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Ecoregion Description


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Species Richness


# of Endemic Species


Threats

519: Western Equatorial Crater Lakes

Major Habitat Type:

montane freshwaters

Author:

Uli Schliewen, Zoologische Staatssammlung München, Munich, Germany and Emily Peck and Neil Burgess, Conservation Science Program, WWF-US, Washington, DC, USA

Countries:

Cameroon

Boundaries:

Situated in southwestern Cameroon along the “Cameroon Line,” a volcanic ridge running southwest-northeast, this ecoregion’s crater lakes host endemic species flocks of cichlid fishes, as well as endemic insects and shrimps. The ecoregion lies adjacent to the Atlantic Ocean and extends inland along the northwestern side of Cameroon’s interior plateau. 

Drainages flowing into:

Gulf of Guinea, Atlantic Ocean

Main rivers or other water bodies:

Lakes of the ecoregion include Barombi Mbo, Bermin, Dissoni (Soden), Ejagham, Kotto, and Mboandong, among many others. The largest is Lake Barombi Mbo, which is situated at 300 m asl and has an open water area of about 5 km². It has a mean depth of 69 m and a maximum depth of 111 m (Hughes & Hughes 1992). The most recent lava flow into the lake was almost exactly 1 million years ago, dating the lake to be at a minimum that old (Cornen et al. 1992). Just south of Lake Barombi Mbo is Lake Barombi Kotto. It lies 110 m asl and covers about 3 km². Lake Dissoni (Soden), with an area of 3.6 km², lies just north of Lake Barombi Mbo, and Lake Bermin is situated still further north with an area of little more than 0.5 km² (Hughes & Hughes 1992; Stiassny et al. 1992). Lake Ejagham is located in western Cameroon and has a surface area of about 0.5 km² and is about 18 m deep. Unlike the other lakes of this ecoregion, Ejagham is of non-volcanic origin; probably a solution basin produced by groundwater and was most likely formed during the last Ice Age glaciation of these highlands (Schliewen et al. 2001).

Topography:

This area has been continuously producing calderas over the past 25 million years or so (Fitton & Dunlop 1985), and some of these have become filled with water.

Climate:

The Crater Lakes ecoregion lies within the Tropical Humid Soudanian climatic zone (Kling 1987) and experiences a single, well-defined rainy season (Stiassny et al. 1992). Rains fall from May through August, tapering off in September, and the dry season extends from October through April (Stiassny et al. 1992). The mean annual rainfall for the ecoregion is approximately 1570 mm (Hughes & Hughes 1992).

Freshwater habitats:

Approximately 36 crater lakes are now known from Cameroon. Most have no outflows or extremely steep outflows (Kling 1987), rendering them isolated from nearby river systems. Lake Ejagham represents a special case, because it is not a volcanic crater lake but is ecologically and geomorphologically very similar to those that are. The lake’s outlet is isolated from the nearby Munaya River by a waterfall that is insurmountable for cichlid fishes. Its oval-shaped lake basin (approx. 1050 x 700 m diameter) has a maximum depth of 18 m. 

Terrestrial Habitats:

Vegetation in the ecoregion consists of submontane forests between 900 and 1,800 m, and at higher elevations a mixture of montane elements including distinct montane forests and patches of montane grasslands, bamboo forests, and subalpine communities. Five species of tree characterize the forested montane zone: Nuxia congesta, Podocarpus latifolius, Prunus africana, Rapanea melanophloeos, and Syzygium guineense bamendae.These trees become increasingly covered with an epiphytic flora, especially orchids and mosses, at higher altitudes (Letouzey 1985).

Fish Fauna:

The western equatorial crater lakes of Cameroon support a highly endemic aquatic fauna with over 75% endemism in fish. In particular, Lakes Barombi Mbo and Bermin have experienced extensive species radiations of cichlids (Stiassny et al. 1992), resulting in an index of endemic fish per area that is unrivaled on earth. 

Description of endemic fishes:

Lake Barombi Mbo is the most studied of the crater lakes in this ecoregion. At present, 15 fish species have been found in the lake, 12 of which are endemic. Except for the clariid catfish, Clarias maclareni, all endemics are tilapiine cichlid fishes. Four of the five tilapiine genera are endemic: Konia (two species), Stomatepia (three species), Pungu (one species), and Myaka (one species).

