Marginal habitats of chimpanzees

Abstract

(1) This paper aims to define the marginal habitat of the chimpanzee before the advent of man-made bushfires and agriculture. (The numbers in this abstract refer to the sections of the paper). (2) Written inquiries and personal interviews resulted in a map of the geographical range of the chimpanzee. (3) The rainfall data produced by McGrew et al. (1981) are inadequate because they are based upon short-term measurements during the years of the Sahel drought. On a long-term basis, Mt Assirik probably receives more annual rainfall (1200–1300 or even 1400 mm) than Gombe (∼1100–1300 mm?). The driest chimpanzee sites are Falémé-Bafing in Mali (∼1000 mm) and Ugalla in Tanzania (∼800–900 mm). The annual amount of rainfall is, however, not the decisive factor determining the geographical range. (4) The climatically determined belts of natural vegetation in tropical Africa are described and defined. The ecological limit of the chimpanzee runs obliquely through the Isoberlinia soudanien belt, i.e. from the southern part of the dry soudanien belt in Senegal and Mali to the relatively humid soudano-guinéen belt in the Sudan. This lack of fit between vegetation belts and chimpanzee range cannot be explained by the presence or absence of drinking water in the dry season. (5) Before the advent of agriculture, the chimpanzee's geographical range was covered predominantly by rain forests in the equatorial zone, and by dry forests in the soudanien belt. Man has converted these into mosaic landscapes of savannas, woodlands, secondary forests and relict forests. (6) However, before that time, the social herbivores (from elephants to locusts) had a more or less similar mosaic-forming impact, though to a lesser degree. Statements on ecosystems and their fauna should, therefore, be preceded by and based upon an analysis of the status and dynamics of the vegetation. (7) In mosaic landscapes in Guinea and Senegal, chimpanzee population densities at different sites appear not to be correlated with the local floristic composition in terms of percentages of drought-adapted vs humid flora, but they are correlated with the degree of floristic richness and diversity at these sites. In the dry parts of the chimpanzee range in Tanzania the apes depend on a combination of drought-adapted and humid flora. Apparently their need for a diversified diet determines their ecological limit. The oblique course of the range boundary across the soudanien belt can perhaps be explained by increasing floral impoverishment towards the East, owing to man's activities. The surprising absence of chimpanzees East of the Western Rift system must be attributed to ecological barriers and palaeo-conditions. (8) A confusing phenomenon is that drought-adapted types of arboreal vegetation have, in general, higher powers of regeneration than humid (rain forest) types. (9) The comparison between chimpanzees and Bushmen as made by McGrew et al. is inappropriate because the vegetation of Mt Assirik is degraded whereas that of the Kalahari is natural.