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# of Endemic Species
521: Lake Victoria Basin
Major Habitat Type:
Dalmas Oyugi, Kenya National Museum, Nairobi, Kenya
Lauren Chapman, Department of Zoology, University of Florida, Gainesville, Florida, USA
Burundi; Democratic Republic of Congo; Kenya; Rwanda; Tanzania; Uganda
This ecoregion is defined by the basins of Lakes Victoria, Edward, George, Kyoga, and the small, but deep, Lake Kivu. Lake Victoria is the largest lake in Africa and contains one of the world’s most important examples of rapid species radiations among its endemic halpochromine cichlid fauna.
Main rivers or other water bodies:
With an area of approximately 68,800 km2, Lake Victoria is the largest tropical lake in the world as well as the second largest freshwater lake in the world (Spigel & Coulter 1996). Stretching 412 km from north to south and 355 km from west to east, Lake Victoria spans the borders of Tanzania, Uganda and Kenya. Its massive catchment (over 193,000 km2), reaches well into Rwanda and Burundi (Hughes & Hughes 1992). Lake Victoria occupies a shallow depression 1,134 m asl, between the west and east African rifts. The water balance of the lake is maintained primarily through rainfall and evaporation, rather than inflows and outflows (Spigel & Coulter 1996). Due to this dependence on rainfall and evaporation, the residence time of water in Lake Victoria is 23 years (Cohen et al. 1996; Spigel & Coulter 1996). Lake level in Victoria has varied by about 2 meters in the last century in response to changes in rainfall and evaporation. The Lake basin itself is about 400,000 years old, but several recent studies suggest that the lake was completely dry for several thousand years and re-filled only 15,000 years ago (Johnson et al. 1996; Johnson et al. 2000; Talbot & Laerdal 2000).
Numerous rivers and streams drain into Lake Victoria. The principal affluent is the Kagera River, which enters the lake along its western shore, draining the highlands of Burundi and Rwanda. The Kagera River is about 360 miles long and the Ruvuvu River is its principal tributary. A series of swamps (2-18 km wide) and small lakes occur along the course of the Kagera River with several water falls in its upper reaches (De Vos et al. 2001). The Nzoia River is also a perennial affluent of Lake Victoria. It drains the Elgon Massif, the Cherangani Hills and Sergoit, entering the lake in the northeast. Inflows from rivers in the northwestern and southeastern portion of the ecoregion constitute the remainder of the riverine input. Rivers entering the lake from the northeast tend to be swift flowing whereas, rivers of the northwest tend to be sluggish and perennial (Hughes & Hughes 1992).
The only outlet from Lake Victoria is the Victoria Nile River, which flows through Lake Kyoga and then to Lake Albert, to the north and northwest, respectively (Hughes & Hughes 1992). Passing through extensive areas of swampland, the Victoria Nile enters Lake Kyoga from the south. Lake Kyoga is part of a permanently flooded series of shallow lakes and swamps called the Kyoga Lake-Kwania Swamp complex, which contains 3,416 km² of open water and shallow lakes and 2,184 km² of permanently flooded swamp (Hughes & Hughes 1992).
Located on the border of Rwanda and the Democratic Republic of Congo (DRC), Lake Kivu lies in the Western Rift Valley, at an altitude of 1,463 m asl and is bordered by steep slopes that rise to elevations of over 2,000 m. It is 100 km long, and has a maximum width of 50 km. The lake is deep with a depth of about 480 meters and it is the most completely stratified lake in Africa. Lake Kivu formerly drained into Lake Edward. Volcanic activity severed this connection and Lake Kivu now drains into Lake Tanganyika via the Ruzizi River (Worthington & Lowe-McConnell 1994).
