The arms race between herbivores and plants has lead to countless antiherbivore defenses. It is assumed that these defenses are costly and that plants have evolved to streamline their defenses so that only effective non-redundant defenses are retained. One example that has received much attention in temperate ecosystem plants is induced defense. These defenses are thought to be more adaptive because they allow plants to invest energy in growth when herbivores are absent but shunt energy to defense when herbivores are present. In contrast, constitutive defenses are expressed continuously regardless of herbivore presence. Induction has been widely document in temperate plants but has not been reported from tropical plants. The current hypothesis predicts that induction will be beneficial when herbivore pressure is intermittent with predictable cues. Most tropical plants, however, have high, constant herbivore pressure. Therefore it is predicted that plants under these circumstances should invest in constitutive defense rather than induced defense. This hypothesis was tested using the tropical tree genus Inga (Leguminosae) which utilizes chemicals and ant bodyguards to defend its leaves. Using natural herbivores of Inga saplings on Barro Colorado Island, Panama, herbivore and no-herbivore treatments were crossed with ant and no-ant treatments to determine their effects on leaf defense chemistry. Leaf samples were collected, fractionated, and quantified gravimetrically.
Results/Conclusions
Chemical defenses were induced in the young leaves that experienced a range of low and high herbivore pressure, but, were constitutive in the young leaves that were constantly under higher herbivore pressure. Inga marginata induced phenolics when herbivores were present but did not induce in response to ant presence. Inga multijuga did not induce in response to any of the treatments. In the tropics, 70% of herbivory occurs on the young leaves because they are soft and full of nutrients. We suggest that because the young leaves of I. marginata expand so rapidly (less than a week) and spend such a short time in the young vulnerable state, they are unlikely to be discovered by herbivores and thus have a sufficiently low and variable herbivore pressure to make induction an adaptive strategy. In contrast, young leaves of I. multijuga are more susceptible to herbivore attack because they expand much slower (three weeks). Therefore, constitutive defenses are more adaptive for I. multijuga. These results are the first to demonstrate induced defenses in tropical trees and suggest that induced defenses are not necessarily more adaptive than constitutive defenses.
