Cattle grazing effects on California coastal prairie and associated annual forbs

California's coastal prairie evolved with a variety of disturbances, including ungulate grazing and fire. In the last 300 years, cattle grazing has become a dominant component of disturbance regimes, and exotic species have increasingly dominated these grasslands. In the last 20 years, there has been increasing concern about cattle grazing impacts on biological diversity but little research about grazing impacts on coastal prairie and the associated native annual forbs, many of which are rare or endangered. In order to inform this issue, I conducted complementary large-scale surveys and manipulative experiments of coastal prairie vegetation.; First, I surveyed the vegetation with a focus on native annual forbs in paired cattle grazed and ungrazed coastal prairie sites over a broad geographical range in north/central, coastal California. Results indicated that native annual dicot species richness and cover were higher in grazed sites and grazing increased both exotic grasses and exotic forbs.; To explain some of the trends in the surveys, I conducted a manipulative experiment using factorial treatments within cattle exclosures. In these experiments, I tested effects of primary disturbances of different clipping frequencies in combination with secondary disturbances of litter accumulation, phytomass removal, and soil disturbance. I analyzed the effects of these treatments with respect to (1) the existing coastal prairie community and (2) a number of life history stages of an endangered annual forb species: Holocarpha macradenia (Santa Cruz tarplant). Results indicate that more frequent clipping increased exotic forbs while decreasing exotic grasses, with little effect on native species. Holocarpha macradenia germination was higher in the more frequently disturbed plots, and, in some years, seedling survival increased with increasing disturbance frequency. Secondary treatments did not differentially affect community composition or Holocarpha macradenia survival.; Overall, this research suggests that cattle grazing favors native annual forbs, and that frequent vegetation canopy reduction (clipping) may be a factor in this relationship. These results suggest that land managers stewarding California's coastal prairie should consider cattle grazing as a management practice, particularly in areas with native annual forbs. Landscape-level cooperation to vary disturbance regimes may further clarify these relationships while conserving species diversity.