Anthropogenic Pressures, Rather Than Plant Vigour, Promote Insect Herbivory Rates on <i>Securidaca longepedunculata</i> Along Elevation in a South African Woodland

Understanding why some plants experience greater herbivory than others is central to predicting population dynamics and ecosystem resilience. We tested the plant vigour hypothesis, the resource concentration hypothesis and the role of anthropogenic disturbance in shaping herbivory of a heavily harvested medicinal plant in South Africa. Plant height and canopy size were positively associated, indicating coordinated growth, yet these traits responded differently to environmental gradients: Larger canopies were more common at higher elevations and closer to human settlements, whereas taller plants occurred at lower elevations and farther from settlements. Root harvesting was negatively size-dependent, with taller individuals harvested less intensively; however, harvesting pressure increased significantly at lower elevations and near settlements, reflecting strong disturbance effects. Insect herbivory increased with harvesting intensity but was unrelated to plant vigour, providing little support for the plant vigour hypothesis in this system. We suggest that anthropogenic pressures may increase herbivory through multiple pathways, including increased plant exposure following disturbance, stress-induced reductions in plant defence and potential disruption of plant-insect dynamics. Cumulatively, our findings show that herbivory risk is structured more strongly by anthropogenic disturbance and spatial environmental gradients than by intrinsic plant vigour, highlighting the interactive roles of ecological and human drivers in shaping plant-insect dynamics.