Hawaiian ant–flower networks: nectar-thieving ants prefer undefended native over introduced plants with floral defenses

Abstract

Ants are omnipresent in most terrestrial ecosystems, and plants have responded to their dominance by evolving traits that either facilitate positive interactions with ants or reduce negative ones. Because ants are generally poor pollinators, plants often protect their floral nectar against ants. Ants were historically absent from the geographically isolated Hawaiian archipelago, which harbors one of the most endemic floras in the world. We hypothesized that native Hawaiian plants lack floral features that exclude ants and therefore would be heavily exploited by introduced, invasive ants. To test this hypothesis, ant–flower interactions involving co-occurring native and introduced plants were observed in 10 sites on three Hawaiian Islands. We quantified the residual interaction strength of each pair of ant–plant species as the deviation of the observed interaction frequency from a null-model prediction based on available nectar sugar in a local plant community and local ant activity at sugar baits. As predicted, flowers of plants that are endemic or indigenous to Hawaii were more strongly exploited by ants than flowers of co-occurring introduced plants, which shared an evolutionary history with ants. We also found that the percentage of plant species with ant-visited flowers was much higher in Hawaii than in other island and continental systems, even reaching 100% in habitats dominated by endemic species. We showed experimentally that the absence of ants on flowers of most introduced and few native plants species was due to morphological barriers, repellent floral scents, and to a lesser extent, unpalatable nectar. Analysis of floral volatiles, however, revealed no consistent ant-repellent “syndrome” attributable to negative responses by ants, probably due to the high chemical variability within the floral scent bouquets. Results from a molecular phylogeny imply that floral defenses against ants were convergently lost in native Hawaiian plants. Exploitation of floral nectar by ants may be an important threat to Hawaiian ecosystems, reducing nectar resources available to native flower visitors and potentially reducing the reproductive success of the endangered endemic flora.