Vegetation change on San Clemente Island following the release from feral grazing pressure

Includes bibliographical references (p. 66-77)Aim: Island ecosystems worldwide have been heavily impacted by the introduction of exotic plant and animal species. The aim of this study is to assess changes in cover, richness, and diversity of native and non-native plant species following the removal of feral herbivores from San Clemente Island, California. Location: San Clemente Island, California, USA. Methods: Fourteen transects, six located in coastal sage scrub and eight in grassland vegetation, were sampled in 1986, 1988, 1990, 1992, and 2011 for ground cover for all vascular plants intercepted along each transect. Due to the inherent differences in structure and composition of coastal sage scrub and grassland vegetation types, data from the two communities were analyzed separately in the study. A repeated measures ANOVA was used to assess statistically significant differences in mean cover of growth forms (i.e., subshrubs, herbs, and vines) and individual species, percent similarity, richness, and diversity between sample periods. Additionally, mean cover over time by growth forms was differentiated by native and non-native species. I used a t-test to determine significant differences between mean native and non-native cover, species richness, and diversity within each sample year. Regression analysis was used to determine the statistical relationship between herbaceous cover and annual precipitation. Results: Mean vegetation cover of both communities varied over the course of the study period. Cover on grassland sites was increasingly dominated over time by non-native annual grasses. However, this trend was complicated by individual species' responses to variation in annual amounts of precipitation. Mean species richness and diversity values did not differ significantly over the study period. Sites of coastal sage scrub experienced a significant increase in ground cover of native subshrubs and herbs. Some non-native herbs that were common from 1986 through 1992 were non-existent on transects in 2011 while Bromus madritensis ssp. rubens, a non-native annual grass, showed a dramatic cover increase. Ground cover and compositional trends of herbs on sage scrub sites also corresponded strongly to amounts of annual precipitation. Main Conclusions: Study results indicate that non-native herbaceous cover increased at the expense of native herb cover on grassland sites following the release from grazing pressure. However, not all of the changes can be ascribed to the removal of the feral herbivores as annual precipitation amounts also influence amount of herbaceous cover of both native and non-native herbs. The sites of coastal sage scrub showed a marked increase in subshrub cover over the course of the study. Artemisia nesiotica, a native subshrub once thought to be a co-dominant on the island, has been observed over the study's course spreading from protected canyon sides into grassland areas on the island's southern half