| Plant-herbivore interactions are common and have been the focus of much research. Ecologists have realized that in order to better understand the generality of plant-herbivore interactions, there is a need to study factors that mediate these interactions. Here, I first address whether plant-herbivore interactions are affected by omnivores, second test whether latitudinal variation in plant quality might explain variation in herbivore body size, and finally examine whether geographic origin of plants and herbivores affects herbivore performance.;These results suggest that the prevalence of omnivory in food webs and the geographic location in which food webs are studied could both affect plant-herbivore interactions. To better understand plant-herbivore interactions, scientists must consider factors such as these that alter the nature of interactions in the field.;First, I examined the top-down impacts of an omnivorous crab, Armases cinereum, on the shrub Iva frutescens and its arthropod fauna. My hypothesis was that Armases would benefit Iva through a trophic cascade, and that this benefit would be stronger than the direct negative effect of Armases on Iva. Field and laboratory experiments both supported this hypothesis. Although Armases is an omnivore, it produced strong top-down forces and a trophic cascade because it fed preferentially on herbivores rather than plants when both were available. Second, I tested the hypothesis that latitudinal variation in plant quality might help explain Bergmann's rule. I found that three herbivores grew bigger or faster when fed plants from high- versus low-latitude regions. For Prokelisia and Aplysia, plant quality alone could explain the variation in herbivore body size observed in the field. Third, I sampled plant and herbivore traits in the field to demonstrate that increased plant palatability at high latitudes likely reflects better plant quality for herbivores. I then conducted laboratory experiments to examine how the geographic origin of plants and herbivore would affect herbivore performance. The geographic origin of plants affected herbivore performance for 4 of 6 herbivores, and had a marginal effect for a fifth. The geographic origin of herbivores affected herbivore performance in all 6 cases, but the pattern was mixed with respect to which geographic region was best. |
