Centres of species richness

Species distribution maps for fishes, molluscs, odonates and the selected aquatic plants were overlaid to identify those river basins holding the highest richness of species across all four species groups combined. Crabs were not included in the analysis due to the relatively low number of species (19) found in the southern Africa region. 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.

Distribution of river basins containing exceptionally high numbers of species from all taxonomic groups. The map represents those Hydro1K level 3 sub-basins holding at least 20% of the total species complement for each of the fishes, molluscs, odonates and plants.

Three river basins were identified as meeting this threshold for all four species groups. A small number of Data Deficient species were not able to be mapped and so could not be included in this analysis.

Area 1: The area of the upper Zambezi at the confluence of the upper Zambezi, Cuando and Chobe rivers above the Victoria Falls at the eastern end of the Caprivi Strip supports an estimated 81 species of fish (23% of the regional total), 89 species of odonate (32% of the regional total), 124 of the selected aquatic plants (28% of the regional total), and 23 mollusc species (26% of the regional total). This area is also important for a number of freshwater dependent mammals, and waterbirds.

The diverse fauna closely reflects the geological history of the region. Linked to the disruption of Gondwana, doming of the continent over major mantle plumes created elevated ground providing headwaters for a new post-Gondwana drainage system, in which the Okavango, Cuando and Upper Zambezi were major south-east-flowing tributaries of the Palaeo-Limpopo River system in the early Cretaceous. The Kafue at that stage was also an Upper Zambezi tributary, which itself had links with the Palaeo-Chambeshi, today part of the upper Congo system. These tributaries, including the Zambezi, then became an endorheic drainage system supplying sediment to an inland Kalahari Basin. The timing of capture of the Upper Zambezi by continued headward erosion of the Middle/Lower Zambezi is tentatively placed in the lower Pleistocene, when there was a complex drainage reorganisation in the “Four Corners” area where Namibia, Botswana, Zimbabwe and Zambia meet. The Makgadikgadi Pans in northern Botswana are relics of a major lake, Lake Palaeo-Makgadigkadi. There is evidence of other lakes in the region, such as Lake Caprivi, thus there was a complex history with evidence of the Upper Zambezi having been diverted at least twice. Uplift along the Linyanti fault diverted the Cuando River to the north east towards the Upper Zambezi, and uplift of the Chobe fault led to the development of the Chobe floodplain. The Cuando River, draining off the Angolan highlands, enters Namibia as the Kwando and in wet years links up with the Chobe River in Botswana, which then joins the Zambezi. The terrain in this area is so flat that, depending on the relative height of the water in the Kwando and the Zambezi, the flow of the Chobe may reverse and, when the Zambezi is high, water is pushed back into the Chobe Marsh and into the ephemeral Lake Liambezi.

This area is therefore centred round the point where a number of major southward draining rivers joined in the past. The fauna reflects links between these rivers up to and including southern Congo tributaries. Wetlands were much more extensive in the past, and are likely to become smaller or more confined in the future, especially given the predictions of 15-20% lower rainfall in the region over the next 50 years owing to global climate change.

At present there are few major threats to the fauna in this area. The rivers upstream are in rural areas with little current disturbance. The Cuando River in Angola is lightly populated following the long civil war, while in Botswana rainfall is erratic and not conducive to intensive development. The Upper Zambezi itself is little impacted by human activities with the exception of locally intensive fishing in areas such as the Barotse Floodplain. In the Chobe area in general, wildlife-based tourism is the major industry and this has little impact on wetland biodiversity and health.

Area 2: Two adjacent sub-basins, the Komati and Crocodile, in the upper part of the Incomati River Basin in Mpumalanga, support an estimated 56 species of fish (16% of the regional total), 75 species of odonate (27% of the regional total), 202 of the selected aquatic plants (45% of the regional total), and 24 molluscs (27% of the regional total).

The Incomati river basin incorporates the Mpumalanga Province in South Africa, part of northern Swaziland, and a small part of southern Mozambique. The main river descends from the highland plateau in South Africa and Swaziland and flows through the coastal plains of Mozambique to the Indian Ocean. Dams with more than 2.060 Mm3 storage capacity have been built in the Incomati basin in South Africa and Swaziland, used primarily for irrigation. Two major dams, Driekoppies Dam in South Africa and the Maguga Dam in Swaziland, fall under the control of the Komati Basin Authority (KOBWA), which has responsibility for the Komati River Basin Development Plan.

As pointed out by Vaz and van der Zaag (2003) “Water use is intense, with 50 percent of the water generated in the basin being withdrawn. Water scarcity has been evident since the mid-1980s, and has become more severe in the last decade. Competition over water is real, and water abstractions are fast approaching the limits of sustainability. The effects of droughts, but also floods, become ever more pronounced.”

The intensive use of the water of the Incomati system for irrigation has impacted on the health of the river system and the use of the river for intensive cultivation of water-hungry crops such as sugar-cane has resulted in criticism. The recently-constructed Maguga Dam has no provision for fishways and is a total barrier to upstream movement of fish. The health of the river system as a whole is also threatened by extensive coal mining across the headwaters with resultant risk of pollution by acid mine waters.

Area 3: The adjacent Mbuluzi basin, also draining into Maputo Bay, supports an estimated 58 species of fish (17% of the regional total), 62 species of odonate (22% of the regional total), 91 species of the selected aquatic plants (20% of the regional total), and 27 species of mollusc (30% of the regional total). The Mbuluzi river basin is the major source of water for agricultural activities, particularly for sugar estates supplied through Mnjoli dam, and for domestic rural and urban water supplies, including Swaziland’s capital, Mbabane. Erosion and consequent siltation as a result of intensive agriculture is a problem throughout Swaziland.

Distribution of threatened species

The Olifants and Berg river systems, both in the Western Cape area, support the highest numbers of regionally threatened species.

The distribution of regionally threatened species of fishes, molluscs, odonates, plants and crabs based on known and inferred species’ distributions mapped to river catchments.

The Olifants river system: The Olifants river basin located in the Western Cape on the west coast, north of the coastal city of Cape Town, supports 18 threatened species including seven fishes; nine plants and two odonates. The river is approximately 285 km long with a catchment area of 46 220 km2. Agriculture comprised approximately 5% of the catchment land-cover of the Olifants system.

The Berg river system: The Berg river system is on the west coast of the Western Cape, north of Cape Town. The river is approximately 294 km long with a catchment area of 7,715 km2. The Berg river basin has the next highest number of threatened species with a total of 11 species recorded including two fish; two odonates, and seven plants. Approximately 65% of the Berg catchment is under agriculture, mostly temporary commercial dryland agriculture, permanent commercial irrigated agriculture, and commercial forestry.

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 Hydro1 K data layer. The average area of a level 3 basin is approximately 36,000 km2.

Most restricted range species were found in South Africa within the eastern seaboard and Western Cape.

Richness of species restricted to single Hydro1K level 3 sub basins. The map shows the numbers of species restricted to each of the level 3 river basins.