A Generalized Model of the Effects of Grazing by Large Herbivores on Grassland Community Structure

Current disturbance models do not adequately account for the wide range of responses by grassland plant communities to grazing by large generalist herbivores. The evolutionary history of grazing, an important factor in the response of grasslands to grazing, has not been explicitly addressed. Grazing history alone, however, is not a good predictor of plant-herbivore interactions. Interactions occur along gradients of convergent to divergent selection pressures with increasing environmental moisture and of intolerance to tolerance of grazing with increasingly long evolutionary histories of grazing. We suggest that feedback mechanisms between plants and grazing animals are well developed in grasslands with long evolutionary histories of grazing. Feedback mechanisms are manifest in the rapid switching capabilities (of plant species and modes of competition) of subhumid grasslands with long evolutionary histories of grazing and divergent selection pressures. Switching capabilities do not exist in semiarid grasslands with long evolutionary histories of grazing and convergent selection pressures. Rather, for heavily grazed dominant species dominance increases. Feedback mechanisms are not well developed in systems with short evolutionary histories of grazing. In these cases, the differences in response to grazing by semiarid and subhumid situations arise primarily from differences in the grazing tolerance of plants adapted to semiaridity or of plants adapted to competition for light and from the different effects of grazing on canopy structure.