Field experiments show contradictory short- and long-term myrmecochorous plant impacts on seed-dispersing ants
Some interactions previously describe as mutualistic were revealed to be commensal or parasitic in subsequent investigations. Ant-mediated seed dispersal has been describe as a mutualism for more than a century; however, recent research suggests that it may be commensal or parasitic. Plants demonstrably benefit from ant-mediated seed dispersal, although there is little evidence available to demonstrate that the interaction benefits long-term ant fitness. Field experiments were conducted in temperate North America focused on a key seed-dispersing ant. All herbaceous plants were removed from a forest understorey for 13 years, and supplemented ant colonies with large elaiosome-bearing seeds aiming to examine potential long-and short-term myrmecochorous plant benefits for the ants. If elaiosome-bearing seeds benefit ants, suggesting a mutualistic relationship, it is expected that there would be greater worker and/or alate abundance and greater fat reserves (colony lipid content) with seed supplementation (short-term) and in areas with high understorey herb abundance. Short-term seed supplementation of ant colonies did not result in an increase with respect to numbers or fat stores, although it did prompt the production of colony sexuals, which is a potential fitness benefit. In the long term, however, there was no positive effect on the ants and, instead, there were negative effects because the removal of elaiosome-bearing plants corresponded with greater colony health. The data obtained in the present study suggest that the ant-plant interaction ranged from occasionally beneficial to neutral to overall negative for the ant partner. Such results did not support considering the interaction as a mutualism. Collectively, the data suggest the need to reconsider the nature of the relationship between these ants and plants.