Cross-Sanaga-Bioko coastal forests
The Cross-Sanaga-Bioko Coastal Forests ecoregion comprises the lowland and coastal forests of southeastern Nigeria, southwestern Cameroon, and the lowlands of the island of Bioko. The flora includes high numbers of endemic genera, indicating a long history of forest cover. These forests contain some of the highest rates of animal species richness of any African forest, especially in terms of forest-restricted mammals, birds, and butterflies. Many of these animals are endemic to this ecoregion or to the contiguous Atlantic Coastal Forests Ecoregion, but others are found more widely across the Congo Basin. The ecoregion is heavily impacted by human use, including logging and plantation agriculture.
Location and General Description
Geographically this ecoregion extends from the left bank of the Cross River in southeastern Nigeria, follows the coast as far south as the Sanaga River in Cameroon, and extends inland up to 300 kilometers (km). It also includes the lowland forests of the Island of Bioko. The forests above 800 meters (m) on Mount Cameroon and Bioko, and above 900 m on other inland Cameroon mountains, are placed in the Mount Cameroon and Bioko Montane Forests and the Cameroon Highlands Forests ecoregions, respectively.
The area is one of low topographic relief at the eastern and western margins, but of increasingly rugged topography in the foothills of the Nigerian-Cameroonian mountains. Mangroves occur in two large and distinct patches on the coasts at the two extremities of the ecoregion, and are included in the Central African Mangroves ecoregion.
This entire ecoregion falls within the humid tropics. In the southwestern foothills of Mt. Cameroon and on southwest Bioko, rainfall can exceed 10,000 millimeters (mm) per year with little seasonal variation. Away from the montane influence however, rainfall averages 3,000 mm per year along the coast falling to around 2,000 mm inland and there can be a short but sometimes severe dry season of two to three months. Humidity is always high, rarely dropping below 90 percent. Temperatures range from a maximum of 27 to 33 °C, to a minimum of 15 to 21 °C, with very minor seasonal differences. As in the Atlantic Equatorial Coastal ecoregion, diurnal temperature ranges often exceed annual variations.
Geologically, the majority of the ecoregion is found on the Precambrian African shield. These basement rocks have been weathered for millions of years and are now overlain by thick layers of heavily leached, red earth soils. Seawards of the shield rocks, silt, and sand deposition has led to formation of beaches and mud banks that can be quite extensive. Mt. Cameroon and Bioko are volcanoes, and in consequence adjacent parts of the ecoregion have soils, rocks and black sand beaches derived from pyroclastic lava and ash. Bioko was most recently separated from the mainland some 12,000 years ago when rising sea levels following the end of the last Ice Age isolated it.
A number of river systems drain from this ecoregion into the Atlantic Ocean. Notable among these are the Cross River, the northern boundary of the ecoregion, and the Sanaga River, its southern boundary. Other important rivers include the Meme, Wouri, Kwa, Ndian, and Nyong Rivers.
According to phytogeographic vegetation classification, this lowland forest ecoregion falls within the Lower Guinea block of the Guineo-Congolian regional center of endemism. The principal vegetation is hygrophilous coastal evergreen rainforest, with mixed moist semi-evergreen rainforest further inland in the drier regions. Forest trees reach up to 50 meters (m) in height, and there are several distinct vegetation layers. The flora here shares affinities with other forest types in the Lower and Upper Guinea blocks of the Guineo-Congolian lowland forest. The most important families (in terms of number of species) are Annonaceae, Leguminosae, Euphorbiaceae, Rubiaceae and Sterculiaceae. In general the forests are mixed, but tend to be dominated by caesalpinoid legumes.
