Herbivorous insect and plant communities respond in complex ways to rainfall manipulation in an oak savanna grassland

Changes in precipitation due to climate change will have consequences for plant and herbivore communities alike. Multiple hypotheses exist to explain how changes in plant diversity and productivity may cause changes in herbivorous insect community composition. However, the effects of water availability on the relationships between plant and herbivore communities are less well understood. For my M.Sc. thesis, I used a long-term rainfall manipulation experiment in a remnant patch of Garry oak (Quercus garryana) savanna to examine how plant community composition and productivity have responded to changing levels of precipitation over a 5-year period. This highly endangered ecosystem is predicted to experience significantly wetter springs and drier summers by 2080. I also investigated plant-mediated, indirect effects of manipulated precipitation on the diversity and abundance of herbivores, drawing on multiple hypotheses that describe the relationships between the composition of plant and herbivore communities. I found that plant productivity increased significantly with increasing growing season soil moisture. Plant productivity was also influenced by plant diversity, with plant productivity increasing with increasing plant diversity but only in drier plots, highlighting a context dependent effect. Further, I found that the effects of soil moisture on the herbivore community were mediated by plant productivity. Herbivore abundance decreased with increasing plant productivity, refuting the more individuals hypothesis. This indirect effect of soil moisture was in addition to a significant negative effect of soil moisture directly on herbivore diversity. The results of this study show that drought will result in significant decreases in plant productivity in this ecosystem and both direct and indirect impacts on the composition of the herbivore community. This study provides valuable insight into the effects of climate change on a highly endangered ecosystem and how future changes in precipitation with climate change will effect plant and herbivore communities.Arts, Faculty ofGeography, Department ofGraduat