AN ECOLOGICAL PERSPECTIVE ON HUMAN DENSITIES IN THE CENTRAL AFRICAN FORESTS

1. The factors determining human densities in the central African fore sts are examined and compared with the predictions from an agronomic m odel by the Food and Agriculture Organization, United Nations Fund for Population Activities and International Institute for Applied Systems Analysis (FAO/UNFPA/IIASA). 2. Published data on crop yields, wild an d domestic herbivore densities and human densities are compared along the rainfall gradient and between three broad geological classes repre senting low, medium and high soil nutrient availability. 3. Crop yield s show no increase with rainfall; both manioc and maize may decrease a t higher rainfall levels. The lowest yields are seen on the geological class which underlies the central African forests. 4. Wild herbivore biomass increases steeply with rainfall on volcanics and marine sedime nts and also on sedimentary rocks (high and medium soil nutrient avail ability, respectively). On basement and Kalahari sand (low soil nutrie nt availability), most sites show a decrease in herbivore biomass at h igh rainfall, suggesting that soil nutrients are limiting. 5. There ar e large differences between geological classes in cattle density. With in each class cattle densities increase along the rainfall gradient an d then decline. The lowest densities are in central Africa. 6. Within each geological class human densities increase with rainfall. The stee pest slope is on volcanics and marine sediments. The slopes on the oth er two classes are less steep. The lowest human densities are on basem ent and Kalahari sand. Geology and rainfall together explain three-qua rters of the variance in human densities over much of sub-Saharan Afri ca (33 out of 40 countries). 7. Cultivation of carbohydrate staples by forest-dwellers in north-eastern Gabon could theoretically support 20 -28 people km(-2). Game resources support 0.3 people km(-2). 8. The pr edictions from FAO/UNFPA/IIASA's agronomic model are very different fr om the actual patterns revealed by the analyses of crop yields, wild a nd domestic herbivores and humans in relation to geology and soils. 9. The carrying capacity of the forest for humans is governed by soil nu trients, agricultural pests and the low protein harvest from game. Gam e resources are the limiting factor for humans. 10. Understanding the relationships between soils, rainfall, biological resources and human populations has important implications for development projects, the r eplacement of forests by cattle pastures and the management of refugee s. The illusion of rain forest fertility, propagated by the FAO/UNFPA/ IIASA model, will result in inappropriate land use policies for the fo rest zone.