Interactions between the invasive Argentine ant and coast barrel cactus: multifaceted disruption of a protection mutualism

In southern California, the coast barrel cactus (Ferocactus viridescens) produces extrafloral nectar (EFN) attractive to ants, including the invasive Argentine ant (Linepithema humile). Ants discourage a prevalent herbivore, a leaf-footed bug (Narnia wilsoni), from feeding on cacti, and increasing bug presence reduces cactus seed mass, a variable closely linked with plant fitness. In areas without L. humile as many as 18 native ant species visited cacti, but ant diversity is greatly reduced in invaded areas. Individual cacti are typically controlled by one ant species at a time. Turnover in the identity of tending ants occurs on scales of weeks to months and likely results from how species with differing environmental tolerances respond to seasonal variation. The cumulative number of ant species on individual cacti over time, which is greatly reduced in invaded areas, is associated with increased ant presence, reduced herbivore presence, and increased seed mass. An additional negative consequence of the Argentine ant concerns their effects on cactus pollinators. Unlike the native ants at our study sites, the Argentine ant forages in cactus flowers, where they harass key pollinators (especially native cactus bees (Diadasia spp.)). Pollinator harassment likely explains why cactus set fewer seeds per fruit when the Argentine ant controls cacti compared to when native ants control cacti. These results illustrate the potential for invasive ants to disrupt protect mutualisms through multiple pathways.