Lakes Edward and George lie in the western portion of the ecoregion. Surrounded by extensive swamps, Lake George straddles the equator and is fed by numerous rivers, which drain the eastern slopes of the Rwenzori Mountains, the highlands of the Western Rift Valley and the Virunga Massif. In general, Lake George is well mixed, though it does have a diurnal stratification cycle (Thompson 1976; Hughes & Hughes 1992). Lake George’s main inflows are the Rivers Nsong, Mbuku, and Bumlikwesi (Beadle 1981). The enormous swamp surrounding Lake George is dominated bypapyrus (Cyperus papyrus), which typically comprises over 95% of the plant biomass (Thompson 1976). Lake Edward is connected to Lake George by the Kazinga Channel, which 40 km long and has a maximum width of less than 1 km (Beadle 1981). Just 5 km off its western shore in the DRC, Lake Edward plummets to a depth of 112 meters, and then slopes gradually up to its eastern shore in Uganda (Hughes & Hughes 1992). The major outflow from Lake Edward is the Semliki River and its main inflows are the Rivers Nyamugasani, Ishasha, Rutshuru and Rwindi, with a smaller contribution by the Kazinga Channel (Beadle 1981).
The Lake Victoria ecoregion is also endowed with several small satellite lakes. A few of these include Lakes Kanyaboli, Sare, Namboyo in Kenya; Lakes Nabugabo, Gigati, and Agu in Uganda; and Lakes Ikimba and Burigi in Tanzania (Aloo 2003). These lakes may act as valuable sources of biodiversity, since many of them are still relatively undisturbed by human activities.
Lakes Victoria, Edward and George are all part of the greater Nile catchment. Separated from the lower Nile catchment by Murchinson Falls, evidence suggests that a connection between these lakes existed in the recent past, within the last several thousand years (Worthington & Lowe-McConnell 1994). During the uplift of the mid-Pleistocene, it is theorized that the rapid “back ponding” of the Kagera and Katonga Rivers overspilled the basin of Lake Victoria, draining into the Edward basin well into the late-Pleistocene (Beadle 1981). Even today, the Katonga River is able to flow in either direction, enabling species communication between all three lakes (Worthington & Lowe-McConnell 1994). Lakes Edward and George have had a very dynamic history. In the early Pleistocene (2 million years ago), there may have been passable connections between Lake Edward and Albert basins, now separated by a section of rapids (300 m descent). Lake Albert is not included in this ecoregion because this separation has likely caused its fauna to remain distinctly riverine and Nilotic. The faunas of lakes Edward and George were very similar: fossil remains of Lates, Hydrocynus, and Crocodylus niloticus in the Edward basin link the two faunas. Subsequently, the two lakes may have gone through a number of mass extinction and recolonization events (Beadle 1981). The last of these events probably occurred between 8,000 and 10,000 years ago and was associated with the eruption of volcanoes which may have deposited enormous amounts of toxic ash on the lake, killing species such as Lates which require well-oxygenated water (Beadle 1981; Schofield & Chapman 2000). Similarly, the Nile crocodiles (Crocodylus niloticus) were no longer present in the areas of Lakes Edward and George after the eruption of neighboring volcanoes (Beadle 1981), presumably kept from colonizing the lake by difficulties in passing the Semliki rapids. Some species in Lakes Edward and George are also found in Lake Albert (e.g., Bagrus docmac, Oreochromis niloticus), while other species (e.g., Polypterus senegalus, Hydrocynus spp.), and even families (such as Mastacembelidae, Characidae, Schilbeidae) that typify the Nilotic fauna from Lake Albert are absent (Greenwood 1966). Many of the cichlids from Lakes Edward and George are closely related to species in Lake Victoria (Greenwood 1966; Greenwood 1973; Beadle 1981; Kaufman et al. 1997); however, the number and distribution of endemic cichlids in the lakes is uncertain, and holds the key to our understanding of the relationship between land form and evolution of cichlid faunas in the Lake Victoria Basin.
A recent study on the phylogeny of the Lake Victoria-Edward cichlid species flock suggests, in contrast to the results of previous studies, that the flock is derived from the Congolese-Nilotic genus Thoracochromis and not from the East African riverine Astatotilapia (Seehausen et al. 2003). The rivers feeding Lake Victoria were tributaries of the Congo until uplifting of the region to its west about 400,000 years ago and it is possible that ancestry with the Congolese Thoracochromis predates this event. Seehausen et al. (2003) also question the monophyly of the Lake Victoria-Edward flock and raise the possibility that the flock has arisen from hybrid swarms.
