Models of semiarid vegetation dynamics incorporating concepts of equilibrium and stability have been largely replaced by nonequilibrium models in recent years. However, neither equilibrium nor disequilibrium and neither stability nor instability have been formally demonstrated. Most records of semiarid vegetation are inadequate for conclusive tests. We tested for equilibrium at seven study sites in six locations and vegetation types, in grazed semiarid grassland in central Otago, New Zealand, using data collected twice annually over 5––13 yr. Some sites showed overall directional change in composition over time, but others appeared to fluctuate about an average equilibrium state. At one site, an instantaneous perturbation experiment was used to test formally for stability, and to seek evidence for intrinsic stabilizing processes. In this experiment, three herbicide treatments removed or decreased different components of the community, and recovery and convergence with the untreated control was monitored over time. Although all exotic species increased or rapidly re-invaded the treated plots, many native species failed to re-invade, and treatments had not recovered by 194 wk after perturbation. We conclude that the vegetation is not stable, but that some stabilizing processes are evident. Contrary to some nonequilibrium theory, we observed strong competitive interactions between species. We suggest that New Zealand semiarid vegetation exists on a continuum between equilibrium and nonequilibrium dynamics.
Ecology – Ecological Society of America
Published: Mar 1, 2002
Keywords: competitive interactions ; equilibrium ; fluctuation ; nonequilibrium ; recovery from perturbation ; selective species removal ; semiarid grassland ; New Zealand ; stability ; theoretical model
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