Herbivore-induced soil transformation and ecological thresholds in arid Albany Thicket piospheres

Artificial water points in arid rangelands concentrate herbivore activity and can drive localised transformation of vegetation and soil. We developed a conceptual model of the pattern and extent of herbivore-induced transformation of the soil environment beneath bush clumps in an arid variant of Albany Thicket, South Africa. In line with model predictions, we tested for the possible presence of ecological thresholds by examining non-linear changes in the physical and chemical properties of soil along independent piosphere gradients subjected to different periods of rest from herbivory (0, 2, 10 and 50 years). Furthermore, we compared mean soil properties with variation in transformed and untransformed reference sites. Chemical properties, including base cations, nitrogen, phosphorus and pH, declined strongly and non-linearly with distance from water, with the steepest gradients occurring within 100 m of watering points. Many chemical properties near water exceeded the range of variation in the untransformed reference site and, in some cases, approached transformed reference conditions, indicating threshold-like changes in soil fertility and pH. In contrast, physical soil properties showed weaker and less consistent responses. These findings demonstrate persistent, spatially structured soil transformation associated with herbivore concentration around artificial water points.