodwaters of the Niger and Bani Rivers spill over their banks and the Inner Niger Delta in Mali is inundated to an area of 30,000 km2, on average. In contrast, the delta contracts to 3,900 km2 or less during the dry season (Welcomme 1986b; Zwarts & Diallo 2002). However, the surface of the inundated zone is highly variable according to river discharge: from 1956-2002 the maximum flooded area varied between 9,500 km2 (1984 – severe drought) and 44,000 km2 (1957 – high floods) (Quensière 1994).
Rainfall occurs in the Niger’s headwaters from May through September, with a clear peak in August, creating a surge that reaches the inland delta in October (Zwarts & Diallo 2002). As a result of lateral expansion, the flood slows in the delta. It takes one month for the flood to reach Mopti 160 km downstream, and nearly another month to reach Lake Debo where the maximum flood occurs in November-December (Dumont 1987; Laë 1997; Zwarts & Diallo 2002). The extensive swamps and vegetation of the delta filter silt and salt from the water so that the water leaving the delta is clear, low in dissolved salts and silt-free (John et al. 1993). Dry, landlocked Mali is completely dependent on these rivers for its water resources.
Forests are scarce in the delta, having been heavily exploited, overgrazed, and harvested for fire wood. Trees such as Acacia seyal, Diospyros sp. and Kigelia africana grow on higher levées. The northern half of the delta - north of the Debo-complex is characterized by emergent sand ridges; palms like Hyphaene thebaica, and Borassus aethiopum (Gallais 1967).
The fish fauna of the delta is composed of about 130 species, of which the dominant families are Mormyridae, Mochokidae, and Cyprinidae. Many of the fish have life cycles that take advantage of the habitats and resources associated with the floodplain with species migrating upriver and downriver as well as laterally out on to the floodplain as the water rises (Quensière 1994). Waters of the flooded delta are initially well oxygenated, providing favorable habitat for spawning fish, as well as developing eggs and larval fish. The initial flooding of the savanna enriches the water with nutrients from decomposition of vegetation and animal droppings, creating a surge of bacteria, algae, and zooplankton that provides a rich feeding ground for fish. When the waters recede, the fish move upriver or risk becoming trapped in small, isolated ponds. Some fish species can survive in these dwindling pools by aestivating or by breathing air (Lowe-McConnell 1985).
Fish migrations include both lateral movements onto floodplains and long-distance, longitudinal movements. There is anecdotal evidence of several fish moving as much as 440-640 km up the Niger River into the Inner Delta with the onset of floods (Welcomme 1986a). One of the African tetras, Brycinus leuciscus, has been observed moving 50 km from the river mainstream to the edge of the floodplain and may move 125-400 km upstream from the Inner Delta to the Markala dam as floods subside (Lowe-McConnell 1985).
Description of endemic fishes:
Two species are restricted to the delta and the Upper Niger: Synodontis gobroni and the rapids dwelling Gobiocichla wonderi (Welcomme 1986a).
Other noteworthy aquatic biotic elements:
The Inner Niger Delta provides essential habitat for huge numbers of wetland birds, including Afrotropical resident species and migrants that spend the Palearctic winter in Africa (Roux & Jarry 1984; Fishpool & Evans 2001; Wymenga et al. 2002). As the water recedes, after peak levels in October-November, birds concentrate in the central part of the delta (Lac Debo, Walado Debo and Lac Korientzé). More then 500,000 garganey (Anas querquedula) and up to 200,000 Northern pintail (Anas acuta) stay here during the northern winter, along with large numbers of ferruginous duck (Aythya nyroca), white-winged tern (Chlidonias leucopterus), ruff (Philomachus pugnax), black-tailed godwit (Limosa limosa) and other waterbirds (van der Kamp & Diallo 1999; van der Kamp et al. 2001; van der Kamp et al. 2002a; Wetlands International 2002). The total waterbird numbers in the delta depend heavily on water level and can reach more than one million in favorable years. In addition more than one million wetland-related passerines, in particular sand martin (Riparia riparia) and yellow wagtail (Motacilla flava), pass through the delta during their autumn migration to and from their breeding grounds. Despite these impressive numbers, several species in the delta are under serious threat, in particular large Afrotropical species such as the locally rare black crowned crane (Balearica pavonina pavonina), which has a population of only 50 birds (van der Kamp et al. 2002a).