There is exceptional species richness in the rainforests of this ecoregion. Combined with the Atlantic Equatorial Coastal Forests to the south, these two ecoregions support about 50 percent of the 7,000 to 8,000 plants endemic to Tropical West Africa, mainly in the coastal portion of Cameroon. This ecoregion is a center of diversity for the genera Cola (Sterculiaceae), Diospyros (Ebenaceae), Dorstenia (Moraceae), and Garcinia (Guttiferae). There are also many endemic trees in these lowland forests, including: Camplyospermum dusenii, Deinbollia angustifolia, D. saligna, Hymenostegia bakeri, Medusandra richardsiana, and Soyauxia talbotii. Among large trees, the endemic Microberlinia bisulcata is regarded as critically endangered. These biological features, especially the presence of endemic families and genera indicates a long evolutionary past.
In Cameroon's Korup National Park 1,700 species of vascular plants have been recorded, as many as 5 percent of which are narrow endemics. It is estimated that the park may eventually be found to include as many as 3,500 vascular plants. Although floral inventories are incomplete for the Cross River National Park, the lowland forests are similar to Korup, and likely to share the same level of diversity. On Bioko, over 1,000 vascular plant species and 49 site endemics have been recorded in the lowland forests.
The forests of the Cameroon-Nigerian border are also known for harboring the highest forest butterfly species richness in Africa, a finding that may be reflected in other invertebrate groups as well. This area also has very high vertebrate species richness, and contains the highest figures for forest restricted birds and mammals in Africa.
These lowland forests are of particular importance for the conservation of primates, and a number of narrowly distributed species occur, such as the strictly endemic Preuss's red colobus (Procolobus pennanti preussi, EN), and the near-endemic red-eared monkey (Cercopithecus erythrotis, VU), crowned guenon (Cercopithecus pogonias, VU), drill (Mandrillus leucophaeus, EN), pallid needle-clawed galago (Euoticus pallidus spp. pallidus on Bioko, EN), and Pennant's red colobus (Procolobus pennanti pennanti, EN) of Bioko. The Cross River population of lowland gorilla is a critically endangered strictly endemic sub-species, Gorilla gorilla diehli, and this area also supports the endangered chimpanzee subspecies, Pan troglodytes vellerosus. Two small mammals are also considered strictly endemic, the Bibundi bat (Chalinolobus egeria) and the pitch shrew (Crocidura picea, CR). Other near-endemic small mammals include the long-footed shrew (Crocidura crenata) and Eisentraut's mouse shrew (Myosorex eisentrauti, EN).
The protected areas in this ecoregion hold important populations of large mammal species. The forests of the Cross River National Park in Nigeria are a particularly important conservation area, with populations of lowland gorilla and chimpanzee. Korup National Park in Cameroon holds one of the priority populations of forest elephants (Loxodonta africanus cyclotis, EN) in Africa, although this has been much reduced through recent poaching. In some parts of the ecoregion, the areas of forest are sufficiently large and interconnected that the migrations of forest elephant still occur, although severely reduced. While the migration routes may be intact, the elephants are increasingly threatened by hunting for bushmeat and for ivory. The main predator species are leopard (Panthera pardus, VU) and crowned eagles (Stephanoaetus coronatus), both of which appear to be declining due to intense hunting of their prey animals.
No bird species are endemic in this lowland ecoregion, but eighteen bird species are shared only with adjacent ecoregions. Six of these were used to define an Endemic Bird Area in the coastal portion of the Congo Basin forests.
The herpetofauna is highly diverse and contains a some endemic species; Korup alone contains 174 species of reptiles and amphibians. Among the reptiles, the forest chameleon (Chamaeleo camurunensis) and two worm lizards (Cynisca schaeferi and C. gansi) are strictly endemic. The amphibian fauna is exceptionally diverse, but contains few endemics. Strict endemic species include Schneider's banana frog (Afrixalus schneideri), Dizangue reed frog (Hyperolius bopeleti), and Werner's river frog (Phrynobatrachus werneri).
Despite much deforestation, there are still extensive areas of rainforest remaining in this ecoregion, particularly in the border region between Cameroon and Nigeria. Sayer et al present a map of the remaining forest cover from the late 1980s. At that time the main habitat blocks were between Oban and Obubra in southeastern Nigeria, and northeast along the border between Cameroon and Nigeria. An update of this work confirms that the same areas still have the largest remaining patches of forest.
