Large mammalian herbivores can profoundly impact the structure and composition of plant communities, with indirect effects that cascade throughout food webs. However, our understanding of these impacts remain limited because responses of plant communities to herbivory are variable and depend on environmental conditions, herbivore identity and herbivore abundance. As large herbivores are rapidly declining around the world, understanding the context dependence of herbivore impacts on plant communities is increasingly important. Most existing large-scale manipulations of large herbivore abundance consist of a single exclusion treatment, few are replicated across environmental gradients, and even fewer consider the effects of wildlife replacement by livestock. In 2016, we constructed a series of 1-ha herbivore-exclusion plots across a climate gradient in southcentral California. Dubbed “TREE" (Tejon Ranch Exclosure Experiment), this experiment aims to illuminate the effects of native and exotic ungulates on plant communities, and how climate regimes shape the direction and magnitude of these effects. TREE consists of three treatments: total-exclusion (all ungulate herbivores), cattle-exclusion (wild ungulates allowed), and open unfenced control plots. Each treatment is replicated three times at three locations along the climate gradient. Here, we discuss results from the first year of this study.
Results/Conclusions
Our preliminary results emphasize the importance of climatic context in modulating the effects of large mammalian herbivory on plant communities, and the importance of such interactions in understanding effects of wild herbivore declines. Climate influenced the strength and direction of effects of both wild herbivores and cattle on understory plant species richness, species and growth form composition, and physical structure (height, cover, aerial density). We found increasing aridity linked to increasing magnitude of exclosure effects. After multiple growing seasons, we expect livestock and wildlife to have divergent effects on plant communities due to substantial differences in diets, behavior and population densities. These can influence their respective effects through altered competition and facilitation among plant communities, canopy tree recruitment, and nutrient distribution.