HERBIVORE-INDUCED SPECIES REPLACEMENT IN GRASSLANDS - IS IT DRIVEN BYHERBIVORY TOLERANCE OR AVOIDANCE

Herbivore-induced shifts in species composition have been documented f rom grasslands throughout the world, but the mechanism(s) of species r eplacement remains largely unexplored. An experiment was conducted in a transplant garden, on the campus of Texas A&M University, to test th e hypothesis that greater herbivory tolerance of mid-seral species, ra ther than selective herbivory of a late-seral dominant, is the dominan t mechanism contributing to species replacement in a mesic grassland. Individual plants of a dominant, late-seral, perennial grass, Schizach yrium scoparium, were grown in competition with conspecifics and two o ther perennial grasses (Bothriochloa saccharoides and Stipa leucotrich a) known to increase in relative abundance in response to intensive gr azing by domestic herbivores in the southern true prairie. Uniform def oliation of both S. scoparium and neighbors was imposed to test the re lative expression of herbivory tolerance, while selective herbivory wa s imposed by defoliating S. scoparium plants, but not neighbors. After four successive defoliations, mean annual shoot production and cumula tive shoot production of S. scoparium plants were suppressed to a grea ter extent by conspecific neighbors than by either of the two mid-sera l neighbors. Mean basal area of S. scoparium plants was not significan tly suppressed by either intra- or interspecific neighbors; however, t iller number per plant was suppressed by B. saccharoides, but not by S . leucotricha or conspecific neighbors. Defoliation of S, scoparium pl ants, but not neighbors, negatively impacted the late-seral plants. Se lective defoliation of S. scoparium plants significantly reduced tille r variables of mean mass, leaf blade area, and leaf number, but did no t significantly reduce plant variables including mean basal area, till er number, or annual shoot production. Defoliation of both S. scopariu m plants and neighbors increased annual shoot production, mean basal a rea per plant, mean tiller leaf area, leaf number, tiller mass, stomat al conductance to H2O vapor, and plant xylem pressure potential in com parison with S. scoparium plants grown with comparable, nondefoliated neighbors. An increase in both plant and tiller variables in defoliate d S. scoparium plants grown with uniformly defoliated neighbors establ ishes that replacement of a late-seral dominant is not driven by a gre ater relative expression of herbivory tolerance of mid-seral species.T hese results collectively suggest that the late-seral dominant, S. sco parium, possesses a greater competitive ability and a comparable or gr eater degree of herbivory tolerance than the mid-seral species that co mprise the community. Therefore, the initial hypothesis was rejected. It can be inferred that the alternative mechanism, selective herbivory of the late-seral dominant, is the dominant mechanism contributing to species replacement. Herbivore-induced modifications of competitive i nteractions are most likely to drive species replacement in grasslands characterized by high and consistent resource availability. This may partially explain why condition and trend analysis was developed and i nitially implemented in the true and mixed prairie associations of Nor th America and why it is widely used by rangeland managers in these gr asslands.