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

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

published in Journal of Biogeography 29(12) (2002): 1595-1618. (with Rick Rohde).

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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 ‘desertification’ 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, On the relevance of non-equilibrium concepts to arid and semi-arid grazing systems, Ecological Applications, 9, 798–813), 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 ‘normal’ 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 ‘evidence’ 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 ‘system’ from a knowable and desirable ‘climax’ community at equilibrium. On the other, is an emerging idea of such environments as continually driven, or ‘disturbed’, by abiotic factors, primarily rainfall, such that systemic effects of herbivory are relatively unimportant. The two approaches have been termed ‘equilibrium’ and ‘non-equilibrium’,  respectively. The debate is significant for ecological theory because viewing arid and semi-arid environments through these two lenses can influence what, and how, questions are asked about these environments, consequently affecting interpretations drawn from findings of ecological studies. However, its importance is greater than this. Equilibrium thinking in rangeland science has fostered a pervasive and self-referential narrative which holds that degradation and ‘desertification’ are endemic in drylands, particularly those utilized by African livestock herders under communal forms of land tenure. This degradation narrative carries critical implications for the institution of democratic policy and planning in these areas.

The term non-equilibrium was coined by ecologist Wiens in 1984 to describe the dynamics of arid and semi-arid ecosystems. As Illius & O’Connor (1999, p. 799) paraphrase, he argued that ‘…all ecological systems fall somewhere on a continuum from equilibrial to nonequilibrial … the latter … showing weak biotic coupling, independence of species, abiotic limitation rather than resource limitation, density independence and large stochastic effects’. Illius and O’Connor insist that
the validity of non-equilibrium theory depends on showing that non-equilibrium environments are qualitatively different from equilibrial environments governed by density-dependent inter- and intraspecific interactions (Illius & O’Connor, 1999, p. 800). While admitting the failings of rangeland management models predicated on equilibrial concepts of carrying capacity and Clementsian succession, the crux of their position is that a new paradigm for pastoral development in Africa
‘…cannot be predicated on the notion that grazing has a negligible impact in semi-arid systems’ (Illius & O’Connor, 1999, p. 799).

The consequent conclusion drawn by Illius and O’Connor is that non-equilibrium dynamics are less relevant, for both rangeland ecology and policy formulation, than argued in many recent studies. Illius and O’Connor’s primary contention is that ‘…despite the apparent lack of equilibrium, animal numbers are regulated in a density-dependent manner by the limited forage available in ‘‘key resource’’ areas, which are utilized in the dry season’ (1999, p. 809). In other words, ‘…strong equilibrial forces exist over a limited part of the system, with the animal population being virtually uncoupled from resources elsewhere in the system’, such that ‘…spatially and temporally, the whole system is heterogenous in the strength of the forces tending to equilibrium’ (Illius & O’Connor, 1999, p. 809).

We acknowledge that present understandings of non-equilibrium environments are as much conceptual as empirical, and argue that further site-specific studies across a variety of dryland areas should be understood within their own contexts in order that the theory can be applied critically by decision-makers in the interests of social, economic and environmental ‘sustainability’. What follows is our attempt to promote a critical debate by analysing three crucial components of their paper: namely, a review of their interpretation of key terms; a reconsideration of the material they present as ‘evidence’; and an engagement with the policy implications of their arguments.

Since Illius and O’Connor’s paper was published in 1999, it has been cited as providing a serious challenge to a non-equilibrium view of arid and semi-arid rangelands . We thereby offer our comments here in the spirit of intellectual engagement and discussion regarding issues critical for both the discipline and ‘real world’ of ecology, and for the livelihoods of those who utilize dryland environments. …

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