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Western Africa: Southern Nigeria, extending into Benin

Though the Nigerian Lowland Forests ecoregion [4] is relatively small and the rates of endemism are quite low it contains five endemic animal species, including an endangered primate, the white-throated guenon (Cercopithecus erythrogaster) as well as chimpanzees (Pan troglodytes) and elephants (Loxodonta africana). This ecoregion is also one of the most densely populated areas in Africa, and already showed high levels of human activity before colonial times (White and Oates 1999). As a result the forest has been fragmented for a long period of time, a process that has accelerated considerably over the last four decades, leaving only a few remaining forest blocks. Though a number of protected areas have been established they are presently too small, and lack adequate protection. Without increased protection efforts, it is unlikely that the viable sections of forest will survive long into the future.

  • Scientific Code
    (AT0123)
  • Ecoregion Category
    Afrotropical
  • Size
    26,000 square miles
  • Status
    Critical/Endangered
  • Habitats

Description
Location and General Description
The Nigerian Lowland Forest ecoregion [4] is confined to a narrow band along the coast in the southwest of Nigeria, from the eastern margin of the Dahomey Gap in Benin (0240E) to the Niger River (0650 E) in the west. The ecoregion extends more than 100 km inland at its widest extent in the Edo State (formerly Bendel State) of Nigeria, but gets narrower to the west, reflecting the former extent of forest. To the south and southwest, the ecoregion is separated from the coast by a band of mangroves and swamp forest. To the north, forest is gradually replaced by the Guinean Forest-Savanna Mosaic [38] ecoregion, which also forms the eastern boundary. In the northeast the Niger River forms a boundary between the Nigerian Lowland Forests [4] and the Cross-Niger Transition Forests [6], with the Niger Delta Swamp Forest ecoregion [5] forming a partial wedge between the two ecoregions.

The southern part of this ecoregion is situated on a sloping, gently undulating coastal plain that lacks striking features of relief, except for occasional inselbergs (steep-sided hills of ancient crystalline rock). The elevation generally increases gradually northwards, producing a plain with an average elevation of 150 m above sea level. Two prominent scarps are found in the southern area of the ecoregion. The first lies just north of the coastal swamps and extends eastwards into Edo State with an average elevation of 160 m. The second rises over 250 m and extends from near Aiyetoro in the west (Ogun State) past Ore into Edo State. The ecoregion's underlying rocks are part of the Pre-Cambrian crystalline basement complex. These are often exposed in the north but further south are covered by Tertiary-aged sediments, including the Benin Sands. Most of this ecoregion's soils are red and red-yellow ferrasols, that are mostly well drained, moderately to strongly leached, and of low to medium humus content (Rosevear 1954, Barbour et al. 1982).

The distribution of vegetation in West Africa is dependent mainly on the climate, which becomes increasingly drier further inland from the coast. Climatic zones, therefore, run roughly parallel to the coast, widening or narrowing as geographical features alter the steepness of the climatic gradient. This climatic zonation has resulted in a vegetation zonation, comprising the the rain forest zone, the mixed deciduous forest zone and the parkland zone. The first two are climax systems, but the parkland zone is probably caused by anthropogenic conversion of forest and is maintained by annual bush fires (Richards 1939). The natural vegetation of the parkland zone would probably be mixed deciduous forest. Typical mean annual rainfall varies from 2000 to 2500 mm in the rain forest zone near the coast to 1500 to 2000 mm in the mixed deciduous forest zone. While the amount of rainfall may vary the distribution of wet and dry months is uniform throughout the ecoregion. The dry season lasts about three months, from December through February, when each month receives less than 50 mm of rain. A number of sizeable rivers drain this ecoregion. From west to east the most important rivers are the Ogun, the Oshun, the Oni with its tributary the Shasha or Omo, the Owenna and the Osse. None of these rivers flow directly to the sea but discharge into narrow lagoons, which run along the entire coast from the Niger Delta to Benin.