There are also a number of forest blocks around Korup National Park, which is itself one of the largest protected areas in this ecoregion at 1,259 square kilometers (km2). The Cross River National Park in Nigeria (4,000 km2) is made up of the Oban and Okwango divisions, with the Afi River forest reserve nearby. Part of the Gashaka Gumti National Park in Nigeria also contains lowland forest (estimated at 200 km2). Additionally, there are a number of important forest reserves in the ecoregion. The most significant is the Takamanda Forest Reserve in Cameroon, which is contiguous with the Okwangwo division of the Cross River National Park.
A number of spectacular primate species in this ecoregion could make suitable flagships for conservation investment, most notably the isolated population of lowland gorilla and the drill.
Types and Severity of Threats
Over the past 100-200 years, commercial logging and plantation agriculture have been the main causes of deforestation on the mainland, followed by subsistence agriculture, which often occurs after the logging has opened up an area. Lowland forest habitats on Bioko have also been lost through conversion to plantations, and farming activities – except in the southern sector where they are inaccessible due to rugged topography. Despite these losses there is still much more forest remaining than in the adjacent Cross-Niger Transition Forest ecoregion, and other forest ecoregions further west. While the original forest is now highly fragmented in many areas, some large habitat blocks remain, and in the border region between Nigeria and Cameroon these blocks are still well connected.
A major threat to the fauna of the area is the overhunting of larger mammal species for bushmeat. In some areas, this trade is fully commercialized, and supplies protein to major towns. In other areas, certain species such as lowland gorillas are hunted for their religious, magical, and supposed medicinal properties. The wildlife trade is also a cause of species depletion of reptiles. Another threat is the pressure to establish rubber, wood pulp, oil, and palm plantations in the forest zone of Nigeria.
Although more then 10 percent of the ecoregion is officially protected in national parks, in reality, these parks do not adequately protect fauna and flora because of low staffing, inadequate budgets, and lack of political will. Some species of larger mammals in the Korup and Cross River National Parks are severely threatened and populations of elephant, drill, and red colobus have been seriously reduced. The national and international conservation community has not been effective in protecting fauna in this ecoregion, especially the primates and other large forest mammals.
Justification of Ecoregion Delineation
This forest ecoregion is a part of the Guineo-Congolian regional center of endemism. The biogeographical barriers of the Sanaga River in Cameroon and the Cross River in Nigeria define the mainland boundaries of this ecoregion. These rivers are particularly important barriers for primates (e.g., drill (Mandrillus leucophaeus) and red-eared guenon (Cercopithicus eurythrotis)) and amphibians (e.g., Dizangue reed frog (Hyperolius bopeleti). The Bioko lowland forest shares close biological affinities with the adjacent mainland forests because it was connected to the mainland during lower sea levels towards the end of the last Ice Age, and thus is included in this ecoregion.
Additional Information on this Ecoregion
- For a shorter summary of this entry, see the WWF WildWorld profile of this ecoregion.
- To see the species that live in this ecoregion, including images and threat levels, see the WWF Wildfinder description of this ecoregion.
- World Wildlife Fund Homepage
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- Blom, A., A. Kamdem Toham, J. D'Amico, D. O'Hara, R. Abell, and D. Olson in prep. Assessment of biological priorities for conservation in the Guinean-Congolian Forest Region. Washington, DC: World Wildlife Fund
- Bowen-Jones, E., and S. Pendry. 1999. The threat to primates and other mammals from the bushmeat trade in Africa, and how this threat could be diminished. Oryx, 33:233-246.
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- WWF. 2003. Biological Priorities for Conservation in the Guinean-Congolian Forest and Freshwater Region. Proceedings of Workshop held on March 30 - April 2, 2000 in Libreville, Gabon. Kamdem Toham, A., D. Olson, R. Abell, J. D'Amico, N. Burgess, M. Thieme, A. Blom, R. W. Carroll, S. Gartlan, O. Langrand, R. Mikala Mussavu, D. O'Hara, H. Strand, and L. Trowbridge (Editors).
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