Geographic Patterns

Species distribution ranges have been mapped according to the best available information but are largely extrapolated from patchy records for point localities. Electronic copies of all individual species distribution maps are available.

Species richness

Fishes: The greatest concentrations of fish species are within the African Great Lakes, Malawi, Tanganyika and Victoria. The bulk of species in these lakes is from the family Cichlidae and most are endemic to single lakes. Beyond the Great Lakes, the Rufiji/Ruaha, Pangani, Malagarasi, Shire and Tana River basins also have high species richness and Lakes Albert, Edward, Turkana and Kivu also support a large diversity of fish species, again predominantly cichlids many of which are lake endemics.

Given that the river catchment is now widely accepted as the appropriate management unit for freshwater ecosystems, species distributions were also analysed across river catchments. The Malagarasi and Rusizi River catchments adjacent to Lake Tanganyika are highlighted as holding the greatest numbers of fish species. The Lake Victoria, East African coastal rivers and Lower Shire catchments are also rich in species. Note that this map only shows species richness within river catchments and does not include the many lake restricted species.

Molluscs: The highest concentrations of mollusc species are, as for the fishes, within Lakes Malawi, Tanganyika and Victoria. Of these three lakes, Lake Tanganyika is the richest with a species flock of endemic gastropods (Family: Thiaridae) and a total of 62 mollusc species recorded. It should, however, be noted that an estimated 80 mollusc species have previously been recorded in Lake Tanganyika but confusion regarding taxonomy left a number of these taxa unassessed in this study.

The river catchments of the east coast of Kenya and northern Tanzania and the east coast catchments of Lake Victoria hold the greatest numbers of mollusc species.

Fishes and mollusc species richness in eastern Africa

Odonates: The greatest concentration of odonate species was in the south-western part of Uganda where species densities reached a peak of 59 species within a 0.25 degree grid square (28 x 28 km). This apparent centre of species richness could, however, be a reflection of greater survey effort in Uganda. Elsewhere species densities were relatively evenly distributed throughout the region.

Crabs: Thirty seven freshwater crabs are known from Eastern Africa. Point data for the locations of all species were obtained but, given the limited survey intensity throughout the region, it is thought likely that most species are more widespread than the point localities suggest. Crab distributions were again centred on the three largest lakes, namely, Lakes Malawi, Tanganyika and Victoria, and the Pangani and Tana River basins. The 0.25 degree grid employed for other taxa was not considered an appropriate resolution suggesting a higher level of precision than that obtained. The 2 degree grid employed better reflects the suspected wider distributions for these taxa.

Odonate and crabs species richness in eastern Africa

Threatened species

Fishes: Two-hundred-and-fifty-two of the 901 fish taxa assessed at the global level (mostly endemic to the region) are threatened (28% of the total number of fish taxa assessed), with two species (Aplocheilichthys sp. “Naivasha” and Barbus microbarbis) thought to be extinct. This assessment provides a significantly improved picture for the regional level of threat than that previously obtained from the 100 species assessed for the 2003 IUCN Red List of which 87% were assessed as either threatened or extinct. These earlier assessments focused on the Lake Victoria fish community in an effort to highlight the apparent large-scale decline and loss of cichlid species due to the combined impacts of invasive species, eutrophication and possibly overfishing. Clearly this picture was not representative of the threatened status for fish throughout the region. It should of course be noted that, given the high levels of endemism in many of the Rift Valley lakes, a similar catastrophe could arise if the appropriate conservation measures are not put in place.

The main centres of threatened fish species are within Lake Victoria (for the reasons given above), particularly in the most intensively surveyed south-eastern part of the lake, and Lake Malawi. Many of the Lake Victoria cichlids were previously thought to be extinct but, following additional and more extensive surveys, it appears that a number of these species still exist in small pockets in the lesser-known parts of the main lake and in the smaller satellite lakes (e.g., Bisini, Kanyaboli and Nabugabo). The majority of these species are now assessed as either Critically Endangered (where small subpopulations have now been found), or as Critically Endangered – Possibly Extinct where survey intensity is still considered insufficient to confirm that they are truly extinct. In Lake Malawi there are 117 species assessed as Vulnerable D2 on account of their highly restricted distributions (these species are excluded from the map below as Vulnerable D2 species are not directly threatened by ongoing activities but rather by intrinsic factors). In some cases these species may be restricted to a section of rocky shore of less than a few hundred metres length. Such species are assessed as Vulnerable due to the risk from stochastic events that may possibly eliminate entire populations given their highly restricted ranges.

Most of the restricted range species are within the group of mouthbrooding rocky shore cichlids known locally as mbuna. Nine more widely distributed demersal cichlids are threatened by the commercial trawl fisheries operating in the southern parts of Lake Malawi and, in some cases, in Lake Malombe. Finally, three cyprinid species are threatened by heavy fishing pressure during the annual spawning migrations when nets are set across river mouths as they ascend the rivers to spawn in the headwaters. An additional threat to river-spawning species is sedimentation of the spawning gravels in the river headwaters, a product of the large-scale deforestation occurring throughout Malawi.

