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Ecoregion Description


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Species Richness


# of Endemic Species


Threats

546: Kasai

Major Habitat Type:

tropical and subtropical floodplain rivers and wetland complexes

Author:

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

Reviewers:

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

Countries:

Angola; Democratic Republic of Congo

Boundaries:

The ecoregion largely follows the boundaries of the catchment of the Kasai River, which encompasses a vast area (900,000 km2) within southwestern Democratic Republic of Congo (DRC) and the northeast corner of Angola. 

Drainages flowing into:

The mainstem Congo River.

Main rivers or other water bodies:

The Kasai River and its tributaries are the principle rivers in this ecoregion. The Kasai originates on the Lunda Plateau of Angola, flows through channels characterized by falls and rapids, and discharges an average of 1,200 m3/sec where it flows into the Congo River (Bailey 1986). 

Topography:

The headwaters of the Kasai River begin in a series of highland massifs along the northern rim of the Lunda Plateau (with sources at elevations between 1,000 and 1,500 m asl) (Hughes and Hughes 1992). The land tends to slope gently towards the central Congo basin, but as a result of numerous scarps located in the plateau, most of the rivers traverse falls and rapids before reaching the Cuvette Centrale (Marlier 1973; Hughes and Hughes 1992). The Cuvette Centrale is the low-lying portion of the Congo basin that once made up part of an ancient inland lake formed during the Pliocene (Beadle 1981).

Climate:

The Kasai ecoregion experiences a tropical climate, except in the more temperate uppermost Angolan Highlands. Throughout the northern portion of the Kasai basin there is one wet season and a mean annual temperature of 23oC. In the Bandundu, West Kasai, and East Kasai provinces, the daily temperature range is greater in the dry season than in the wet, with mean minima and maxima of 15oC and 32oC in July (dry season) and 19oC and 29oC in December (wet season) (Hughes and Hughes 1992). Where the headwaters of the Kasai originate in northern Angola, mean annual temperature is about 18oC. Mean annual rainfall in the northern half of the ecoregion is about 1,400-1,750 mm; whereas up to 2,000 mm/yr rain falls over the Angolan tributaries.

Freshwater habitats:

Tributaries to the Kasai originate at elevations up to 1,400 m asl in Angola and flow north into the DRC across a plateau of about 1,000 m asl. Most of the Kasai River’s rapids are located upstream from the city of Tshikapa at about 500 m asl (Kamdem-Toham et al. 2003). Above this elevation, the upper reaches of the Kasai and its tributaries are interrupted by falls and rapids along the edge of the Lunda plateau (Marlier 1973). The rivers flow off the plateau and eventually flow into the mainstem Kasai. The Kasai descends to about 300 m asl at its lowest point within this ecoregion and the mainstem river flows through the swamp forests of the central Congo Basin before entering the mainstem Congo River. The mainstem Kasai attains mean widths of 4 km for stretches of up to 700 km, and in its lower reaches it is studded with islands that become inundated in periods of high water. Major tributaries of the Kasai from east to west are the Kwango (Cuango), Wamba, Kwilu (Cuilo), Loange, Lulua, and Sankuru Rivers.

The Kasai has a flooding regime that is unimodal in its upper reaches and becomes bimodal approaching its junction with the Congo River. The annual amplitude of flooding is about 3 m in its lower reaches (Marlier 1973). Seasonally and permanently inundated swamp forests grow in the lowest part of all river valleys in the ecoregion, in strips 100 m to 10 km wide. For example, along the Kwilu River and its tributaries there are 11 major swamps with a total area of about 1,550 km2. Near Sandoa, there is a large wetland of 4,500 km2 associated with the Lulua River (Hughes and Hughes 1992).

