Trade-offs in Antiherbivore Defenses in Piper cenocladum: Ant Mutualists Versus Plant Secondary Metabolites

Abstract

Ant–plant mutualisms may provide indirect evidence for costs of antiherbivore defenses when plants demonstrate trade-offs between allocating resources and energy into ant attractants versus chemical defenses. We tested the hypothesis that ecological trade-offs in defenses are present in Piper cenocladum. This plant possesses two distinct defenses: food bodies that attract predatory ants that destory herbivore eggs and amides that deter herbivores. Previous studies have demonstrated that the food bodies in P. cenocladum are an effective defense because the ants deter herbivory by specialist herbivores. Amides in other Piper species have been shown to have toxic qualities, but we tested the additional hypothesis that these amides have an actual defensive function in P. cenocladum. To test for ecological trade-offs between the two putative defenses, fragments of P. cenocladum were examined for the presence of amides both when the plant was producing food bodies and when it was not producing food bodies. Plants with active ant colonies had redundant defenses, producing food bodies and high levels of amides at the same time, but we detected a trade-off in that they had significantly lower levels of amides than did plants with no ants. To test for the defensive value of P. cenocladum amides, we used an ant bioassay and we examined herbivory results from previous experiments with plants that had variable levels of amides. These tests demonstrated that amides are deterrent to omnivorous ants, leaf cutting ants, and orthopterans. In contrast, the resident Pheidole bicornis ants are effective at deterring herbivory by specialist herbivores that oviposit eggs on the plant but not at deterring herbivory by nonresident omnivores. We concluded that although both amides and food body production appear to be costly, redundancy in defenses is necessary to avoid damage by a complex suit of herbivores.