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Ecoregion Description


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Species Richness


# of Endemic Species


Threats

535: Sudanic Congo - Oubangi

Major Habitat Type:

tropical and subtropical floodplain rivers and wetland complexes

Author:

Emily Peck and Michele Thieme, Conservation Science Program, WWF-US, Washington, DC

Reviewers:

Uli Schliewen, Zoologische Staatssammlung München, Munich,,Germany and David Kaeuper, United States Ambassador to the Republic of Congo

Countries:

Central African Republic; Congo; Democratic Republic of Congo

Boundaries:

The ecoregion lies in the southern portion of Central African Republic (CAR), and includes the western boundary of the Democratic Republic of Congo (DRC) along the Oubangui and Congo Rivers, and the western border of the Republic of Congo (ROC). The northern border of this ecoregion marks the divide between the Congo River and Lake Chad basins and the eastern border is the divide between the Congo and Nile basins. The ecoregion encompasses the entirety of the Oubangui basin (with the exception of the Uelé basin [Ecoregion 874]) and a portion of the mainstem Congo River from its confluence with the Oubangui downstream to Malebo Pool. Located between major river basins, the Oubangui’s freshwater fauna is considered to be transitional between the Nilo-Sudanian and Congolian faunas (Roberts 1973; Bailey 1986; Hughes & Hughes 1992). 

Main rivers or other water bodies:

The Oubangui River, with a catchment of over 777,000 km², is a major tributary of the Congo River. The river drains the savanna-covered, elevated plateaus (500-700 m) of the Central African Republic and then flows into one of the most biologically diverse wetlands in all of Africa — the Cuvette Centrale. 

Climate:

Four principal climatic zones exist within the Oubangui ecoregion. The southern area of the ecoregion (1°N-2°S latitude) experiences a Congolese equatorial climate. With no dry season and little variation in temperature, the climate is hot, humid, and stable. The Congolese equatorial climate has a mean annual rainfall over 1,700 mm and a mean annual temperature of 26°C. Slightly further north lies the sub-equatorial climatic zone, where temperatures vary from 13°C in January to 40°C in March. In this zone, the annual precipitation remains over 1,500 mm; however, a 2-month dry season occurs between December and January. From 4-8°N latitude, the Sudano-Guinean climate zone is characterized by an annual rainfall of approximately 1,400 mm and a 3-5 month dry season from June through September. Mean annual temperature is 24°C, with a range from 18-32°C. In the far north, the tropical Sudanian climate zone has a 5-month dry season from November to March. Less than 1,200 mm of precipitation falls each year. The ecoregion’s wet season varies in length, with rainfall decreasing from south to north (Sayer et al. 1992).

Freshwater habitats:

The headwaters of the Oubangui River begin in the Central African Republic. The Chinko River and its affluent the Vivado, the Ouarra and its affluent the Goangoa, the Kerre, and the Mbokou rivers all begin in the far southeastern corner of CAR and flow down gentle slopes to join the Mbomou River. Floodplains are abundant in this section of the basin and follow the riverbanks for a total of over 1,320 km along these rivers, inundating more than 6,500 km² of land during the wet season. The Mbomou flows along the border of CAR and DRC for several hundred kilometers until it is joined by the Kotto River; below this confluence the river is called the Oubangui. Forested floodplains line the Oubangui as it flows west along the border. The floodplain contracts and the river valley narrows where the river flows over the Kouimba Rapids (4°37’N/20°27’E). Rapids are also located upstream along the Mbomou, Uelé, and Kotto Rivers. Several tributaries join the north bank of the Oubangui River, including the Kouma, Tomi, and Ombella, M’Poko, M’Bali, and Lobaye, from east to west. Inundated swamp forests cover approximately 2,310 km² of the floodplains of these tributaries.

In western CAR the river turns south and flows along the border of the ROC and DRC through virtually unbroken primary rainforest into the Cuvette Centrale. The Oubangui River is slow-flowing and wide (4-15 km) with islands in its channel. Northeast of its confluence with the Congo, the Oubangui is joined by the Giri River and spreads out across a large floodplain, forming the Giri or Bangala Swamp. These swamps have black, acidic waters (pH 3.5-5.2) derived from the surrounding floodplain forests (Roberts 1973; Bailey 1986; Hughes & Hughes 1992). The Oubangui eventually joins the Congo, which, at this point, is a slow flowing, braided maze of alluvial islands, sand banks, and floating beds of grass. Right-bank tributaries of the Congo River below its junction with the Oubangui include the Likouala aux Herbes, Sangha, Likouala, Alima, Nkeni, and Lefini rivers. Swamp forests predominate on the floodplains in this section, creating over 6,800 km² that include parts of the Likouala aux Herbes swamps (Beadle 1981; Hughes & Hughes 1992). The Likouala-aux-Herbes drains Lac Telé, a shallow lake possibly formed by a meteorite crater. The massive Kasai River joins the left bank of the Congo about 150 km upstream of where this ecoregion ends at Malebo Pool.

Terrestrial Habitats:

A variety of vegetation types covers the terrestrial landscape of the Oubangui ecoregion. In the Sudanian zone in the north, savanna predominates with some semi-humid forest, cropland, and gallery forest interspersed. Primary tropical rainforest blankets the landscape in the southern equatorial latitudes; dense evergreen forest occurs in the west-central areas and freshwater swamp forest occurs along the lower course of the Oubangui River (Sayer et al. 1992).

