Persistence or disappearance of plants under grazing pressure has led to their categorisation as grazing increasers or decreasers. We aimed to extend this classical indicator concept in rangeland ecology by interpreting the shape of species responses and trait patterns modelled along continuous grazing gradients at different spatial scales.
Taking transects of two different lengths, we recorded the cover of vascular plant species along grazing gradients in central Namibian rangelands. We used a hierarchical set of ecologically meaningful models with increasing complexity – the HOF (Huisman–Olff–Fresco) approach – to investigate species’ grazing responses, diversity parameters and pooled cover values for two traits: growth form and life cycle.
Based on our modelling results, we classified species responses into eight types: no response, monotonic increasers/decreasers, threshold increasers/decreasers, symmetric unimodal responses, left skewed and right skewed unimodal responses.
The most common category was that of no response (42% of the short and 79% of the long transect responses). At both scales, decreaser responses with higher grazing pressure were more frequent than increaser responses. Monotonic and threshold responses were more frequent along the short transects.
Diversity parameters showed a slight but continuous decline towards higher grazing intensities. Responses of growth form and life cycle categories were mostly consistent at both scales. Trees, shrubs, dwarf shrubs, and perennials declined continuously. Woody forbs tended to show a symmetric unimodal distribution along the gradients, while herbaceous forbs and annuals showed skewed unimodal responses towards lower grazing intensities.
The different grazing response types proposed in this study allow for a differentiated picture of niche patterns along grazing gradients and provide a basis to use species as indicators for a continuum of vegetation states altered by livestock impact. The general decline of plant diversity with increasing grazing intensities highlights the importance of reserves that are less impacted by grazing to support the resilience of the studied system.