Identifying the context dependencies of plant-herbivore interactions across a species’ range

Spatial variation in abiotic and biotic factors creates local contexts that influence the intensity and outcome of plant-herbivore interactions. Previous studies have tried to represent the complexity of these context dependencies using latitudinal clines, but this approach has proven insufficient for many systems. Despite substantial variation in herbivore damage across populations, the relative contribution of local community factors to explaining this variation is less well known. I investigated plant-herbivore interactions across the entire range of Quercus garryana, specifically testing the relative importance of climate and population traits on herbivore communities, leaf traits, and the extent of damage caused by different insect feeding guilds. I performed similar analyses on trees grown in a common garden, allowing me to detect the relative importance of environmental and genetic contributions to leaf defense traits and susceptibility to herbivory. Although I observed no variation in herbivore diversity among populations in the field, the abundance of herbivores declined with increasing latitude, while also responding to variation in population size, leaf traits, and climate. Leaf traits were also influenced by climate factors, but in addition varied with tree size and within the growing season. Differences in herbivore damage were best explained by long-term trends in spring precipitation, leaf traits, and population size, with no relationship to latitude. The relative importance of each of these factors depended on when the damage occurred as well as the insect feeding guild causing the damage. Conversely, the extent of damage in the common garden was constant across trees of different provenance, providing further support for the importance of climate in driving variation in herbivory. The findings of this study demonstrate the importance of climate, irrespective of latitude, on plant-herbivore interactions and in mediating the effects of other community factors. As such, they highlight the importance of conducting studies across diverse ecosystems and climate gradients. Only by understanding the underlying drivers of selection can we begin to draw generalizations and develop a predictive framework of plant-herbivore interactions to inform conservation and effective habitat management.Arts, Faculty ofGeography, Department ofGraduat