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 down stream, 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 the at the close of the Cretaceous and the sea advanced, permitting the deposition of thick sequences of shales and sandstones which were subsequently exposed (Anon. 1954, Buchanan and Pugh 1955). All soils in the ecoregion are ferrasols (old, deep, strongly leached and highly weathered soils), which can be red, red-yellow or yellow (Barbour et al. 1982).
Mean annual rainfall is somewhat higher than the Nigerian Lowland Forest to the west. Typical mean annual rainfall varies from 2000-2500 mm near the coast to 1500-2000 mm further north. The most northern areas typically receive between 1250 and 1500 mm rain per annum (Barbour et al. 1982). 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 rain forest 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 (Oates 1989) 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 (Schiøtz 1967, Oates 1989), 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) (Sayer et al. 1992). 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 (Rosevear 1954). 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 (White 1983).
Happold (1987) indicated that when the mammal faunas east and west of the River Niger are compared, 34 percent of the 97 rain forest species occur only on one side. More recent surveys in the Niger Delta (Powell 1997, Werre 2000) 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 (White 1983).
Archeological evidence indicates that high population densities are not a recent phenomenon in this ecoregion. A rich archeological record, dating as far back as the ninth century AD, shows that levels of human activity were already considerable at that time (Shaw 1977, Barbour et al. 1982). 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 1940's indicated that even by then large animals were extremely scarce (Marchant 1949). Subsequent surveys by Oates (1989) 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 (Oates et al. 1992). 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 (Oates 1996).
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 (White 1983).
Anon. 1954. Geology, water supply and minerals. The Nigeria Handbook. The Government Printer, Lagos, Nigeria.
Barbour, K. M., J. S. Oguntoyinbo, J. O. C. Onyemelukwe, and J. C. Nwafor. 1982. Nigeria in Maps. Hodder and Stoughton, London.
Buchanan, K. M., and J. C. Pugh. 1955. Land and People in Nigeria. University of London Press, London.
Happold, D. C. D. 1987. The Mammals of Nigeria. Oxford University Press, New York.
Marchant, S. 1949. Aspects of the fauna of Owerri Province. Nigerian Field 14: 47-51.
Oates, J. F. 1996. African Primates: Status Survey and Conservation Action Plan, revised edition. IUCN, Gland, Switzerland.
Oates, J. F. 1989. A survey of primates and other forest wildlife in Anambra, Imo and Rivers States, Nigeria. Report to the National Geographic Society, the Nigerian Conservation Foundation, the Nigerian Federal Department of Forestry, and the Governments of Anambra, Imo and Rivers States
Oates, J. F., P. A. Anadu, E. L. Gadsby, I. Inahoro, and J. L. R. Werre. 1992. Sclater's guenon - a rare Nigerian monkey threatened by deforestation. National Geographic Research and Exploration 8: 476-491.
Powell, C.B. 1997. Discoveries and priorities for mammals in the freshwater forests of the Niger Delta. Oryx 31: 83-85.
Rosevear, D. R. 1954. Vegetation and Forestry. The Nigeria Handbook. The Government Printer, Lagos, Nigeria.
Schiøtz, A. 1967. The treefrogs (Rhacophoridae) of West Africa. Spolia Zoologica Musei Hauniensis 25: 1-346.
Shaw, T. 1977. Unearthing Igbo-Ukwu. Oxford University Press, Idaban.
Sayer, J.A., C.S. Harcourt, and N.M. Collins. 1992. The Conservation Atlas of Tropical Forests: Africa. IUCN and Simon & Schuster, Cambridge.
Stuart, S. N., R .J. Adams, and M. D. Jenkins. 1990. Biodiversity in Sub-Saharan Africa and its islands. Occasional Papers of the IUCN Species Survival Commission No. 6. IUCN Publications Unit, Gland, Switzerland.
Werre, J. L. R. 2000. Ecology and behavior of the Niger Delta red colobus (Procolobus badius epieni). Ph.D. thesis, City University of New York, New York.
White, F. 1983. The vegetation of Africa, a descriptive memoir to accompany the UNESCO/AETFAT/UNSO Vegetation Map of Africa (3 Plates, Northwestern Africa, Northeastern Africa, and Southern Africa, 1:5,000,000). UNESCO, Paris.
Prepared by: Jan Lodewijk R. Were
Reviewed by: In progress