An ecological perspective on human densities in the central African forests

1. The factors determining human densities in the central African forests are examined and compared with the predictions from an agronomic model 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 and domestic herbivore densities and human densities are compared along the rainfall gradient and between three broad geological classes representing low, medium and high soil nutrient availability. 3. Crop yields show no increase with rainfall; both manioc and maize may decrease at 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 sediments and also on sedimentary rocks (high and medium soil nutrient availability, respectively). On basement and Kalahari sand (low soil nutrient availability), most sites show a decrease in herbivore biomass at high rainfall, suggesting that soil nutrients are limiting. 5. There are large differences between geological classes in cattle density. Within each class cattle densities increase along the rainfall gradient and then decline. The lowest densities are in central Africa. 6. Within each geological class human densities increase with rainfall. The steepest slope is on volcanics and marine sediments. The slopes on the other two classes are less steep. The lowest human densities are on basement and Kalahari sand. Geology and rainfall together explain three-quarters of the variance in human densities over much of sub-Saharan Africa (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 predictions from FAO/UNFPA/IIASA's agronomic model are very different from the actual patterns revealed by the analyses of crop yields, wild and domestic herbivores and humans in relation to geology and soils. 9. The carrying capacity of the forest for humans is governed by soil nutrients, agricultural pests and the low protein harvest from game. Game 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 replacement of forests by cattle pastures and the management of refugees. The illusion of rain forest fertility, propagated by the FAO/UNFPA/IIASA model, will result in inappropriate land use policies for the forest zone.