Geographic Patterns

Centres of species richness

When the species distributions from the four animal species groups assessed through this project (e.g. fishes, odonates, molluscs, and crabs – plants were excluded as they are mostly mapped to country boundaries) are overlaid, areas with the highest numbers of species are identified. The Figure below shows that the Niger Delta and the Ebrie Lagoon (Côte D’Ivoire) contain the greatest number of species in the region (between 268 and 338 species per grid cell). Other areas of high species richness (222–267 species) include the coastal basins of the Upper Guinea ecoregion (Sierra Leone and Liberia), the lower courses of rivers in Côte D’Ivoire (Eburneo ecoregion) and western Ghana (Ashanti ecoregion), lower Volta and Oueme rivers, the Ogun drainage in western Nigeria, the lower Niger river between the delta and the confluence with the Benue river and the upper Cross river and Bamenda Highlands in south-west Cameroon. Species richness then declines northwards with the exception being the upper Niger river and inland Niger Delta.

Western Africa species richness for all four animal taxonomic groups.

Western Africa species richness for all four animal taxonomic groups. Species richness = species per hexagonal grid cell (289 km2).

This mapping presentation is, however, biased towards highlighting areas rich in the most speciose groups such as the fishes and odonates. In order to therefore highlight areas rich in representation from all taxonomic groups the distribution maps for fishes, molluscs and odonates were overlaid. Crabs were not included in the analysis due to the relatively low number of species (35) found in the western Africa region, and plants were excluded as they were mostly mapped to country boundaries rather than sub-basins. 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 taxonomic groups. Five areas have been identified as meeting this threshold for all three species groups. Moving from west to east the areas are:

i) the southern coastal area of Guinea

ii) the lower river Jong in Sierra Leone

iii) Ebrié Lagoon in Côte d’Ivoire

iv) lower Ogun and Oueme rivers and their coastal lagoons in Benin, and

v) western Nigeria and the Niger Delta to the lower Cross river in southern Nigeria.

Areas containing exceptionally high numbers of species from all taxonomic groups.

Areas containing exceptionally high numbers of species from all taxonomic groups. The map represents those hexagonal grid cells that hold at least 20% of the total species complement for each of the fishes, molluscs and odonates. The sub-basins that contain the grid cells with >20% of total species complement for all three taxonomic groups are highlighted.

Distribution of threatened species

The upper Cross river (west Cameroon highlands) and the Niger Delta contain the highest number of regionally threatened fishes, odonates, molluscs and crabs (20–29 species per grid cell) in western Africa (see below). Other areas that contain relatively high numbers of threatened species (13 – 19 species) are the wider Niger Delta and the lower Cross river, the lower Oshun and Ogun basins (western Nigeria), Pra and Tano (Ghana), upper Cavally (Liberia/Côte d’Ivoire), St. Paul basin in Liberia, the Moa, Jong and Rockel rivers in Sierra Leone and the upper Konkoure and upper Baffing in Guinea.

Threatened fishes, molluscs, odonates and crabs in the western Africa region.

Threatened fishes, molluscs, odonates and crabs in the western Africa region. Species richness = species per hexagonal grid cell (289 km2).

Distribution of restricted range species

Species with restricted ranges were defined as those regionally endemic species restricted to any level 3 river sub-basin as defined in the Hydro1K data layer. Most restricted range species are found in the Upper Guinea region of western Africa. In particular, 74 species are only known from the sub-basin that covers Liberia and most of Sierra Leone, and 19 species are endemic to the sub-basin covering western Guinea to south Senegal (see below). It is important to note that the sizes of the level 3 sub-basins vary quite drastically, from 200,000 km2 to less than 1,000 km2 (average, approximately 31,500 km2), with some representing sub-basins within a catchment while others include multiple river basins.

Number of species restricted to single Hydro1K level 3 sub-basins.

Number 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.

However, similar patterns are evident below, which shows the richness of species endemic to the western Africa region. It shows that the upper Guinea region and the basins in western Ghana and eastern Côte d’Ivoire, with between 66 and 75 species per grid cell, contain the highest levels of regionally endemic species.

Western African endemic fishes, molluscs, odonates and crabs in the western Africa region

Western African endemic fishes, molluscs, odonates and crabs in the western Africa region. Species richness = species per hexagonal grid cell (289 km2).