Neighbor palatability generates associational effects by altering herbivore foraging behavior

Despite increasing evidence that herbivory on a focal plant may hinge upon the identity of its neighbors, it is not clear whether predictable mechanisms govern the nature and magnitude of such associational effects. Using a factorial field experiment replicated at 14 sites across 80,000 hectares, we evaluated the mechanisms driving associational effects between two plant species mediated by grasshopper herbivores. Our experiment manipulated local neighborhood plant density (two levels) and frequency (three levels), nested within two larger-scale habitat contexts (habitats that did or did not have past agricultural land use). We found that the more palatable plant species, Solidago nemoralis, experienced reduced herbivory (associational resistance) when rare due mainly to reduced grasshopper foraging activity. Damage to the less palatable plant species, S. odora, was affected by the interaction between plant frequency and the land-use history of the site: it experienced increased damage (associational susceptibility) in even-frequency neighborhoods, but only in sites with a history of agricultural use. Behavioral assays generally corroborated the results from the field, further supporting the importance of foraging behavior in generating associational effects. In finding that associational effects are contingent upon relative palatability among plants and events in the distant past that modify contemporary habitat structure (i.e., past agricultural land use), our work suggests that foraging decisions made at the neighborhood level are important for generating associational effects and that in some cases these neighborhood interactions also depend on the larger-scale environmental context resulting from legacies of past land-use events.