The delta is also of international importance for several bird species due to the high proportion of their populations that occurs in the delta. More than 27 species have 1% of their population occurring in the delta, including the African cormorant (Phalacrocorax africanus), purple heron (Ardea purpurea), glossy ibis (Plegadis falcinellus), collared pratincole (Glareola pratincola), gull-billed tern (Gelochelidon nilotica) and Caspian tern (Sterna caspia). For example, 20% of the population of the Plegadis falcinellus – breeding in Europe and the Black Sea region – spends the northern winter in this area (van der Kamp et al. 2002a). The northern lakes, including the Ramsar site Lac Horo, often hold a large proportion of the West African wintering population of Aythya nyroca (up to 50 percent), though high numbers of this species have not been recorded recently (Scott & Rose 1996; Girard & Thal 2001). The delta is also nationally important for breeding purple swamp-hens (Porphyrio porphyrio) (Wetlands International 2002).
The Inner Niger Delta is also known for its large waterfowl breeding colonies, with 80,000 breeding pairs of birds within 15 species of cormorant, heron, spoonbill and ibis (Skinner et al. 1987). These colonies contain about 50,000-60,000 cattle egrets (Bubulcus ibis), 18,000-20,000 African cormorant (Phalacrocorax africanus), 1,500-1,800 pairs of great white egrets (Egretta alba) and 250-300 pairs of African darters (Anhinga rufa), among several other species. Breeding occurs during high flood (September-November) and the colonies are situated in flooded forests of (mainly) Acacia kirkii. Of the seven mixed breeding colonies present in 1985-86 (Skinner et al. 1987), only two remain at present (1998-2002). The presence of the remaining two is largely due to conservation efforts by the IUCN in close collaboration with local communities (van der Kamp et al. 2002b).
Several mammal species are closely linked to the wetlands of the Inner Niger Delta. The delta harbors an important but dwindling population of the vulnerable West African manatee (Trichechus senegalensis), which has suffered from hunting and severe droughts. Hippos (Hippopotamus amphibius) are present in the central and southern delta with an estimated population of 40-60 individuals (Stuart et al. 1990; Wymenga et al. 2002). Antelope populations have been seriously reduced by droughts, the bushmeat trade, and conflicts with grazing livestock. Buffon’s kob (Kobus kob kob) was once numerous in the Inner Niger Delta, but is no longer present. This also appears to be the case for roan antelope (Hippotragus equinus), dorcas gazelle (Gazella dorcas), and dama gazelle (Gazella dama). A small population of the vulnerable red-fronted gazelle (Gazella rufifrons) is believed to remain, though little information is available (Wymenga et al. 2002). Species like clawless otter (Aonyx capensis), spotted-neck otter (Lutra maculicollis), African civet (Civettictis civetta), caracal (Felis caracal), serval (Felis serval), striped hyena (Hyaena hyaena), and spotted hyena (Crocuta crocuta) – once recorded from these regions – appear to have been extirpated. During four years of field work and several aerial surveys none of these species nor any other large native mammals, with the exception of hippopotamus, were seen in the delta (Girard & Thal 2001; Wymenga et al. 2002). The vast floodplains still provide habitat for Nile crocodile (Crocodylus niloticus), Nile monitor (Varanus nilotica) and Python sebae. Nile crocodile is believed to be on the edge of extinction, while Nile monitor and python are facing heavy human pressure (Wymenga et al. 2002).
Aquatic plants of the delta are highly adapted to the yearly floods. One of the most important of these species is bourgou (Echinochloa stagnina). Bourgou plays a role in maintaining fish diversity by providing breeding and feeding habitat (Roggeri 1995; Oyebande & Balogun 1996). For example, Marcusenius abadii, Mormyrus macrophthalmus, Polypterus bichir, and P. endlicheri, prefer bourgou mats for breeding (Welcomme 1986a).
The ecoregion is recognized for its continentally important congregations of wetland birds (Fishpool & Evans 2001; Thieme et al. 2005). Maximum densities of wetland birds documented at the Lac Debo-Lac Oualando Debo site exceed one million birds (Fishpool & Evans 2001).
Justification for delineation:
The Inner Niger Delta ecoregion is distinguished due to the important habitat it provides for wetland birds, including both Palearctic migrants and Afrotropical residents, and other aquatic fauna (i.e., fish, mammals, amphibians, etc.) that also depend on the extensive floodplains.
Level of taxonomic exploration:
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