|Environment, Biodiversity and Agricultural Change in West Africa (UNU, 1997, 141 pages)|
|1: General background|
West Africa lies between the Gulf of Guinea and the southern edges of the Sahara desert. I It is made up largely of an ancient Precambrian land mass, rising from under 50 m in the narrow coastal lowlands to about 600 m in the dissected interior plateau. Soils vary from nutrient-deficient coastal sands, plateau laterites and saline desert soils, through waterlogged coastal swampy soils, dry savanna and desert soils, including potentially rich loess, to fertile alluvium and humic forest soils.
Climatically, the region is characterized by consistently warm temperatures and by a rainfall ranging from under 250 mm to over 3,000 mm per annum. Seasonal rainfall trends, including the unimodal and bimodal patterns, are closely associated with the seasonal migration of the dominant north-easterly dry continental and south-westerly humid maritime air masses. Rainfall and humidity decrease northward, and there is a close correlation with the generally latitudinal alignment of the natural vegetation, which varies from forest to scrub.
The region may be divided into the following major ecological zones: rain or humid forest; forest-savanna ecotone; savanna; sahel; and desert (fig. 1.1).
The rain forest is characterized by high humidity, and a yearly minimum 1,500 mm rainfall which occurs in two distinct seasons. Nutrient leaching is prevalent from the rich humus which is derived from the dense forest of lianas, bush and tall trees such as Milicia excelsa. A profusion of fauna helps to make the rain forest the most biologically diverse zone. The inhabitants subsist upon this rich biodiversity, and on the cultivation of tubers, plantains and tree crops, with livestock featuring only as a minor component. More of West Africa's population is concentrated in the rain forest zone, and in the adjoining swampy mangroves and sandy coconut-growing coastal areas, than in any other. This population feature relates to the forest resources, and to the fishing and export-import opportunities offered by the sea. It makes this zone crucially important to protect and to research for viable land use options.
The forest-savanna ecotone is made up of a mosaic of forest and savanna species, and characterized by a bimodal 1,300-1,500 mm annual rainfall. It is found at both high and low elevations in a zone transitional between rain forest and dry savanna. Reportedly once covered by high forest, the forest-savanna ecotone is increasingly dominated by secondary regrowth, by invasive herbaceous species such as the weed Chromolaena odorata, and by savanna grasses such as Imperata cylindrica; hence the frequent designation of this ecological type as "derived savanna." Farming is primarily for seasonal food crops and vegetables, perennial tree crops, and for livestock. This range of land use relates to the transitional nature of the ecosystem, but also underscores the dynamic changes in the biophysical environment which make it essential that the forces for change and continuing threat to biodiversity and the other environmental resources be researched.
The savanna ecosystem, with its mixture of grasses and short trees, notably Parkia clappertoniana, Butyrospemum and Adonsonia digitata, and an annual 800-1,500 mm unimodal rainfall, supports grain farming and livestock on an extensive plateau north of the forest-savanna zone, and on a smaller plain centred around Accra along the coast of Ghana. In this savanna zone is the major cattle belt of West Africa. The low rainfall and the low humic content of the soils limit the yields of seasonal food crops and vegetables, and render this zone one of the most ecologically sensitive and most prone to deleterious change.
The most extensive ecosystem is the sahel, which extends northwards of the savanna zone on an elevated plain with isolated hills, to the southern fringes of the Sahara desert. It receives 200-500 mm of highly unimodal rainfall, has hardy stunted trees, notably Acacia spp., and short grasses, which form the basis of the essentially livestock economy. Beyond this zone, in the extreme north, lies the true desert.
West Africa remains rich in both domesticated and non-domesticated plants and animals which, besides their economic value, perform vital ecological functions such as pollination, seed dispersal, decomposition and biomass recycling. These rich biological resources, together with the time-tested diversified indigenous systems of managing them, have, however, come under increasing threat (Hence 1964; Mountjoy and Embleton 1967; Pritchard 1979; Okigbo 1993; United Nations Environment Programme 1994).