(Lack of) defensive role of ants throughout a broad latitudinal and elevational range of a cactus

Extra-floral nectaries of Opuntia sulphurea. Photo: Marina Alma.

Many plant species offer feeding resources (usually nectar) or nesting sites for ants, which in turn defend plants from herbivore attacks. With Marina Alma (former lab member), Rodrigo Pol (colleague at IADIZA) and Luis (Lucho) Pacheco (colleague at Universidad Mayor de San Andrés, La Paz, Bolivia) we observed ants visiting what seemed extra-floral nectaries in the cactus Opuntia sulphurea. This cactus species has a broad latitudinal distribution, from Mendoza in Argentina to La Paz in Bolivia; it also has a broad altitudinal distribution in the Andes. These facts led us to wonder about the outcome of the ant-plant interaction and its geographic variation: do ants visiting the extra-floral nectaries defend this plant species from insect herbivores, and does the outcome of this interaction vary throughout the geographic distribution of this plant species? We just published a paper in Biotropica reporting the results of a study (actually, Marina’s undergraduate thesis) evaluating this question. We conducted ant-exclusion experiments close to the southern (Mendoza) and northern (La Paz) latitudinal limits of O. sulphurea, and at two contrasting elevations. Our results indicate that ants do not benefit the plant in terms of herbivore damage or fruit and seed production, a result that was latitudinally and altitudinally consistent. Thus, this plant is apparently offering free meals for the ants, with no apparent benefit (nor detriment).