Relating Threshold Changes in Soil Function to Structural Changes in Woody Vegetation Along Herbivore Utilization Gradients with Different Management Histories in an Arid Thicket Mosaic, South Africa

Abstract

The hypothesis that transformation of rangelands by domestic herbivores follows state-and-transition models predicts that vegetation communities will cross ecological thresholds when shifting from one state to another. We test this hypothesis by identifying threshold responses of soil function in Gamka Thicket, a variation of Arid Thicket in South Africa. We relate indices of soil water infiltration, nutrient cycling, and soil retention (reflecting ecosystem function) to distance from artificial watering points in four piosphere treatments differing in rangeland management history. Furthermore, we compare the pattern, extent, and variation in transformation at the end regions of our piosphere treatments to each other and to a transformed and untransformed reference site. The changes to all the indices of ecosystem function support the hypothesis that functional thresholds have been crossed. We propose a conceptual model that suggests that Arid Thicket transformation due to herbivory transcends three vegetation states. We contend that during the process of transformation the crossing of structural thresholds lead to the crossing of functional thresholds that ultimately drive the formation of new vegetation states.