In the moister southern parts of the ecoregion, the forest is dominated by members of the Leguminosae family (Brachystegia spp., Cylicodiscus gabunensis, Gossweilerodendron balsamiferum, Piptadeniastrum africanum), and by the Meliaceae family (Entandrophragma spp., Guarea spp., Khaya ivorensis, Lovoa trichilioides) (Richards 1939, Rosevear 1954, Jones 1955, 1956, White 1983, Sayer et al. 1992). Within the southern rainforests, a number of forest types can be recognized, determined by variations in the underlying soils (Rosevear 1954). One forest type occurs on the Benin Sands, a deep soil so well drained that it is difficult to obtain water away from the rivers (found in sections of Ogun, Ondo, Edo and Delta States). Due to the lack of water the forest here is irregular, rich in Khaya and Guarea spp., with a middle story of dense-crowned, wide-spreading trees and a ground flora that is mainly herbaceous and characterized by an abundance of creepers, mostly Acacia ataxacantha and rattan (Calamus deerratus). Another forest type grows mainly on crystalline rock (northern Ondo State); this is less open, also rich in Khaya spp., but mostly lack Guarea spp., and is dominated by two species, Nesogordonia papaverifera and Mansonia altissima, which are absent in the other forests. The most noticeable difference with the forest of the Benin Sands, however, is that the undergrowth is more shrubby than herbaceous and consists largely of Solanum inconstans. A third type of forest is located in the southern part of Ogun State. These forests are comparatively poor, and emergents are widely separated by relatively sparse woodland of small trees and little ground flora. The dominant tree species are Nauclea diderrichii, Khaya ivorensis, Erythrophleum ivorense, Klainedoxa gabunensis, Brachystegia eurycoma and Terminalia superba. Other than Terminalia these trees are often associated with the edge of swamps and this, as well as the soil, indicates that this forest grows in areas that were once fresh water swamp forest, and before that mangrove.

In the drier northern portion of this ecoregion dominant trees belong generally to the Sterculiaceae family (Cola spp., Mansonia altissima, Nesogordonia papaverifera, Pterygota spp., Sterculia spp., Triplochiton scleroxylon), to the Moraceae family (Antiaris africana, Ficus spp., Milicia excelsa), and to the Ulmaceae family (Celtis spp., Holoptelea grandis). The boundaries between these forest communities are gradual, and soil type plays a significant role in determining species composition.

Biodiversity Features
Despite the discrete biogeographical boundaries formed by the Niger River and the Dahomey Gap, levels of endemism in this ecoregion are low, especially when compared to other lowland forest ecoregions in West and Central Africa. Floristically the ecoregion contains few strictly endemic plant species. However, some of the plant assemblages that contain both Upper and Lower Guinea plants are believed to be unique.

Five strictly endemic animal species are present. The white-throated guenon (Cercopithecus erythrogaster, EN) is only found here and contains two endemic sub-species (ssp. erythrogaster and pococki). Ibadan malimbe (Malimbus ibadanensis, EN) occurs in the northermost forest fringes of this ecoregion (in the parkland zone), and has only been observed in the Ibadan area. The Benin genet (Genetta bini) was described by Rosevear (1974) from a single specimen from the Ohusu Game Reserve north of Benin, and has not been found since (Oates and Anadu 1982, Happold 1987). However, a survey of the Niger Delta revealed the presence of the crested genet (Genetta cristata, EN), which also ranges to Cameroon, and this species may encompass the Benin genet according to Powell (1995). The crested genet itself is often regarded as a sub-species of the servaline (Genetta servalina). The endemic Nigeria crag gecko (Cnemaspis petrodroma) and the Petter's toad (Bufo perreti) were collected in the early 1960's during an expedition in the Idanre Forest Reserve, and further efforts to collect reptiles and amphibians will likely result in the discovery of new endemic species.

In addition to the Ibadan malimbe and the white-throated guenon, other threatened animals include the African elephant (Loxodonta africana, EN) and the chimpanzee (Pan troglodytes, EN) (Hilton-Taylor 2000). There is evidence that chimpanzees in Nigeria and western Cameroon are a different subspecies from those further to the west and southeast. Moreover, there may be a significant difference between populations in western and eastern Nigeria – further research urgently needed (Gonder et al. 1997, Gonder 2000). If this split is substantiated, then the subspecies of chimpanzee found in this ecoregion will be critically endangered. If the Benin genet is proven to be part of the same species as the crested genet then it would also be classified as endangered in the IUCN Red List (Hilton-Taylor 2000). However, if the Benin genet is shown to be a separate species, it would be classified as endangered in its own right.

Current Status
Although most demographic data for Nigeria are not very reliable, it is safe to estimate that approximately 20 percent of all Sub-Saharan Africans live in this country with a population of roughly 120 million people. The Nigerian urban population doubled since 1980, and combined with continuing population growth in the rural areas, human pressure on the remaining forest is tremendous.

