Is extrafloral nectar production induced by herbivores or ants in a tropical facultative ant-plant system?

Background/Question/Methods

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 defense 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 the effects of herbivores and ants on leaf defense chemistry and extrafloral nectar production. Nectar samples were quantified using gas chromatography.
Results/Conclusions

Analysis of nectar samples revealed that Inga species do not induce nectar production in response to herbivores. This supports the hypothesis of constitutive defense being more adaptive in environments where herbivory is constantly high. This result is not due to an inability to change as the plants in this study did alter their defense investment in response to light and ant presence. In gaps, nectar production was higher than in the understory, which follows predictions of the carbon/nutrient balance hypothesis: nectar is predominantly sugar and with excess light sugar production is less limited. In the presence of ants, nectar production was significantly greater than when ants were excluded. Considering these are facultative interactions and plants cannot consistently rely on ant presence for defense, it follows that being able to detect the presence of ants and “decide” to invest in this defense would be beneficial for plants.  Contrary to the dogma in temperate ecosystem plants, these results demonstrate that tropical plants do not induce one type of defense and suggest that the most adaptive defense strategies are different for the two ecosystems.