Fish Fauna:

The tropical rainforests, floodplains, and swamps of the Oubangui ecoregion represent some of the most undisturbed freshwater habitats in central Africa. Though largely unstudied, their biotas are likely among the most biologically diverse in central Africa. Of the nearly 700 species of fish documented within the entire Congo basin, over 140 reside within the Oubangui ecoregion. Further study is expected to reveal many additional species. Major fish families represented in the ecoregion are Alestiidae (Characidae), Amphiliidae, Aplocheilidae, Channidae, Cichlidae, Citharinidae, Claroteidae, Clariidae, Clupeidae, Cyprinidae, Mastacembelidae, Mochokidae, Mormyridae, Polypteridae, Schilbeidae, and Tetraodontidae(Lévêque et al. 1988).  

Description of endemic fishes:

The lower course of the Oubangui River is characterised by lower levels of endemism than other rivers within the inner Congo basin. However, upstream areas and rapids, which are often located in savanna regions harbor some endemics, though these species are known only from small collections. There are twelve endemic fishes known from this ecoregion, including rivulines, cichlids, mochokids, and cyprinids. Recent collections in the Sangha River drainage indicate that some fish species previously considered to be endemic to the Oubangui ecoregion (e.g., Haplochromis oligacanthus) are in fact shared with the Sangha basin.

Other noteworthy aquatic biotic elements:

The Likouala-aux-Herbes and Lac Télé areas, which are connected to swamps of the Congo-Sangha-Oubangui-confluence, are poorly investigated. However, preliminary surveys and historical collections indicate high species richness in these blackwater habitats, with a fauna that is similar to that of blackwater habitats within the Cuvette Centrale ecoregion [818].

Five species of turtle and 3 species of crocodile (Crocodylus cataphractus, C. niloticus, and Osteolaemus tetraspis) have been documented. Among these, O. tetraspis is endemic to the lowland equatorial rainforests of West and Central Africa. It inhabits shallow river basins bordered by swamps; thus, the Likouala swamps of the Oubangui ecoregion provide prime habitat for this increasingly hunted species (Riley & Huchzermeyer 1999). Of over 40 species of frogs, 4 are endemic: Hyperolius brachiofasciatus, H. schoutedeni, Xenopus pygmaeus, and Ptychadena straeleni. Allen’s swamp monkey (Allenopithecus nigroviridis), a Congo basin endemic, lives in Lac Télé (Wetlands International 2002).

The Oubangui also supports a rich avifauna. The wetlands of Lac Télé and Likouala aux Herbes are especially important for migratory bird species in the families Ciconiidae, Areidae and Pelicanidae with about 250 bird species known from the area (Wetlands International 2002). Recent surveys by the African Waterfowl Census revealed over 20 species of waterbirds living in the Likouala aux Herbes swamps. Species present in notable congregations included African jacana (Actophilornis africanus), cattle egret (Bubulcus ibis) and great egret (Ardea alba) (Dodman et al. 1999). Additionally, the Central African Republic, which covers much of the northern portion of the ecoregion, has particularly high bird richness, with approximately 700 species recorded, including the locally rare shoebill Balaenicepts rex, which inhabits riverine swamps of the Oubangui (Sayer et al. 1992).

Justification for delineation:

High habitat diversity and hydrogeographic barriers have shaped the ichthyofauna of the Oubangui drainage. Numerous waterfalls separate upstream and downstream sections of the main river and its tributaries. Some of these rapids may have been formed after river capture of Nilo-Sudanic catchments by the Oubangui drainage. Evidence for river capture comes from several Nilo-Sudanic fish species or subspecies occurring in the Oubangui drainage that are otherwise absent in the Congo basin, except in catchments further northeast with a similar history (e.g., the Lindi and Aruwimi). Nilo-Sudanic species include the cichlid fishes Sarotherodon galilaeus galilaeus and Tilapia zillii. It should be noted that the Uele River, a tributary to the Oubangui, defines a separate ecoregion [536] because isolation by rapids and waterfalls has created a distinct fauna there.

Level of taxonomic exploration:

Fair

References/sources:

Bailey, R. G. (1986)"The Zaire River system" In Davies, B.R.;Walker, K.F. (Ed.). The ecology of river systems. (pp. 201-214) Dordrecht, The Netherlands: Dr W. Junk Publishers.

Beadle, L. C. (1981). "The inland waters of tropical Africa" England: Longman Group Limited.

Dodman, T., Béibro, H. Y., et al. (1999). "African waterbird census 1998" Dakar, Senegal: Wetlands International.

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

Lévêque, C., Bruton, M., et al. (1988) Biology and ecology of African freshwater fishes. Paris: ORSTOM.

Riley, J.,Huchzermeyer, F. W. (1999). "African dwarf crocodiles in the Likouala swamp forests of the Congo basin: habitat, density, and nesting" Copeia 199(2) 313-320.

Roberts, T. R. (1973)"Ecology of fishes in the Amazon and Congo basins" In Meggers, B.J.;Ayensu, E.S.A.;Duckworth, W.D. (Ed.). Tropical Forest Ecosystems in Africa and South America: A Comparative Revue. (pp. 239-254) Washington, DC, USA: Smithsonian Institution Press.

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

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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