Lake Bermin has an endemic radiation of nine tilapiine cichlids, all belonging to the subgenus Coptodon — a taxon that is only distantly related to the Sarotherodon of Barombi Mbo. Except for one species, Tilapia bemini, all other species in this lake were described in 1992 (Thys van den Audenaerde 1972; Stiassny et al. 1992). Although most of the tilapiines feed on detrital material, at least two trophic specialists are currently recognized: T. imbriferna (a phytoplanctivore) and T. spongotroktis (a sponge eater). 

In contrast to Barombi Mbo and Bermin, this lake was colonized both by Coptodon and Sarotherodon. Coptodon gave rise to at least five different species, whereas Sarotherodon split into two species. Except for Tilapia deckerti, all of these species are still undescribed (Thys van den Audenaerde 1968; Schliewen et al. 2001).

Lake Dissoni in the Rumpi Hills harbors one endemic poeciliid (Procatopus lacustris) and probably an endemic Barbus, and an endemic Clarias (Trewavas 1962, 1974; Schliewen 1996b).

Other noteworthy fishes:

The only non-endemic cichlid genus from Lake Barombi Mbo is Sarotherodon (four species) (Trewavas 1962; Trewavas et al. 1972; Schliewen 1996a) Lake Bermin’s non-cichlid ichthyofauna includes a small cyprinid of the Barbus aboinensis group and an aplocheilid of the Fundulopanchax mirabilis group (Stiassny et al. 1992). 

Other noteworthy aquatic biotic elements:

In addition to the endemic fishes, Barombi Mbo harbors at least one endemic sponge (Corvospongilla thysi) and one endemic but undescribed caridinid shrimp (Caridina spec.) (Trewavas et al. 1972; Roth, pers. comm.). The ecoregion also hosts an endemic and species rich amphibian fauna, with about one-third of the nearly 60 species of frogs endemic to the area, primarily the surrounding forests. The invertebrate fauna of Lake Bermin includes various species of freshwater crab, one rotifer, one cladoceran, one copepod, as well as the fish-eating colubrid snake, Afronatrix anoscopus (Stiassny et al. 1992). Additionally, the ecoregion hosts one endemic aquatic mollusc, Bulinus camerunensis.

Evolutionary phenomena:

TheWestern Equatorial Crater Lakes are considered globally distinctive for their higher-level taxonomic endemism (Thieme et al. 2005). Four genera are endemic: Konia, Stomatepia, Pungu, and Myaka.

Justification for delineation:

This ecoregion is distinguished by its crater lakes that host endemic species flocks of cichlid fishes, as well as endemic insects and shrimps. Most of the crater lakes known from this ecoregion have no outflows or extremely steep outflows such that they are isolated from nearby river systems. This isolation has let to evolution of highly endemic aquatic faunas.

Level of taxonomic exploration:

Fair

References/sources:

Cornen, G., Bandet, Y., et al. (1992). "The nature and chronostratigraphy of Quaternary pyroplastic accumulations from Lake Barombi Mbo (West-Cameroon)" Journal of Volcanology and Geothermal Research 61 367-374.

Kling, G. (1987). "Comparative limnology of lakes in Cameroon, West Africa. Master's Thesis" Unpublished Thesis. Duke University.

Letouzey, R. (1985). "Notice de la carte phytogéographique du Cameroun au 1: 500 000: region Afro-montagnarde et etage submontagnard" Toulouse, France: Institute de la carte internationale de la végétation.

Schliewen, U. K. (1996) "Barombi Mbo: Summary of the background knowledge of the natural history of the lake and recommendations for conservation". Germany. Gesellschaft für technische Zusammenarbeit (GTZ).

Schliewen, U. K. (1996) "Ichthyological survey of the Rumpi Hill waters". Germany. Gesellschaft für technische Zusammenarbeit (GTZ).

Schliewen, U. K., Rassmann, K., et al. (2001). "Genetic and ecological divergence of a monophyletic cichlid species pair under fully sympatric conditions in Lake Ejagham, Cameroon" Molecular Ecology 10 1471-1488.

Thys van den Audenaerde, D. F. E. (1972). "Description of a small Tilapia (Pisces, Cichlidae) from West Cameroon" Rev Zool Bot Afr 85 93-98.

Thys van den Audenaerde, D. F. E. (1968). "Addendum to the freshwater fishes of Fernando Po" Rev. Zool. Bot. Afr. LXXVIII(1-2) 123-128.

Trewavas, E. (1962). "Fishes of the crater lakes of the northwestern Cameroons" Bonner Zoologische Beitraege 13 146-190.

Trewavas, E., Green, J., et al. (1972). "Ecological studies on crater lakes in West Cameroon, Fishes of Barombi Mbo" Journal of the Zoological Society of London 167 41-95.

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