Background/Question/Methods Plant-herbivore interactions have significant impacts on the composition and function of both plant and herbivore communities. Herbivorous insects in particular play a critical role in transferring energy and nutrients from primary producers to higher trophic levels. Multiple hypotheses attempt to explain how changes in plant diversity and productivity may cause changes in herbivore community composition. However, how abiotic conditions affect the relationships between plant and herbivore communities is less well understood. Predicted changes in abiotic conditions due to climate change, such as water availability, will have consequences for plant and herbivore communities alike. In this study, we used an ongoing rainfall manipulation experiment in a low-productivity, temperate oak-savanna ecosystem (part of the International Drought Experiment), to examine how plant community composition and biomass have responded to both increased and decreased precipitation over a 5-year period. In addition, we investigated plant-mediated effects of manipulated precipitation on the diversity and abundance of herbivores throughout the growing season, drawing on multiple hypotheses that describe the relationships between the composition of plant and herbivore communities. Results/Conclusions We found that average growing season (mid-April to mid-June) soil moisture had significant effects on both plant diversity and plant productivity (measured as total above ground biomass). Decreased soil moisture in the drought treatment had a significant negative impact on plant diversity, while plant biomass increased with increasing soil moisture. In addition, we found that the drought and irrigated treatments had weak positive and negative impacts on herbivore community composition, respectively. These effects were mediated by changes in plant diversity – herbivore and abundance significantly increased with increasing plant diversity. For example, in the drought treatment, plant diversity had a significant positive effect on herbivore diversity throughout the growing season. These results contribute to our broader understanding of plant community responses to changes in precipitation and our understanding of the bottom-up effects of abiotic conditions on plant-herbivorous insect interactions.