Archeological evidence indicates that human pressure on the forest of this ecoregion is not a recent phenomenon. A study conducted in Okomu National Park found an extensive layer of charcoal and pottery below the forest, suggesting that present forest regenerated over the last 700 years (Jones 1955, White and Oates 1999). Human pressure on the ecoregion's forest grew after that time, and by the turn of the nineteenth century the British colonial administration established a forestry department to deal with the alarming rate of destruction (Rosevear 1954). When Richards (1939) arrived in this ecoregion to study the vegetation he indicated that the narrow strip of Nigerian Lowland Forest was under such heavy pressure that the remaining forest formed a series of disconnected blocks rather than a continuous belt.

In the early part of the 20th century, most of these patches of forest were declared as forest reserves by the colonial administration. These areas continued with the same status after independence. The forest reserves were established as a measure to prevent uncontrolled forest destruction, and to allow the establishment of plantations. Initially the emphasis was on logging of African mahogany (Khaya spp.). Over time, however, the pressure on forest reserves increased, controlled management measures became less sustainable, and forestry budgets and staff became inadequate. The last comprehensive survey of most remaining forest reserves and protected areas in the ecoregion was conducted in the early 1980s (Oates and Anadu 1982). Nearly all forest reserves visited were farmed, transformed into single-dominant plantations of exotic tree species (rubber (Hevea braziliensis), Gmelina arborea and Tectona grandis), heavily exploited for their remaining timber, and contained ample evidence of hunting.

The only native forests protected in this ecoregion are in a few, mostly small, protected areas, including the Omo Biosphere Reserve, Akure-Ofosu, Ala and Owo Strict Nature Reserve, Orle River Game Reserve, Ifon, Kwale and Gilli-Gilli Game Reserves, and Okumu National Park (WCMC 2000). Omo's Strict Nature Reserve was established in 1949 and approved as a Biosphere Reserve in 1977. Despite international recognition of Omo's importance, the actual protected area is only 46 km2 with the surrounding Forest Reserve functioning as buffer zone. In the early 1980s Okomu was identified as one of the largest and least disturbed Forest Reserve, with the largest surviving population of white-throated guenons (Oates and Anadu 1982). Proposals for a conservation effort resulted in the creation of a 67 km2 wildlife sanctuary in 1985, but before it became effective loggers were allowed to remove many large trees. In 1987 the Nigerian Conservation Foundation (NCF) took over management, and a one-mile wide buffer zone was added, increasing the total area to 114 km2. In May 1999 the NCF handed over management to the Nigeria National Parks Service (NNPS). Though the NNPS apparently increased the protected area to 181 km2, the legal status of Okomu as a park appears to be ambiguous as no boundaries have been listed in a formal Federal Government Gazette notice (Oates, pers. comm.). Ifon was selected by the Ondo State Wildlife Unit as the most suitable site for a Game Reserve since other Forest Reserves suffered heavy logging, conversion to plantations, road building and hunting (Oates and Anadu 1982). Though it is not a prime rain forest site the lack of relatively undisturbed habitat in this ecoregion increases the conservation value of this game park, warranting an investigation into its present status. Gilli-Gilli Game Reserve lies 30 km southwest of Benin City. About two-thirds is lowland forest with the remainder swamp forest. Kwale Game Reserve covers only 3 km2 and was established in 1932.

Despite the small size of these forests blocks, they represent the only sizeable remaining lowland forest in Nigeria, outside the Cross-Sanaga-Bioko Coastal Forests ecoregion [7].

Types and Severity of Threats
Human population density in this ecoregion is high; two decades ago large sections already had population densities between 250 and 400 persons per km2, with the remainder having population densities between 100-200 persons per km2 (Barbour et al. 1982). Given the rapid population growth in Nigeria since this period these densities must have increased dramatically. Farming and logging are the two most important human activities in the region. Agriculture is usually a form of shifting cultivation, mostly the yams-maize-cocoyams-cassava farm system, interspersed with oil palm, cacao, and rubber plantations. Therefore the demand for land is high and as a result the number and size of the few remaining forest blocks is decreasing (Sayer et al. 1992). Other threats are logging and hunting.

