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

Many plants use induced defenses to reduce the costs of antiherbivore defense. These plants 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 documented in temperate plants but has not been reported from tropical plants. Most tropical plants have higher, more constant herbivore pressure than temperate plants. In this situation, it is hypothesized that constitutive defenses rather than induced defense would be favored. Using natural herbivores of four species of Inga saplings on Barro Colorado Island, Panama, herbivore presence was crossed with ant presence to determine their effects on extrafloral nectar production. Analysis of nectar samples revealed that Inga species do not induce nectar production in response to herbivores. This result is not due to an inability of the plants to respond, as the plants in this study increased nectar production in response to light and ant presence. Contrary to most induction experiments with temperate ecosystem plants, these results demonstrate that tropical plants do not induce one type of defense, and they suggest that the most adaptive defense strategies are different for the two ecosystems.