HOW HERBIVORY AFFECTS GRAZING TOLERANT AND SENSITIVE GRASSES IN A CENTRAL TEXAS GRASSLAND - INTEGRATING PLANT-RESPONSE ACROSS HIERARCHICAL LEVELS

The hypothesis that herbivore selectivity, rather than plant response to defoliation, is the overriding factor in determining community level responses to grazing was tested in a Texas grassland. We monitored defoliation intensity and subsequent regrowth and reproduction of herbaceous dominants Schizachyrium scoparium and Paspallum plicatulum at the individual tiller, population, and community levels of organization. While long-term observations indicate that Schizachyrium declines and Paspalum increases in response to herbivory, individual Schizachyrium tillers exhibited little response to any level of defoliation and were able to fully compensate for lost tissue on a seasonal basis. Conversely, individual Paspalum tillers responded negatively to defoliation throughout much of the growing season, failing to compensate for leaf tissue lost to herbivory. Defoliation intensity was not proportional to availability. Schizachyrium tillers were selected by herbivores in preference to Paspalum tillers regardless of season or stocking rate. Herbivore pressure required to achieve uniform defoliation exceeded levels required to allow grazers to maintain adequate forage intake. New tiller recruitment per unit area declined over a four-year period as herbivore pressure increased for Schizachyrium. In contrast, Paspalum tiller recruitment remained constant over four years. The inability of heavily defoliated Schizachyrium plants to recruit new tillers and the ability of Paspalum to avoid defoliation and recruit new tillers into the population was manifest in measurements of standing crop over the same period. The ratio of Schizachyrium to Paspalum standing crop increased from 1.3 to 1.5 in protected areas. In treatments with extreme herbivore pressure the ratio declined from 1.3 to 0.6. Our hypothesis that herbivore selectivity, rather than defoliation tolerance, is the overriding factor mitigating species change was supported by these experiments and we suggest that predictions of competitive interactions within a lifeform based on individual and species responses to disturbances such as defoliation may not provide a valid assessment of community level responses.