Geographic Patterns

Centres of species richness

Species distribution maps for the priority taxonomic groups of fishes, molluscs, odonates and crabs were overlaid to identify those river basins holding the highest combined species richness. Aquatic plants were  excluded from the analyses as a lack of specific range information meant many species could only be mapped to a country level. The area of greatest species richness is clearly defined by the channel of the Congo  River, and its tributaries the Ubangi River and the Kasai River. The Congo River has the highest species diversity of any freshwater system in Africa, and is second in species richness globally, after the Amazon Basin. The river passes through a number of different habitats, and the Ubangi River in particular is thought to be one of the most undisturbed areas within central Africa.

Central Africa freshwater species richness

Central Africa species richness for fishes, molluscs, odonates and crabs, mapped to river sub-catchments.

Overall species richness is heavily weighted by the high fish diversity in the region. By overlaying the distribution maps for each taxonomic group, those river basins holding the greatest richness across all groups could  be identified. Centres of overall species richness were identified as those sub-basins holding at least 20% of the total numbers of mapped species within each of the four taxonomic groups. This approach now also identifies the coastal river basins as areas of high species richness across all taxonomic groups. The two basins of particular high species diversity are that of Malebo Pool and the Upper Congo Rapids.

Areas of high respresentation across four freshwater taxonomic groups

Distribution of river basins containing exceptionally high numbers of species from all freshwater taxonomic groups, mapped to river sub-catchments. The map represents those HYDRO1k level 6 sub-basins holding at least 20% of the total regional species compliment for each of the fishes, molluscs, odonates, crabs and plants.

Malebo Pool is a large shallow pool within the lower reaches of the Congo River. It has a surface area of approximately 500 km2, and the depth fluctuates considerably within a range of 3–10 m. It supports an estimated  341 species of fish (28% of the regional total), 47 species of mollusc (28% of the regional total), 149 species of odonates (33% of the regional total), 107 species of the selected aquatic plants (27% of the regional total),  and six species of crab (16% of the regional total). Malebo Pool lies between the two capitals of Brazzaville and Kinshasha. This is an area of high urbanisation, along with a large and over-exploited fishery supplying both sides of the river. The central island of Ile Mbamou is of increasing agricultural importance, resulting in an escalation of erosion and runoff of sediments and pollutants into Malebo Pool. The Boyoma Falls (formerly  known as Stanley Falls) are an area of rapids where the river falls by 60 m along a stretch of 100 km. This area delineates the point where the Lualaba River ends, and the Congo River begins. Although the Boyoma Falls can be sited as the largest waterfalls globally by volume, each of the seven cataracts that make up the rapids drops by no more than 15 m. It supports an estimated 281 species of fish (23% of the regional total),  44 species of mollusc (27% of the regional total), 154 species of odonates (34% of the regional total), 75 species of the selected aquatic plants (19% of the regional total), and four species of crab (11% of the regional  total). The area is heavily impacted by artisanal mining, and 95% of industrial discharge and sewage in the region is dumped directly into the river. The continued civil unrest within the region includes struggle for control over the natural resources in the surrounding area, and also prevents the instigation of environmental protection within the region.

Distribution of threatened species

The areas supporting the highest number of regionally threatened species are the coastal zones of Northern, Central and Southern West Coastal Equatorial ecoregions, the Lower Congo Rapids, and to a lesser  extent, the Bangweulu-Mweru system in the southeast. The coastal areas of Cameroon are forested plains and swamps, which support a high number of endemic species. The region’s habitats are  threatened by widespread logging, and offshore oil exploration and pollution is impacting the estuaries. One of the areas sustaining the greatest number of threatened species is the Western Equatorial Crater Lakes  region of southwestern Cameroon. Here a collection of 36 crater lakes lies along a volcanic ridge, the largest of which is Lake Barombi Mbo with a surface area of just 5 km2. Most of these lakes are completely isolated from nearby water systems, and support an extremely high level of endemism, particularly of fishes. All life within the lakes is under constant threat of lake ‘burping’; seismic activity causing the release of  carbon dioxide into the lake, turning it anoxic. In addition to this constant underlying threat, anthropogenic activities are, however, a cause for more immediate impacts to species. Increased agriculture results in  deforestation and severe run-off carrying sediments, pesticides and fertilisers. In some lakes water levels have been lowered by water extraction, but those lakes most heavily affected by overfishing may now benefit  from recent bans on commercial fishing.

The second highest centre of threatened species is the Lower Congo Rapids ecoregion. Located near to the mouth of the River Congo, this stretch of water receives all the pollutants collected from the length of the  river, in particular from the areas around Brazzaville and Kinshasha situated just north of this area, at Malebo Pool. Water pollutants include sewage and domestic waste, agricultural run-off, and extensive industrial  and mining seepage. Water samples taken in the area have shown lead levels to be four times higher than the US Environmental Protection Agency’s recommended levels for children. This ecoregion also contains the Inga dams. There are currently two dams, Inga I and II, which generate 351 and 1,424 MW in capacity respectively. Although no environmental impact studies have been  conducted, it is thought that some species, particularly endemic waterfall molluscs, are already threatened by the hydropower station. Plans are in place for Inga III (1,344 MW), however long term plans for Grand Inga Dam (39,000 MW), which if completed would have been double the size of the Three Gorges Dam, currently the largest energy-generating body ever built, have recently been repealed due to the catastrophic impacts both at a local and global scale. This has instead been replaced with designs for Grand Inga Cascades, which are ‘run of the river’ structures intended not to disrupt the river flow.

Threatened freshwater species richness in Central Africa

The distribution of regionally threatened species of fishes, molluscs, odonates and crabs within central Africa, mapped to river sub-catchments.

Distribution of restricted range species

Species with restricted ranges were defined as those regionally endemic species restricted to any level 3 river basin as defined in the HYDRO1k data layer. It is worth noting that basin sizes in the region do vary quite largely, from 98 km2 to a maximum of 306,952 km2, with an average of 32,726 km2. The greatest density of restricted range species is found in the north-western area of the central Africa region, in the vicinity  of the Lower Cross River and the Western Equatorial Crater Lakes. Other areas supporting a large number of range restricted species are the Upper and Lower Congo Rapids.

Restrcited range freshwater species in central Africa

Number of species of fishes, molluscs, odonates and crabs restricted to single HYDRO1k level 3 sub-basins. The distributions are mapped to the level 6 basin level.