Cross-Niger transition forests
This West African rainforest ecoregion has lost most of its original forest cover and native flora and fauna. Human population densities everywhere are extremely high, and little or no natural vegetation remains in most areas. Unfortunately the few protected areas (mainly Forest Reserves) in this ecoregion have also been converted to plantations of exotic species with the exception of Stubbs Creek Game Reserve in Akwa Ibom State. Throughout the ecoregion there are a number of communities that protect sacred groves and their associated fauna. Unfortunately this form of protection is also rapidly disappearing with the erosion of village traditions. The only chance for the survival of representative habitat and species assemblages of this ecoregion lies in the continued protection and expansion of the Stubbs Creek Game Reserve, and in efforts to assist communities to maintain their sacred groves.
Location and General Description
The Cross-Niger Transition Forests ecoregion is separated in the east from the Cross-Sanaga-Bioko Coastal Forests by the Cross River. In the west the Niger River forms a boundary with the Nigerian Lowland Forests, and moving downstream, with the Niger Delta Swamp Forests. The ecoregion's northern boundary merges into the Guinean Forest-Savanna Mosaic and in the south into the Central African Mangroves.
Most of the ecoregion's relief is low and undulating with few notable topographic features, but its northern section becomes hilly. Bedrock dating to the Upper Cretaceous underlies the northern part of the ecoregion, while to the south the deposits are from the Tertiary. The Tertiary deposition started when southern Nigeria subsided at the close of the Cretaceous and the sea advanced, permitting the deposition of thick sequences of shales and sandstones which were subsequently exposed. All soils in the ecoregion are ferrasols (old, deep, strongly leached, and highly weathered soils), which can be red, red-yellow or yellow.
Mean annual rainfall is somewhat higher than the Nigerian Lowland Forest to the west. Typical mean annual rainfall varies from 2,000-2,500 millimeters (mm) near the coast to 1,500-2,000 mm further north. The most northern areas typically receive between 1,250 and 1,500 mm rain per year. Though there is variation in the amount of rainfall in this ecoregion, the seasonal distribution of wet and dry months is uniform. A dry season lasts about three months, from December through February. Other than the Niger and Cross Rivers only two large rivers, the Imo and the Kwa Ibo, drain this ecoregion.
As in the rest of West Africa the distribution of vegetation depends mostly on the change in climate, which becomes drier with increasing distance from the coast. As with the Nigerian Lowland Forests ecoregion, the rainforest zone, the mixed deciduous forest zone, and the parkland zone cross this ecoregion. The ecoregion's high population densities long predate colonial times, hence little of the original vegetation remains and there is little data available on the natural forests of this ecoregion. It is, however, clear that the ecoregion is unique because it harbors species typical of the Upper Guinea Forest Region to the west and the Cross-Sanaga-Bioko Coastal Forests to the east, and is therefore transitional between Upper Guinea and Lower Guinea forest types.
The drier northern sections of this ecoregion were probably dominated by the same tree species as those in the Nigerian Lowland Forests; Sterculiaceae (Cola spp., Mansonia altissima, Nesogordonia papaverifera, Pterygota spp., Sterculia spp., Triplochiton scleroxylon), Moraceae (Antiaris africana, Ficus spp., Milicia excelsa) and Ulmaceae (Celtis spp., Holoptelea grandis). With precipitation increase toward the south, the forest would have become dominated by members of the Leguminosae (Brachystegia spp., Cylicodiscus gabunensis, Gossweilerodendron balsamiferum, Piptadeniastrum africanum) and the Meliaceae (Entandrophragma spp., Guarea spp., Khaya ivorensis, Lovoa trichilioides). An old description of the forest in the Mamu Forest Reserve, located on the northern edge of this ecoregion, detailed the tree species found there. Emergents, though infrequent, were mostly Ceiba pentandra and Milicia excelsa, while most of the canopy was dominated by Cola gigantea, Lannea welwitschii, Ricinodendron heudelotii and Terminalia superba, and the middle story mostly by oil palms (Elaeis guineensis). In drier sections Trilepisium madagascariense and Albizia zygia were more common. In the swamp forest along the Mamu River Hallea ledermannii, Symphonia globulifera, and Raphia palms (Raphia spp.) were common.
Today, cultivation and fire has destroyed much of the rainforest. Like other heavily used forest zones in Africa, the landscape has been converted into a mosaic of farmland, degraded remnant forest patches, tall grasses, and secondary thicket/forest. Regular burning favors grasses and fire-hardy, gnarled trees. Common grasses include Andropogon gayanus, A. schirensis and A. tectorum, with Annona senegalensis, Afzelia africana and Borassus aethiopum being some of the more common trees.
