Alternative Defenses against Herbivores in Inga (Fabaceae: Mimosoideae) over an Elevational Gradient

Inga densiflora and Inga punctata have foliar nectaries that promote protection against herbivores by ants. Nectar—drinking ants were found to be less active at higher elevations than at lower elevations in Costa Rica, and ant defense less effective in upland populations of these trees. Herbivore damage to leaves was greater at higher elevations than at low elevation, although the abundance of lepidopteran larvae was not different. This study tested the hypothesis that lack of ant defense in Inga at high elevations is compensated for by alternative defenses. Upland I. densiflora and I. punctata had higher concentrations of phenolics in their leaves than their lowland counterparts. Extrafloral nectaries were found to be present and actively secreting nectar on plants at high as well as at low elevations. Adult wasp and fly parasitoid presence at nectar sources was greater at high elevations in the absence of ants, and parasitization o caterpillars reared from upland Inga was significantly higher than in those reared from lowland Inga. Thus, upland caterpillars on Inga are more vulnerable than those at low elevations to predators and parasitoids attracted to nectar, because at high elevations less of the nectar is taken by ants. The antiherbivore properties of upland Inga represent a novel complex of facultative defenses in the absence of protection by ants.