The protected areas in this ecoregion have all been created from existing Forest Reserves and have seen varying degrees of human exploitation. Other than incursions by farmers, the major threats are logging and hunting. Given the inadequate number of protected areas their protection and the identification of other suitable forest blocks for some kind of protection is imperative. However, little information is available on the present status of potential areas other than Okomu and Omo. The 1982 survey by Oates and Anadu was the last attempt at a comprehensive effort for possible conservation measures in the area, which due to the long period of time that has passed, is likely to be inaccurate. For Omo core area is largely undisturbed though an old logging road indicates past activities. The remainder of the Forest Reserve suffers from increasing human activity. There is also evidence of hunting and the conversion of large sections of forest to Gmelina arborea plantations. This, combined with a new road from Shagamu to Benin that passes through the Forest Reserve provide the greatest threats, other than inadequate size, to the integrity of the core area. At Okomu, effective protection around 1990 had resulted in a decrease of poaching and other human activities. However, a development program for the surrounding population and its associated new economic opportunities attracted immigrants from large distances. As a result human pressure increased, affecting the sanctuary negatively (Oates 1995). Ifon Game Reserve has no villages or roads, and because it lies on the boundary between this ecoregion and the Guinean Forest-Savanna Mosaic it contains little valuable timber. As a result it was last exploited in 1913 by selective felling. Gili-Gili Game Reserve has been extensively logged and few large trees remain. Extensive hunting and farming along its eastern edge are also problematic (Oates and Anadu 1982). Since 1966 two thirds of Kwale Game Reserve has been given over to farming and tree plantations. In its present form it is unlikely to provide enough protection for the survival of populations of large mammals (Oates and Anadu 1982).

Justification of Ecoregion Delineation
The Nigerian Lowland Forest ecoregion is based on the 'lowland forest – drier type' vegetation unit of White (1983), with slight modifications based on AVHRR imagery (e.g: including the sliver of swamp forest that originally separated the lowland forest area from the mangrove) (Loveland et al. 2000). This forest unit is bordered on the west by the drier habitats of the Dahomey Gap, and to the east by the Niger River Delta. The Nigerian Lowland Forest ecoregion contains much lower rates of plant and animal endemism than other West and Central African lowland forests, but has one important endemic primate, white-throated guenon (Cercopithecus erythrogaster).

References
Barbour, K. M., J. S. Oguntoyinbo, J. O. C. Onyemelukwe, and J. C. Nwafor. 1982. Nigeria in Maps. London: Hodder and Stoughton.

Gonder, K. M., J. F. Oates, T. R. Disotell, M. R. J. Forstner, J. C. Morales, and D. J. Melnick. 1997. A new West African chimpanzee subspecies? Nature 388: 337.

Gonder, K. M. 2000. Evolutionary genetics of chimpanzees (Pan troglodytes) in Nigeria and Cameroon. Ph.D. thesis, City University of New York, New York.

Happold, D. C. D. 1987. The Mammals of Nigeria. New York: Oxford University Press

Hilton-Taylor, C. 2000. 1998. The IUCN 2000 Red List of Threatened Species. IUCN, Gland, Switzerland and Cambridge, United Kingdom.

Jones, E. W. 1955. Ecological studies of the rain forest of southern Nigeria IV. The plateau forest of the Okomu Forest Reserve, Part 1. The environment, the vegetation types of the forest, and the horizontal distribution of species. Journal of Ecology 43: 564-594.

Jones, E. W. 1956. Ecological studies of the rain forest of southern Nigeria IV. The plateau forest of the Okomu Forest Reserve, Part 2. The reproduction and history of the forest. Journal of Ecology 44: 83-117.

Loveland, T.R., B.C. Reed, J.F. Brown, D.O. Ohlen, Z. Zhu, L. Yang, and J.W. Merchant. 2000. Development of a global land cover characteristics database and IGBP DISCover from 1 km AVHRR data. International Journal of Remote Sensing. 21:1303-1330.

Oates, J. F. 1995. The dangers of conservation by rural development – a case-study from the forests of Nigeria. Oryx 29: 115-122.

Oates, J. F., and P. Anadu. 1982. The status of wildlife in Bendel State, Nigeria, with recommendations for its conservation. Report to Bendel State Ministry of Agriculture and Natural Resources, Benin City.

Powell, C. B. 1995. Wildlife Study 1. Report submitted to the Environmental Affairs Department, Shell Petroleum Development Company of Nigeria, Ltd.

Richards, P. W. 1939. Ecological studies on the rain forest of Southern Nigeria. I. The structure and floristic composition of the primary forest. Journal of Ecology 27: 1-61.

Rosevear, D. R. 1954. Vegetation and Forestry. In: The Nigeria Handbook. Lagos, Nigeria: The Government Printer.

Rosevear, D. R. 1974. The Carnivores of West Africa. London: British Museum (Natural History).

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, 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.

WCMC 2000. Protected areas database. http://www.wcmc.org.uk/protected_areas/data/

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.

White, L. J. T., and J. F. Oates. 1999. New data on the history of the plateau forest of Okomu, southern Nigeria: an insight into how human disturbance has shaped the African rain forest. Global Ecology and Biogeography Letters 8: 355-361.

Prepared by: Jan Lodewijk R. Were
Reviewed by: In progress