The Lake Victoria ecoregion has an equatorial climate. In the north there are two rainy season; one during April/May, and the other during October/November, whereas the south experiences one long rainy season from December to March (Burgis & Symoens 1987; Lowe-McConnell 1987). Average annual rainfall is 650-900 mm, with peak rains in April and a dry season between June and September (Beadle 1981). Temperature of the lake varies from 23-27 °C with a mean temperature of 25 °C throughout the year (Witte & Van Densen 1995). Wind speed is generally low during the wet season (0-3.5 m/s), due to the protection of the Rwenzori mountains in the north and the Rift escarpment to the east and south. Low wind speeds allow for the build up of a thermocline, which results in stratification in the lake (Talling 1966). Lake Victoria experiences annual stratification and overturn with an associated upwelling of nutrients, as in temperate lakes (Lowe-McConnell 1987). Wind speeds increase during the dry season, between May and June when strong southerly winds exceeding 15 m/s blow over the ecoregion (Spigel & Coulter 1996). These strong southerly winds cause high evaporation rates, water mixing and a decrease of surface temperatures. Seasonal algal blooms coupled with shallow water upwellings as a result of wind action, affect seasonal transparency patterns in the lake (Witte & Van Densen 1995).
The ecoregion’s lakes and shallow bays are home to many types of emergent and submerged vegetation (macrophytes). In the Lake Victoria basin, the dominant macrophytes include; Cyperus papyrus, Miscanthidium violaceum, Phragmites mauritanius, and Typha domingensis,among others. The most extensive papyrus swamps (C. papyrus)in East Africa occur along the perimeter of Lake Victoria and along the perimeters of the other lakes within this ecoregion (Chapman et al. 2001). The valley swamps fringing the rivers flowing into Lake Victoria are dominated by Miscanthidium violaceum. These two swamp types also support a diversity of other plant species, with both types reported to contain upwards of thirty other species (Chapman et al. 2001). The interface between open water and permanent swamp sustains a distinctive plant and animal community. However, many fewer species are adapted to the often oxygen poor environment of the permanent swamps and their dense stands of fibrous papyrus mats.
Grasses and trees also grow on the seasonal floodplains and stands of Acacia occur throughout the landscape adjacent to the lake and inflowing rivers (Hughes & Hughes 1992).
Lake Victoria’s endemic haplochromine fauna is one of the world’s most outstanding examples of explosive speciation and adaptive radiation (Greenwood 1981; Kaufman 1992; Kaufman et al. 1997). Recent studies have shown that sexual selection is likely the driving force behind reproductive isolation within the haplochromine cichlids of the Lake Victoria basin and that the flexible and versatile pharyngeal jaw apparatus of these fishes has also played a role in the rapid diversification of these fish (Kaufman et al. 1997; Galis & Metz 1998; Seehausen & Van Alphen 1998, 1999; Seehausen et al. 1999). Over 600 endemic fish are known from Lake Victoria alone, although estimates of species numbers vary widely (Seehausen 1996; Kaufman et al. 1997; Turner et al. 2001). The total number of cichlid species within Lakes Edward and George is about 80, with nearly sixty of these being endemic (L. Chapman, pers. comm. 2003). Currently, 28 fish species are known from Lake Kivu and its affluents. Nineteen of the 28 are cichlids, 9 are non-cichlids.
In addition to Cichlidae, the lakes of the Victoria ecoregion also host fish fauna from the families Alestiidae, Amphiliidae, Clariidae, Cyprinidae, Mochokidae, Mormyridae, Poeciliidae and Protopteridae. About one-third of the approximately 90 non-cichlid fish species are endemic to the ecoregion.In addition to the lacustrine species there are also many riverine fish. For example, at least 55 fish species live in the Rwandan portion of the Kagera River. Several of these are anadromous fishes like Labeo spp., Clarias spp., Bagrus spp., and Barbus spp., among others, which utilize the river for spawning purposes (Okedi et al. 1974).