The catchment for the Malagarasi River system flowing into the north-eastern side of Lake Tanganyika is shown to hold the greatest number of threatened species where the major recorded threats are loss of habitat due to agricultural encroachment into wetland areas and the eutrophication and sedimentation of the riverine habitat. It is suggested that the current boundaries of the Ramsar site, which encompass the lower Malagarasi-Muyovozi Wetland, might be extended to provide a more comprehensive cover for the catchment.

With the exclusion of Vulnerable D2 species the Malagarasi River catchment and the southern and western drainages to Lake Victoria hold the greatest numbers of threatened species. The Pangani and Lake Kyoga/Victoria Nile catchments also hold high numbers of threatened species most of which are threatened by overfishing and sedimentation. Overfishing is also the main recorded threat to the many fish species in the southern Lake Malawi catchment and the Shire River.

Molluscs: Twenty-five species of mollusc (16% of the total assessed) are globally threatened. These species are mainly found in the north-eastern margins and catchment of Lake Victoria, the shores of Lakes Malawi and Tanganyika, and in the East Coast river drainages near the Kenya-Tanzania border. The main threats identified are increased sedimentation and habitat loss due to deforestation of river catchments, drainage of wetlands, and agricultural encroachment. No particular family is more threatened than any other. Of the five critically endangered species three, Gabiella candida, G. parva, and Incertihydrobia teesdalei are in the family Bithyniidae. G. candida is only known from a small stretch of shoreline at Butiaba in Lake Albert where it is subject to increasing levels of water pollution, and sedimentation. G. parva is only known from south-western Uganda in Lake Bunyoni which is fast developing as a popular tourist destination and where the water quality of the lake is declining due to the associated pollution and sedimentation from increasing agriculture. I. teesdalei is only found in Lake Jilore (Kenya) where it is subject to rapidly increasing siltation of the lake (mainly due to livestock) which is now reported to dry out completely at times. The other two critically endangered species, Bulinus tropicus torensis (at a crater lake near Fort Portal) and Eupera crassa (Dagusi Island), are only found in single locations and have been impacted by a decline in water quality.

Threatened fish and mollusc distributions in eastern Africa

Odonates: Twenty-one species of odonate (7% of the total assessed) are assessed as globally threatened with the greatest concentration of threatened species in the Eastern Arc Mountain Range in Tanzania. The high level of threat to these montane species is thought to be a reflection of the extensive deforestation that has taken place throughout the area, most likely impacting on both the terrestrial and aquatic phases of the species. The other major threat to odonates was identified as the drainage of swamps. Those species inhabiting lakeshore habitats are generally less threatened with the exception of Platycypha pineyi which is assessed as Critically Endangered due to its limited distribution and reliance on diminishing lakeshore forests. The Eastern Arc Mountain Range, identified as one of the Global Hotspots by Conservation International, is the focus of a significant research and conservation initiative through the Critical Ecosystem Protection Fund (CEPF). It is therefore anticipated that new information will become available for species in this area.

Crabs: Twenty species of freshwater crab (54% of the total assessed) are assessed as threatened. More extensive survey is required to establish distribution ranges which are currently limited to point localities. Based on this limited knowledge the greatest numbers of threatened species are known from the area around Lakes Kivu (Rwanda) and Mutanda (south-western Uganda), and the headwaters of the Tana River in, or near to, the Kora National Reserve and Meru National Park in Kenya. The threats to these species remain largely unknown but many are thought to have highly restricted ranges and their scarcity in museum collections has been used to infer low or declining populations.

 Threatened odonata and crab distributions in eastern Africa

Restricted range species

Extent of Occurrence (EOO) is defined as the area contained within the shortest continuous imaginary boundary which can be drawn to encompass all the sites of occurrence of a taxon (IUCN 2001). No thresholds have yet been set for defining restricted range for freshwater taxa, and this is the focus of ongoing work by the IUCN Species Survival Commission. For the purposes of this study a threshold for restricted range of EOO less than 2,000 km2 was used as it was found to select an appropriate proportion of the total species set.

Fishes: The majority (61%) of the 297 restricted range fish species are found in the shoreline habitats in the southern part of Lake Malawi and its islands such as Likoma and Chizumulu. Most of these species are rocky shore mouthbrooding cichlids with very restricted larval and adult dispersal. In the river basin flood plains a number of killifish species (Cyprinodontiformes) are restricted to small temporary pools where they are able to survive the dry season as their fertilised eggs remain viable until the rains return to re-flood the pools.

Molluscs: One-hundred-and-eighteen species of mollusc are thought to be endemic to the region and an additional 38 species might also be endemic but this is yet to be confirmed. Of these, 55 species have recorded distribution ranges of less than 2,000 km2. A number of the lacustrine taxa are restricted to very short stretches of coastline as many of the species-rich groups, such as Lavigeria, brood their young and so have limited dispersal. A large proportion of these restricted range species (19 species) are gastropods (predominantly belonging to the family Thiaridae) found on the shores of Lake Tanganyika.

Restricted range fish and molluscs in eastern africa

Odonates: Twenty-six species are endemic to the region of which 15 species have recorded ranges of less than 2,000 km2. Many of these species are found in Uganda and along the Kenya-Tanzania border.

Crabs: Crab distributions are only known from point data at this time so it was not possible to determine or map restricted range species.

Restricted range odonates in eastern Africa