Introduced ants frequently enter into mutualisms with plants and other insects to obtain carbohydrate-rich resources. This phenomenon can disrupt mutualisms to the detriment of mutualist partners as well as give rise to ecological effects external to the mutualism itself. Observations of Argentine ant workers harassing native bees in the flowers of island morning glory, a plant endemic to the California Channel Islands, suggested that pollination services provided by bees could be compromised by the presence of non-native ants. To test this hypothesis, we employed a before – after, control – impact experimental design that involved a two-year field experiment on Santa Cruz Island in which we removed the Argentine ant from an 8-ha area. We address the following questions. (1) Do the floral visitors of island morning glory differ in their species composition and visitation frequency in areas invaded by the Argentine ant, and do they revert to an ‘uninvaded’ state after experimental removal of the Argentine ant? (2) Do island morning glory flowers experience pollen limitation as a result of pollinator harassment, and does the degree of pollen limitation decrease after Argentine ant removal?
Results/Conclusions
Invasion by the Argentine ant reduced the diversity, altered the species composition, and decreased the visitation rate of arthropods visiting the flowers of island morning glory. Argentine ant removal in the second year of the study led to an increased diversity of floral visitors, elevated visitation rates, and shifts in species composition away from that typical of plants growing in invaded sites. Although the Argentine ant and native ants both visited flowers of the island morning glory, the Argentine ant was much more likely to behave aggressively towards floral visitors. Presumably as a consequence of such interference behavior, island morning glory experiences a higher level of pollen limitation in areas invaded by the Argentine ant compared to the level measured in uninvaded areas. Moreover, Argentine ant removal decreased the level of pollen limitation relative to that observed in areas where the invader was still present. Our study demonstrates how non-consumptive, aggressive behavior on the part of an introduced ant can change diversity, composition and rate of visitation of floral visitors to the detriment of plant reproduction.