Other noteworthy aquatic biotic elements:
The ecoregion’s swamps and wetlands also support numerous waterbirds. Among these, the vulnerable papyrus yellow warbler (Chloropeta gracilirostris), the vulnerable white-winged warbler (Xenoligea montana), the locally rare papyrus gonolek (Laniarius mufumbiri) and shoebill (Balaeniceps rex), and the more common Carruther’s cisticola (Cisticola carruthersi), great egret (Ardea alba) and Baillon’s crake (Porzana pusilla), inhabit wetlands bordering Lake Victoria (Bennun & Njoroge 1999, 2001). Congregations of Chlidonias leucopterus, Egretta garzetta, Phalacrocorax africanus, P. carbo, Larus cirrocephalus also occur in marshes, bays, islands, and swamps along the margins of the lake (Baker & Baker 2001; Byaruhanga et al. 2001).
The ecoregion is also rich in other taxa, including aquatic-dependent reptiles, amphibians, and mammals, plankton and freshwater molluscs. In particular, there is a high species richness of frogs, with over 60 species known from this ecoregion, one-quarter of which are endemic (and mainly confined to forest habitats). Aquatic obligate vertebrate species in the Lake Victoria environs, include five species of freshwater turtle, two aquatic snakes, monitor lizard, Nile crocodile (Crocodylus niloticus), three species of otters (Aonyx capensis, A. congicus, and Lutra maculicollis), and hippopotamus (Hippopotamus amphibius)(Hughes & Hughes 1992). The waters of Lake Victoria and other lakes are rich in plankton species, for example with around 80 species of planktonic diatoms (Lung’aya in press). The ecoregion’s waters also support an abundant mollusc fauna comprising 54 species, with about one-fifth endemic to the ecoregion (Brown 1994).
From a biodiversity perspective, Lake Edward is very important. Preliminary surveys of the fish fauna of the lake (1996) have yielded many new species of cichlids (L. Chapman, pers. comm.). Additionally, Lake Edward is the least disturbed of those great lakes that contain endemic faunas since much of the lake lies within national parks in Uganda and Democratic Republic of the Congo. Finally, the riverine faunas in the tributaries of the Edward-George system are very rich and include the antecedents to the lacustrine radiations. However, many of the tributaries remain unexplored. The system is currently undergoing natural and anthropogenic turmoil. In addition to increasing pressures on environmental resources from fishing villages inside the protected areas, Nile crocodiles have become reestablished in the system after an ~8,000 year absence and hippopotamus populations are increasing in response to one decade of protection within the parks.
This ecoregion is outstanding for its cichlid species radiation with 22 endemic cichlid genera and 3 additional endemic genera (Xenoclarias [Clariidae], Cynopanchax, Laciris [Poeciliidae]) (Thieme et al. 2005).
Justification for delineation:
This ecoregion is defined by the basins of Lakes Victoria, Edward, George, Kyoga, and Kivu, and is characterized by a lacustrine fauna with cichlid species radiations typical of those within the Great Lakes bioregion. The mountains of the Karamoja district enclose the ecoregion in the northeast. In the east, the highlands of the Eastern Rift Valley are comprised of the Cherangani Hills, Elgeyo Escarpment and Mau Escarpment. The Rwenzori Mountains and the Virunga Massif are the highlands that enclose the ecoregion in the west. The southwest is dominated by a chain of mountains in Rwanda, along the rim of the Western Rift Valley (Hughes & Hughes 1992).
Level of taxonomic exploration:
Good. The Lake Victoria Region has provided a model for relationships between land form and evolution of fishes. Our understanding of the evolution of these extraordinary faunas is increasing as faunal and paleolimnological surveys continue in the region. But, there is a huge missing piece in the puzzle, the Lakes Edward-George region. Lake Edward is one of the less explored ichthyofaunas in Africa. Lake George is better known, although the extensive wetland around the swamp remains relatively unexplored. The Lake Edward-George system is very exciting biogeographically because it represents the confluence of the Albertine and Victoriine faunas.
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