Induced responses and developmental changes of direct and indirect defenses in the young leaves of the neotropical tree genus Inga

Plant-herbivore interactions are the most common macroscopic interactions on the planet and are an important mechanism for maintaining species coexistence. Tropical young leaves are under higher and more constant herbivore pressure than their temperate zone counterparts. Therefore tropical leaves are predicted to invest in constitutive defenses (always expressed) over induced defenses (expressed only when herbivores are present). However, there is little empirical evidence to support this hypothesis. In addition, little is known about developmental changes in defense investment during tropical young leaf development regardless of herbivore presence. In this dissertation, I demonstrated that younger leaves are better defended than more mature leaves in four species of the Neotropical tree genus Inga. In addition to chemical defenses, Inga species produce nectar on their leaf surfaces to attract ants that in turn patrol the leaves and remove herbivores. Through natural observations and experimental nectar manipulation I demonstrated that plants with higher nectar production rates received more ant bodyguards. I then showed that investment in both ant defense and chemical defense were highest in the youngest leaves. While investment in both of these defenses continued during young leaf development, the relative investment per leaf tissue decreased significantly as leaves expanded. Through experimental manipulation of ant presence and herbivore presence I also demonstrated that tropical young leaves have a limited capacity to induce their defenses. I showed that plants can increase their nectar production but not their defense chemistry when ants are present. However, I showed that there was no change in nectar production and a very limited change in chemical defenses when herbivores were present. This evidence supports the hypothesis that tropical young leaves are primarily protected by constitutive defenses. This is the first report of tropical plants investing in constitutive rather than induced defenses, and supports predictions of the induced resistance hypothesis that are rarely tested. Furthermore, divergent defenses among Inga species appear to be species level traits and not plastic responses. Thus, my results support the hypothesis that a diversity of antiherbivore defenses among plant species is an important mechanism in maintaining species coexistence.