Variable Responses of a California Grassland to the Reintroduction of Tule Elk

Although large mammalian herbivores are known to have had large effects on the ecology and evolution of terrestrial plant communities, many herbivore populations have been dramatically reduced or extirpated from their pre-historic ranges due to hunting and other human-caused alterations to the landscape. Over the last century, there have been many successful reintroductions of mammalian herbivores throughout the world. However, reintroduction sites are often highly altered by exotic plant species, habitat fragmentation and grazing by domesticated herbivores. One such herbivore is tule elk (Cervus elaphus nannodes), which was on the brink of extinction 150 years ago, but now has several stable populations throughout California due to successful conservation and reintroduction efforts. Here, I summarize results of a five-year exclosure experiment that explores the effects of reintroduced tule elk populations on a coastal grassland in northern California. I demonstrated that the invasive grass, Holcus lanatus, experienced reduced levels of elk herbivory when associated with the native shrub, Baccharis pilularis so I also found that elk decrease shrub cover, suggesting that these herbivores may play an important role in maintaining open grasslands and reducing colonization by shrubs. I also documented that elk promoted the successful establishment of Lupinus arboreus juveniles, a native shrub that is known to have large effects on soil nitrogen availability and community composition. My results indicate that the reintroduction of elk increased plant species richness and abundance, with these effects due primarily to the response of annual, rather than perennial species. In summary, although my work shows that tule elk have positive effects on the native species composition, this comes at the cost of increasing the richness and abundance of the exotic taxa in the community. These results demonstrate that a reintroduced herbivore species can have profound effects on ecosystems, and that land managers will need to not only monitor the status of the reintroduced herbivore population, but also develop an in-depth understanding of the diverse impacts of reintroduced herbivores on different components of the community.The National Science Foundation (NSF Grant: DEB-9981663