UC San Diego

In this study we investigate the effects of the invasive Argentine ant (Linepithema humile) on the food-for- protection mutualism between native ant species and the San Diego Barrel Cactus (Ferocactus viridescens). Mutualistic interactions may be disrupted by non-native species with possible detrimental effects for one or both partners. In the presence of L. humile, plant protection may be enhanced in the short term as a consequence of the Argentine ant's aggressive nature, yet may be compromised by pollinator harassment and abandonment of the cactus. Consequently, in the long term, this invasion may decrease the fitness of cacti by shifting the net effects of the interaction from a mutualism to one that is detrimental. To quantify ant protection, we used an established surrogate herbivore assay to compare protective services offered by Argentine ants with those offered by a common native ant C. californica. Our results support the existence of a mutualism between ants and F. viridescens. Ant presence discouraged leaf-footed bugs, the most commonly observed herbivores, from feeding on plant reproductive structures. Herbivory reduced mass per individual seed and seed mass per plant. This study adds to a growing body of work concerning the variation found in mutualistic interactions, allows for predictive measures of plant benefits with respect to partner identity, and examines how these interactions are affected by a behaviorally dominant and aggressive invasive ant. Furthermore, it explores the consequences of reduced ant richness, and the impacts that result from the loss of functional redundancy in the F. viridescens food-for- protection mutualism