Happold indicated that when the mammal faunas east and west of the River Niger are compared, 34 percent of the 97 rainforest species occur only on one side. More recent surveys in the Niger Delta have indicated that the true number is a bit lower, but the Niger River remains a formidable zoogeographic barrier. The Cross River, which forms this ecoregion's eastern boundary is much smaller than the Niger River and less of a barrier. The absence in this ecoregion of a number of species found in the Cross-Sanaga-Bioko Coastal Forest ecoregion may, therefore, be more an artifact of the high level of deforestation in this ecoregion than a genuine biogeographical feature.
The Cross-Niger Transition Forest has extremely low rates of endemism for a tropical forest ecoregion. There are only two near-endemic species, Sclater's guenon (Cercopithecus sclateri, EN) and the crested chameleon (Chamaeleo cristatus). There are no near-endemic amphibians. However, the near-endemic bird, Anambra waxbill (Estrilda poliopareia, VU), is considered to be typical of Niger-Cross region. Although data are harder to find for the plants, it also seems to be true that there are very few species of restricted distribution found here and strict endemics are either absent, or extremely few in number.
Archaeological evidence indicates that high population densities are not a recent phenomenon in this ecoregion. A rich archaeological record, dating as far back as the ninth century AD, shows that levels of human activity were already considerable at that time. During colonial times the levels of deforestation had already progressed to such a degree that logging companies did not take much of an interest in this ecoregion and forestry departments did little to establish forest reserves.
Wildlife has been heavily depleted in this ecoregion, to the extent that bats and frogs, animals that are generally avoided in Africa, have become part of the local diet. The few remaining species of mammals that thrive in farmland are sold for exorbitant prices: a greater cane-rat (Thryonomys swinderianus) sells for more than the average Nigerian earns in two weeks. The remaining native animal populations are restricted to narrow bands of riverine forest, but hunting pressure here is intense and these gallery forests are even more threatened by loggers.
No significant sections of forest remain in this ecoregion, although there are a number of forest reserves: Anambra (194 km2), Mamu River (70 km2), Osomari (115 km2), Akpaka (296 km2), and Stubbs Creek (c. 80 km2). These have mostly been converted in plantations of exotic species. Stubbs Creek forest reserve contains one of the few remaining larger forest blocks where approximately 80 km2 is under conservation management. Sclater's guenon occurs here, and this may well be the only remaining forest block of some size in this ecoregion that has a chance of being preserved. The other intact forest patches are mainly in traditionally protected sacred groves. Unfortunately traditional beliefs are eroding fast, meaning it may not be feasible to pursue the protection of this ecoregion's flora and fauna through the conservation of sacred groves. Another conservation opportunity would be to protect the remaining strips of riverine forest. Unfortunately this is also difficult, not only because of the strips' physical shape, but also because they run through many different communities, thus making the coordination of any conservation effort extremely difficult.
Types and Severity of Threats
This ecoregion has a long history of high-density human settlement. Conversion to agriculture and depletion of the native fauna for bushmeat is long-standing and presents a severe threat to this ecoregion. Anthropogenic fires have also altered and destroyed native vegetation.
It is believed that the high level of deforestation is one of the reasons why a number of threatened large mammals no longer occur. Reports from the 1940s indicated that even by then large animals were extremely scarce. Subsequent surveys by Oates supported these findings confirming that larger mammals were extremely rare, and that hunting pressure was intense. Sclater's guenon, though endangered, still occurs in the ecoregion mostly due to the presence of sacred forest groves. In a number of communities small sections of forest are protected for traditional reasons, and the community protects the populations of Sclater's guenon associated with these groves. Although these monkeys are also found in a few non-protected locations it is unlikely that they will survive over the long term, with the possible exception of the Stubbs Creek forest reserve. The Nigerian Conservation Foundation has initiated a conservation project with the Akwa Ibom State Government at this site. Continued protection of Sclater's guenon in the villages where the monkey is sacred may also decline as village traditions are beginning to lapse.
Justification of Ecoregion Delineation
Located between the Cross River and the Niger River, this ecoregion contains remnant forests with low species richness and endemism relative to adjacent ecoregions. The biota is transitional between the Upper Guinean and Lower Guinean/Central Congolian forest blocks.
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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