The theoretical underpinnings of range management, predicated upon the concept of Clementsian succession and an equilibrium theory of ecosystem functioning, have been challenged for arid and semi-arid environments in which rainfall variability appears to be a potent determinant of system change (Westoby, Walker & Noy-Meir 1989; Behnke & Scoones 1993). Traditionally, stocking rate has been considered the tool whereby range managers can adjust the successional trend of vegetation. It is now argued that grazing has minimal or no influence on the vegetation dynamics of arid environments because of pronounced interannual rainfall variability, such that these grazing systems are considered to be non-equilibrial (Ellis & Swift 1988). The notion that grazing has a minimal impact on vegetation has been extended to the pastoral systems of the semi-arid savannas of Africa (Behnke & Scoones 1993). Certainly, rainfall variability can exert a strong influence on production (Deshmukh 1984; Le Houerou, Bingham & Skerbek 1988) and species composition (O’Connor 1985), although the slight effect of grazing on species composition in one year can accrue over time to manifest as a striking impact (Milchunas & Lauenroth 1993; O’Connor & Roux 1995). Although large ungulate biomass is positively correlated with mean annual rainfall for the savannas of Africa (Fritz & Duncan 1994), implying a relationship between animal biomass and food supply, it is not a logical sequitur that interannual variation in animal biomass in a local area is regulated by density dependence, but may rather be a direct response to rainfall variability (Ellis & Swift 1988). Alternatively, drought episodes may potentiate herbivory and result in tight plant–herbivore coupling during these episodes (Illius & O’Connor 1999).
Degradation is an unambiguous indicator of whether grazing adversely affects system functioning, when degradation is defined as ‘an effectively permanent decline in the rate at which land yields livestock products under a given system of management. This definition excludes reversible vegetation changes even if these lead to temporary declines in secondary productivity. It includes effectively irreversible changes in both soils and vegetation’ (Abel & Blaikie 1989). This definition precludes reversible changes in soil resources that would impact both primary and secondary production (Abel 1993a, b; Biot 1993). This definition is demanding of research because long-term data sets are required for the detection of an irreversible decrease in primary or secondary production, although Wilson & Macleod (1991) have identified that departure from a linear form of the relation between animal production and stocking rate can constitute evidence of degradation. Even the occurrence of erosion at a point in the landscape need not be evidence of degradation because eroded material may become redistributed within the landscape (the concept of erosion cell mosaic; Pickup 1985) such that, although primary production of the eroded area is decreased, there may be no decrease of production over the landscape because of a corresponding increase in productivity in the areas receiving eroded material (Scoones 1992). Empirical support of this claim is, however, lacking (Illius & O’Connor 1999).
The non-equilibrium school has further challenged the reliance of the traditional school on botanical composition as an indicator of range condition for semi-arid African savanna (Abel 1993b) because, in some cases, transformation of a ‘climax or subclimax’ grassland (normally assumed to support the greatest animal production) to early seral stages may result in increased animal production (Harrington & Pratchett 1974), and because it is difficult to extricate the effects of animals on vegetation from those of rainfall (Abel 1993b). Compositional change in response to grazing is far more widely studied in African savannas than production or yield (O’Connor 1985), such that a key question is whether changes in composition are of any significance for primary production.
Illius & O’Connor (1999) have disagreed about the non-equilibrium behaviour of semi-arid African savannas based on theoretical and empirical reviews of density dependence in animal populations, and of the effects of grazing on species composition, primary production, water balance within the system, erosion and animal performance. Grazing management of semi-arid African savannas therefore awaits a resolution of the theoretical conflict between equilibrium and non-equilibrium schools of thought, or at least clarification of the controls of vegetation and animal dynamics for systems across a rainfall gradient.
This study was based on manipulation of stocking rate for cattle, and examined changes in vegetation composition and production, and animal production, over a period of 10 years, including extremes of rainfall. The design of the study allowed us to compare the efficacy of equilibrium and non-equilibrium concepts for this semi-arid savanna (we examine only one position along the rainfall gradient) and to explore the concept of degradation. Specifically, the aim of the study was to account for the relative contribution of (i) rainfall; (ii) stocking rate; (iii) dependence of stocking rate on rainfall; and (iv) botanical composition on changes in (a) botanical composition, (b) herbaceous biomass and (c) animal performance.