Ecoregion Description

534: Sangha

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

Countries:

Cameroon; Central African Republic; Congo

Boundaries:

The Sangha ecoregion spans the borders of three central African countries — Cameroon, Central African Republic (CAR), and Republic of Congo (ROC). It encompasses the Sangha River basin, extending from the headwaters of the Mambéré River in the north on the Cameroon-CAR border, almost down to Malebo Pool in southern ROC. The watershed for the Congo basin, roughly following the Congo-Gabon border, defines the western boundary of the ecoregion, with the Massif du Chaillu marking the southwestern edge and the headwaters of the Boumba River marking the northwestern extent (Hughes & Hughes 1992; Sayer et al. 1992). The rivers and streams of the Sangha ecoregion run through some of the densest forests in the Afrotropical realm and support an impressive array of aquatic species.

Drainages flowing into:

The mainstem Congo River.

Main rivers or other water bodies:

The headwaters of the Sangha include the Kadey River and its tributaries.The Kadey joins the Sangha in the southernmost part of CAR, along the border with Cameroon. Further south, the Ngoko River flows along the Cameroon-ROC border to converge with the Sangha at Ouesso, ROC. The Sangha River then flows south through the Cuvette Centrale, eventually draining into the mainstem Congo River. Several additional rivers, all of which eventually flow into the mainstem Congo River, drain the central and southern parts of the ecoregion. These include the Likouala River and its tributary the Kouyou, as well as the Alima, Nkeni, and Lefini Rivers (Hughes & Hughes 1992).

Climate:

The Sangha River and its tributaries travel through four principle climatic zones. The northernmost climatic zone (6-8ºN) receives a mean annual rainfall of 1,500 mm, with peak rainfall occurring in August and a dry season occurring from December-January. The second climatic zone and is located from 1-6ºN and has an annual rainfall of approximately 1,700 mm. The wet season extends for several months, with most rainfall during August, which is also the coolest month. Here, too, a dry season exists during December and January. The mean monthly temperature for this region ranges from 13ºC, recorded in January, to 40ºC, recorded in March. The central portion of the ecoregion, between 1ºN-2ºS, experiences a true equatorial climate. There is no dry season and little variation throughout the year from the mean annual temperature of 25ºC. The mean annual precipitation ranges from 1,700-2,000 mm. Further south, the Baleke Plateau in south-central Republic of Congo has a subtropical climate with a mean annual rainfall of 1,700 mm, ranging from 1,300 mm/yr along the Congo River to 2,200 mm/yr in the hills along the border of Gabon. The mean annual temperature in this climatic region ranges from 18-32ºC. A distinct dry season exists from June–September and a wet season from October–May (Hughes & Hughes 1992; Sayer et al. 1992). 

Freshwater habitats:

Permanent swamps increasingly dominate the landscape in the central and southern portions of the ecoregion. Raphia swamps proliferate along the main course of the Sangha as it passes south through the Cuvette Centrale and meets the Congo River. The shallow water of these swamps is deep brown to black, deoxygenated and highly acidic (pH 3.5-5.2) (Hughes & Hughes 1992; Laraque et al. 1998). The Likouala and Kouyou Rivers south of the mainstem Sangha also course through extensive swamp forest. Large areas of floating grasses often occur within the rivers. Further south, a mosaic of marshy savanna, boggy steppe, and temporarily inundated swamp forest exist along the Alima, Nkeni, and Lefini rivers (Hughes & Hughes 1992; Sayer et al. 1992; Wetlands International 2002).

Terrestrial Habitats:

Periodically inundated swamp forest, large expanses of dense evergreen and deciduous rainforest and savannas characterize the terrestrial landscape of the ecoregion. Extensive closed canopy forests and large areas of natural savanna woodland occur in the north and central portion of the ecoregion  Savanna occurs primarily in the north of the ecoregion where the headwaters of the Sangha River begin. Evergreen rainforest grades into swamp forests as the eastward-flowing rivers of this ecoregion descend in elevation. This landscape grades into the inundated floodplains, permanent swamp forests, and swamps of the Cuvette Centrale in the south (Hughes & Hughes 1992; Sayer et al. 1992).The riparian swamp forests of the upper Sangha and its tributaries range in width from a few meters up to 7 km (Hughes & Hughes 1992).

