Prominent models of herbivore regulation focus on predators, low plant biomass, or poor resource quality as factors that limit herbivore populations. We examined predictions of these tritrophic models for herbivores on the understory shrub, Piper cenocladum, which is defended by mutualistic ants (Pheidole bicornis) and three amide secondary metabolites. To examine sources of variation in P. cenocladum amide content and to compare the effects of amides vs. ants on herbivores, we used three linked experiments in the field and in shadehouses. We manipulated light, nutrient availability, and presence of symbionts for experimental plant fragments and shrubs and then quantified leaf amide concentration. We also examined relationships between amide content and damage by the three most common groups of folivores on P. cenocladum: generalist orthopterans, specialist coleopterans, and specialist lepidopterans. For all experiments, enhanced resources and absence of symbionts caused higher levels of amides. These increased chemical defenses had strong effects on generalist herbivores in this system, while the ant predators were more effective at depressing herbivory by specialists. The negative effects of amides on specialist lepidopterans were small, suggesting that these herbivores are adapted to chemical defenses in their host plant. It is possible that our results are part of a more general trend where top-down effects are stronger against specialist herbivores, while chemical defenses are more effective against generalists. We concluded that different models of herbivore regulation were supported by components of the P. cenocladum arthropod community, depending on resource availability and on the portion of the web examined.
Available at: http://works.bepress.com/john_stireman/39/