UC San Diego

In this study we have examined a food-for-protection mutualism and its invasion by the Argentine ant (Linepithema humile). The San Diego Barrel Cactus (Ferocactus viridescens) produces extrafloral nectar to attract ants, which in turn deter herbivores. By utilizing both invaded and uninvaded sites, we examined the effects of Argentine ant invasion on cactus reproductive output (seedset) and pollinator visitation. Argentine ants were found in flowers at a much higher rate than native ants in our system and cacti tended by the most prominent native ant in our system, Crematogaster calilfornica, were found to have increased seed set (when controlling for fruit number) when compared to Argentine ant tended cactus. This result may be due to pollen limitation as pollinators (96% of which were bee spp.) spent significantly less time in flowers of cacti tended by L. humile. Thus invasion by an aggressive cacti-tending ant may have long term effects on its host plants by shifting the rewards and consequences of an previously established food-for-protection mutualism