Consequences of leaf-cutting ants on plant fitness: integrating negative effects of herbivory and positive effects from soil improvement

Abstract

A better understanding of plant–herbivore relationships should integrate negative and positive effects of consumers on plant fitness. We studied the effect of a major insect herbivore (leaf-cutting ants, LCA) on plant fitness in several species of Monte Desert, assessing both the direct negative effect of ant defoliation and the indirect positive role of ants as soil improvers. To estimate the negative and positive effects on plants, we sampled 7000 plant fragments carried by the ants and analyzed the nutrient contents of refuse dumps in 15 ant nests. We estimated plant fitness measuring ~10,000 flowers and ~3600 fruits of 122 individuals from 7 of the most common plant species within the ant foraging area. First, we compared the fitness between plants growing on the nutrient-rich refuse dumps and co-specifics on adjacent non-nest soils. Second, using hierarchical confirmatory path analysis model, we analyzed how herbivory and nutrient content of refuse dumps affected plant fitness. We found that the fitness of plants growing on refuse dumps (a) was similar than those growing on adjacent non-nest soils, and (b) was unaffected by ant harvesting or by increments in nutrient content in a consistent way. In this dry habitat, the effect of LCA on plant fitness was complex and species-dependent. To explain this pattern, we discuss the role of plant compensation to foliar damage, the limited plant response to soil nutrients due to water stress and the potential high abundance plant consumers on ant refuse dumps. Our results illustrate the complexity of herbivore-plant interactions emphasizing the need of measuring direct and indirect effects of herbivory in field conditions to validate greenhouse experiments.