On non‐equilibrium in arid and semi‐arid grazing systems

Abstract

A critical comment on A. Illius and T. G. O'Connor (1999) On the relevance of non-equilibrium concepts to arid and semi-arid grazing systems. Ecological Applications, 9, 798-813.A debate in ecology rages over the sources and types of dynamic behaviour driving ecological systems. Drylands have become a particular focus of this debate. In these environments extreme and unpredictable variability in rainfall are considered to confer non-equilibrium dynamics by continually disrupting the tight consumer-resource relations otherwise considered to pull a system towards equilibrium. This implies that livestock grazing in drylands, widely thought to cause degradation and ÔdesertificationÕ through bad management practices leading to overstocking, might not be causing irreversible ecological change through over-use of vegetation. An article recently published in Ecological Applications (Illius & O'Connor, 1999), however, argues that variability in arid and semi-arid grazing systems is not the outcome of qualitatively different dynamical behaviour, and that livestock do cause negative change through ÔnormalÕ density-dependent relations. The authors maintain that these operate primarily in key resource areas and during drought periods.We contest these arguments on several grounds: that key terms are poorly applied in ways which suggest inconsistencies in the internal logic of the arguments; that the paper is unjustifiably selective in the choice and interpretation of ÔevidenceÕ on which it builds; and that the authors do not engage critically with the crucial policy implications of the debate, particularly as they relate to pastoral landuse by African herders in areas under communal tenure and management. We do not suggest that degradation never occurs in arid and semi-arid rangelands, but that Illius and O'Connor's analysis of the mechanisms via which this might take place is misleading and at times, we feel, theoretically bizarre. In particular, we suggest that drought periods may be the times when density-dependent mechanisms are least likely to occur and that key resource areas exist because of ungrazeable reserves which effectively cannot be degraded, although subject to heavy grazing. We also attempt to draw out the theoretical implications of attributing change caused by biotic-abiotic effects to biotic-biotic interactions. We contend that non-equilibrium concepts remain crucial for both natural and social science approaches to understanding dryland environments and their multiple, dynamic uses by pastoralists.Illius and O'Connor's critique of non-equilibrium concepts as a means for understanding ecosystem behaviour in African drylands seeks to address a deepening conceptual fault line separating factions of the rangeland science and livestock development community. Epitomizing one side of this divide are understandings of semi-arid and arid environments in terms of relationships between their biotic components, emphasizing the potential for grazing by domestic livestock to perturb the ÔsystemÕ from a knowable and desi...

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