Relevance of Key Resource Areas for Large-scale Movements of Livestock

Semi-arid rangelands show much spatial heterogeneity, with some parts producing more and better quality food for herbivores. The concepts of ‘Key Resource’ and ‘Key Resource Area’ have been developed to describe a resource that ‘provides good-quality forage’ and that ‘reduces (inter-)annual variation in forage supply’. Illius and O’Connor (1999) formalised these concepts, arguing that in key resource areas herbivores experience a density-dependency relation with food resources, generally during the dry season. In other areas, generally during the wet season, non-equilibrium conditions govern the relation between herbivores and their food resources. They further argued that it is implicit that key resources show lower inter-annual variability than occurs on the (alternative) dry-season range, buffering livestock densities from climatic conditions. Key resource and outlying areas must further operate in a source–sink manner. In this chapter, we discuss the various assumptions and conclusions regarding key resources and key resource areas, using the floodplains of the Sahel, especially those of Waza-Logone in Cameroon, as examples. Sahelian floodplain grasslands are intensively exploited during the dry season, with cattle densities on a year-round basis about five times as high as in surrounding drylands. We come to the conclusion that the inter-annual variability in the quantity of the forage production of the Sahelian floodplains is not less, but often greater than that of surrounding areas. Forage quality, however, may be more constant. The model of Illius and O’Connor would be more realistic if it included intra-annual variability in forage availability, variability in accessibility of that forage, and associated differences therein between the dry-season range and the wet-season range. The importance of a resource varies from year to year, depending among other things on inter-annual variability in rainfall in the wet-season grazing range and in (the catchment upstream of) the dry-season grazing range. When it is of great importance, it may be considered a ‘key resource’, but in another context the same resource is not necessarily a key resource. Because of this spatial and temporal variability in rainfall and forage availability, there is no unequivocal source–sink relationship between the Sahelian floodplains and the associated wet-season grazing ranges. Forage in a key resource area does not necessarily provide the only key resource in the grazing system. Water, for instance, can be important as well. We end by discussing what our findings mean for the key resource area concept of Illius and O’Connor, and by presenting a new definition of key resource area which is also relevant to other trophic systems.