Fish Fauna:

Most of the rivers and streams of this ecoregion have been little explored from a biological standpoint, but recent surveys in Cameroon and CAR indicate very high fish species richness. These investigations have documented more than 200 species only in the Cameroonian portion of the Sangha drainage below the Nki falls, and around 300 species in the entire drainage, including the Upper Dja and portions in the Cuvette Centrale (Schliewen, pers. comm.; Hughes & Hughes 1992; Sayer et al. 1992)(Hughes and Hughes 1992; Sayer et al. 1992). 

Dominant fish families identified within the swamp forests of this ecoregion include Alestiidae, Aplocheilidae, Cichlidae, Claroteidae, Cyprinidae, Mochokidae, Mormyridae, and Schilbeidae. Schooling species of Clupeidae (Microthrissa and Odaxothrissa), as well as small species of Barbus,live in open water habitat(Hughes & Hughes 1992). Within the permanently inundated, low oxygen and acidic waters, lives a specialized fauna adapted for this habitat. Adaptations include swimbladder lungs to assist respiration in Polypterus and Protopterus species, and accessory respiratory organs above the gills in Clarias (Lowe-McConnell 1987; Kingdon 1989).

Maximum water levels in the Cuvette Centrale are reached during the rainy season between November and January. At this time, fish move into the flooded forests along the river channels to feed and breed.

Other noteworthy aquatic biotic elements:

The Sangha ecoregion also provides important habitat for freshwater crabs, amphibians, reptiles, and aquatic mammals. According to Cumberlidge and Bokyo (2000), eight species of freshwater crabs live along the upper Sangha. Their research discovered two new species, S. africanus and S. sangha. It also confirmed two little-known species of Potamonautes and two species of Sudanonautes (Cumberlidge & Bokyo 2000). Over 67 species of aquatic dependent frogs live in the ecoregion, including 6 endemics. Three crocodile species occur in mainstem Congo River wetlands: Crocodylus cataphractus, C. niloticus, and Osteolaemus tetraspis. Among these, O. tetraspis is endemic to lowland equatorial rainforests and is also considered imperiled (Riley & Huchzermeyer 1999). Aquatic and semi-aquatic snakes in the ecoregion include Boulengerina annulata, Natriciteres olivacea, Naja melanoleuca, and Python sebae. Populations of Allen’s swamp monkey, Allenopithecus nigrovirdis, endemic to the Cuvette Centrale, as well as giant otter shrew, Potamogale velox, and two species of otter (Aonyx spp.) also depend upon the rivers of the ecoregion (Hughes & Hughes 1992; Wetlands International 2002).

Justification for delineation:

This ecoregion largely follows the watershed of the Sangha River basin. The river systems of the Sangha ecoregion are permanent and have existed since before the major earth movements of the Miocene. The flat central part of the ecoregion, the Cuvette Centrale, is part of the ancient continental Congo basin, invaded by the sea during the Mesozoic. Following this invasion by the sea, the peripheral land surrounding the Cuvette Centrale experienced massive uplift, accentuating the basin (Beadle 1981). The Dja, an upper tributary of the Sangha, is completely isolated from the Ngoko/Sangha by a large waterfall, the Nki Falls in Cameroon. The fauna upstream of Nki Falls is completely different (more like that of the Nyong and other basins in the Southern Gulf of Guinea Drainages [533] from that of the downstream portion of the river (pure Congo-fauna). Thus, the upper Dja is included in the Southern Gulf of Guinea Drainages [533] ecoregion instead of the Sangha.

Level of taxonomic exploration:

Fair

References/sources:

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

Cumberlidge, Neil,Bokyo, Christopher B. (2000). "Freshwater crabs (Brachyura: Potamoidea: Potamonautidae) from rainforest of the Central African Republic, Central Africa" Proceedings of the Biological Society of Washington 113(2) 406-419.

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

Kingdon, Jonathan (1989). "Island Africa: The evolution of Africa's rare animals and plants" Princeton, NJ, USA: Princeton University Press.

Laraque, A., Mietton, M., et al. (1998). "Impact of lithological and vegetal covers on flow discharge and water quality of Congolese tributaries from the Congo River" Revue Des Sciences De L'eau 11(2) 209-224.

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

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.

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)