The Effects of Aridity and Herbivore Assemblage on Plant Functional Traits and Diversity

Large herbivore communities are rapidly changing globally, with populations of large wild herbivores declining while large domestic herbivore populations are increasing exponentially. Along with climate change, these alterations in large herbivore communities are two of the most important impacts on ecosystem functioning in terrestrial systems. While large herbivores and climate are known to independently influence the cover and composition of herbaceous plant communities, manipulative experiments have rarely captured the interaction between realistic patterns of large herbivore assemblage change and climatic conditions. Critically, no studies to date have experimentally explored how these realistic changes in herbivore assemblages interact with climate to affect functional traits and functional diversity of herbaceous communities, even though these metrics have been demonstrated as some of the most useful proxies for ecosystem function.Here, we use a multi-factor large herbivore exclosure experiment to simulate the three most common herbivore change scenarios globally, replicated along a topo-climatic gradient. We then explore the interaction between climate and herbivore assemblage on community-level functional traits and functional diversity of herbaceous plant understories in an oak savanna ecosystem. Our findings demonstrate interacting effects between changes in large herbivore assemblages and climate. We found a shift between more drought tolerance traits to drought avoidance traits with increasing aridity, and found that plant community responses to grazing changed from an herbivore avoidance strategy at arid sites to a more herbivore tolerant strategy at mesic sites. Even over a relatively large temperature gradient within this landscape, we observed that the effects of herbivores on community-level traits can sometimes counteract those of climate. Finally, we found that cattle and large wild herbivores can differ in the magnitude and direction of effects on functional traits and diversity