Threshold changes in the structure and composition of bush clumps in piospheres in an arid thicket mosaic, South Africa

Abstract

Arid Thicket transformation by domestic meso-herbivores is purported to follow a state-and-transition model. Our study represents a first attempt to verify this hypothesis in Pruim-Spekboomveld, a variation of Arid Thicket. We correlate structural and compositional attributes of thicket bush clumps to distance from artificial watering points in four piosphere treatments that have had a different rangeland management history. We fit linear, exponential and sigmoid regression models to our data and use Akaike's Information Criterion to select the best fitting model. Furthermore, we compare the end regions of our data in the piosphere treatments to each other and to similar data collected in a transformed and untransformed reference site. We use both the pattern and extent of transformation within and between the treatments and the reference sites, to verify the existence of stable states and ecological thresholds. Our results indicate a threshold reduction in species richness, functional type diversity, succulent shrub diversity, succulent shrub cover and palatable shrub cover; and a corresponding threshold increase in perennial alien invasive and ephemeral forb cover; with an increase in herbivore-induced transformation. These findings suggest that our treatments have crossed a degradation threshold and therefore represent an alternative stable state to the untransformed reference site.