Arid vegetation in disequilibrium with livestock grazing: Evidence from long-term exclosures

In recent decades, the conventional equilibrium paradigm for explaining rangeland vegetation dynamics has been challenged. Proponents of an alternative non-equilibrium paradigm argue that in variable rangeland environments, external climatic events are critical to vegetation dynamics and there is little opportunity for plantherbivore interactions to reach equilibrium. Understanding which paradigm more effectively describes an ecosystem has important consequences for management. In particular, some authors have argued that a focus on reducing stocking rates in non-equilibrium systems may be futile, and management should be opportunistic in response to unpredictable rainfall events. We measured herbaceous biomass and plant species richness and abundance at five 14-year exclosures on Innamincka Regional Reserve. Four were situated in the dunefields land system, and one on the Cooper Creek floodplain. We did not detect any significant differences between grazed and ungrazed treatments in total species richness or abundance, life form richness or abundance, or herbaceous biomass. Only one species, Portulaca oleracea, showed differences in abundance between treatments at more than one site, but the direction of these differences was not consistent. These results suggest that the non-equilibrium paradigm more accurately describes vegetation dynamics in the dunefields and floodplains of north-eastern South Australia. It is possible that some species had been lost from the study area prior to the establishment of the exclosures, thereby precluding recovery with protection from grazing; however, a regional analysis of the flora reveals little evidence of this. We argue that the dominance of ephemeral species confers resilience by limiting the development of strong feedbacks between grazing intensity and vegetation dynamics. Current grazing practices seem consistent with the conservation of plant species diversity across the dunefields and floodplains. Future studies should focus on the impacts of cattle grazing on areas of the landscape dominated by palatable perennials, as well as the small number of rare and potentially grazing-sensitive species identified.