Terrestrial Habitats:

With the exception of swamp forests in the lowermost river reaches, the vegetation of the ecoregion is primarily savanna with gallery forests lining the river valleys. A Julbernardia-Brachystegia complex dominates the vegetation of savannas in the south. Gallery forests are tall and dense and include species such as Anthocleista vogelii, Mitragyna ciliata, and Phoenix reclinata (Hughes and Hughes 1992). These forests become more extensive as the Kasai flows towards the central Congo basin. In addition to gallery forests, extensive peat bogs exist in some valleys and shallow depressions adjacent to the rivers. Permanent swamp forests include the oil palm Uapaca guineensis, and in deep narrow valleys and ravines, Mitragyna stipulosa and Sarcophrynium schweinfurthianum dominate swamp forests directly adjacent to streams (Hughes and Hughes 1992).

Fish Fauna:

This ecoregion has an incredibly rich fish fauna with over 200 known species, about one-quarter of which are endemic. However, few biological studies have been completed in recent years and further surveys should reveal additional new species and endemics. The fish fauna is far from uniform throughout the ecoregion. Certain fish are characteristic of the savanna streams (for example Aplocheilichthys and Hypsopanchax species) while others are more common in the lower reaches that are surrounded by swamp forests (such as Aphyosemion and Epiplatys species) (Hughes and Hughes 1992).

Description of endemic fishes:

As an example, Angolan tributaries on the Lunda Plateau harbor an impressive array of endemic fishes, including the dwarf distichodontid Dundocharax bidentatus and one or more poeciliids, cyprinids, mormyrids and cichlids (Poll 1967). The Fwa River, a short (20 km) tributary of the Lubi located in the middle of the ecoregion, also possesses five endemic cichlid species. Two of the endemic cichlids (Cyclopharynx fwae and C. schwetzi) belong to a genus only found in the Fwa (Roberts and Kullander 1994). The Fwa’s stable environment might have facilitated speciation.

Other noteworthy fishes:

In rapids of the lower Kasai River, specialised fish assemblages known only from a few historical specimens include the cichlids Steatocranus rouxi and Teleogramma monogramma. Interestingly, these taxa often have their closest relatives in Pool Malebo and the rapids of the Lower Congo, but not in other parts of the Kasai drainage. 

Other noteworthy aquatic biotic elements:

The ecoregion is also extremely rich in frogs, with about 60 species known from the Kasai. There are also about five known endemics, although both richness and endemism may be expected to increase upon further research. Although little studied, the aquatic insect fauna is also thought to be rich, with many endemics. Historical collections from the rapids of the Kasai upstream from Tshikapa reveal a large number of species, including some endemics (Kamdem-Toham et al. 2003). 

Evolutionary phenomena:

Cichlid species radiation in Fwa River (described above).

Justification for delineation:

The ecoregion largely follows the boundaries of the catchment of the Kasai River. The many falls and rapids along the course of the Kasai River and its tributaries restrict the movement of fish. This may explain the high level of endemism in this ecoregion (Beadle 1981). Rapids have prevented the dispersal of species to the lower reaches of rivers as well as headwater colonization by potentially competing species (Lévêque 1997). A few of the uppermost tributaries of the Kasai are included within the Zambezian Headwaters ecoregion [555] due to their historic connection with the rivers of that ecoregion and the Zambezian affinity of their fauna (Poll 1967).

Level of taxonomic exploration:

Poor

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.

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

Kamdem-Toham, A., D'Amico, J., et al. (2003) "Biological priorities for conservation in the Guinean-Congolian forest and freshwater region: Report of the Guinean-Congolian forest and freshwater region workshop, Libreville, Gabon, March 30-April 2, 2000". Libreville, Gabon. WWF.

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

Marlier, G. (1973)"Limnology of the Congo and Amazon rivers" In Meggers, B.J.;Ayensu, E.S.;Duckworth, W.D. (Ed.). Tropical forest ecosystems in Africa and South America: A comparative review. (pp. 223-238) Washington, DC, USA: Smithsonian Institution Press.

Poll, M. (1967). "Contribution à la faune ichthyologique de l´Angola" Lisbon: Diamang.

Roberts, T. R.,Kullander, S. O. (1994). "Endemic cichlid fishes of the Fwa River, Zaire: systematics and ecology" Ichthyological Explorations of Freshwaters 5(2) 97-154.

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