Book excerpt: Feeding by herbivorous arthropods often induces phenotypic responses in their host plants. Such responses are increasingly found to be herbivore-specific, as both their strength and type can vary with herbivore identity. To date, most research has focused on discerning the causes, whether functional or evolutionary, of this specificity. However, recent studies have suggested that it can also have strong ecological effects on the distribution and occurrence of herbivores. Using Solanum dulcamara (bittersweet nightshade), I established a consistent ecological pattern within its herbivore community that is driven by specificity in plant responses: early-season differences in the identity of colonizing herbivores induce divergent plant phenotypes, producing heterogeneity in resource quality and a predictable distribution of late-season herbivores. These priority effects (i.e., differences in later herbivore occurrence caused by the identity of initial herbivores) seem to have their temporal basis in a physiological tradeoff within the plant, as it is unable to sequentially respond to different herbivore species. The responses associated with a particular herbivore occur only when it is the first species to feed on the plant. If its feeding follows that of a different herbivore, these responses are no longer induced. Chemical antagonism between different signaling pathways in the plant may underlie these results, in which case we may be able to predict the occurrence of priority effects based on knowledge of the response pathways induced by different herbivores. However, the spatial scale of these interactions can also influence their strength. I found that the presence of multiple plant phenotypes has a greater effect on herbivore occurrence within stands of S. dulcamara than it does between them. Fragmentation of S. dulcamara populations reduces the likelihood of herbivores moving between plants, thereby reducing the degree to which they can discriminate between different plant phenotypes. Overall, therefore, a combined knowledge of the chemical basis and ecological context of plant-herbivore interactions may be essential when examining the dynamics of herbivore communities.