Invasive species, and particularly invasive ants, threaten native communities, in part, through their potential to alter mutualisms. However, invasive species may also facilitate native species. The red imported fire ant (Solenopsis invicta) is one of the most conspicuous invasive ants in North America and its high densities, combined with its potential to displace native ants, have led to concerns that it may alter ant-plant seed dispersal mutualisms. We examined the potential of fire ants to disperse seeds in the longleaf pine ecosystem by comparing the removal of elaiosome-bearing seeds by fire ants and native ants. We determined rate and distance of seed removal, as well as likelihood of dispersal to the nest. During 2006-2007, we observed seed removal from 180 experimental caches of Piriqueta cistoides and Polygala grandiflora. At the same sites, we used monthly pitfall trapping to determine the relative abundance of fire ants and native ants.
Results/Conclusions
Fire ants transported more than half of all elaiosome-bearing seeds that were removed. Thirteen native ant species were also observed removing seeds, with Pheidole sp. serving as the second most common seed remover. While fire ants were the dominant seed remover in this system (removing 50% of seeds), they did not remove significantly more seeds than would be expected based on their population density (46% of ground-dwelling ants). Additionally, fire ants were similar to native ants with respect to the quality of the seed dispersal service provided, with no differences in rate of discovery, distance removed, or frequency of moving seeds back to the nest. Areas with naturally higher fire ant densities were found to have higher rates of seed removal by ants without a subsequent drop in seed dispersal by native ants, suggesting that fire ant-invaded areas may experience overall higher levels of seed dispersal. Thus, fire ants may actually be facilitating the dispersal of elaiosome-bearing plant species in the longleaf pine ecosystem. However, further study is necessary to determine the fate of these seeds.