Effects of large ungulate herbivory on tallgrass prairie plant population and community structur

A long-term study was initiated in 1992 to evaluate the effects of grazing systems and cattle stocking densities on tallgrass prairie plant population and community composition, species abundances, diversity, and stability. Treatments include sites grazed seasonally (SLS; May 15 to October 15) at three stocking intensities (3.8, 2.8 and 1.8 ha/AU), ungrazed (control) sites, and pastures under a late-season rest rotation (LSRR) grazing system at this same range of stocking intensities. Three populations of forbs (Aster ericoides, Ruellia humilis, and Amorpha canescens) were sampled in the three SLS and ungrazed treatments to determine population-level responses to increasing grazing intensity. Plant communities were sampled on the major range sites twice each season using a modified step-point procedure and a nested random sampling design. Forb populations generally decreased in growth and reproduction in response to various levels of herbivory, although the plant-level growth and reproduction responses measured in this study do not explain the grazing-induced changes in population abundances within these tallgrass prairie communities.;Results from the first four study seasons of the plant community show plant species richness increases with increasing grazing intensity and is positively related to stability of plant species abundances. Species diversity, evenness, and growth form diversity increase monotonically with increasing stocking density under SLS relative to ungrazed, and low and moderate LSRR treatments. Differences between grazing systems disappear at the high stocking density. Species diversity was positively correlated with stability of plant species abundances, however both diversity and richness were negatively related to compositional stability. The maintenance of high native plant species richness and diversity across varying ungulate densities in this study indicates high stability of tallgrass prairie plant communities under a range of grazing pressures. These results suggest that the diversity of the tallgrass prairie plant community responds differentially to various temporal-spatial patterns of herbivory associated with different grazing systems under low and moderate stocking densities, with high stocking density overriding effects of grazing system. These patterns of plant diversity produced in response to various grazing regimes may in turn influence the long-term stability of plant species composition and productivity.