Impact of the red imported fire ant, Solenopsis invicta Buren, on ant-seed mutualism

The red imported fire ant, Solenopsis invicta, is a nonnative ant that has been spreading unabetted through the southern United States since the 1930s. I found that clear-cutting of deciduous forests facilitates S. invicta's spread by increasing its populations while decreasing native ant abundances. Invasion by fire ants has negative impacts on individual plant and animal species, but little is known about how S. invicta affects complex mutualistic relationships. In some eastern forests of North America, 30% of herbaceous species have seeds dispersed by native ants. These plants have seeds with external attachments, elaiosomes, which attract ants. Both native ants and ant-dispersed plants benefit from this interaction, and accordingly, this relationship is considered to be a mutualism. I found that plots infested with fire ants had significantly lower populations of Crematogaster ants. Because Crematogaster spp. are associated with ant-seed mutualisms, fire ants can disrupt plant populations by reducing effective seed dispersers. Fire ants can also adversely impact ant-seed mutualisms by destroying seeds of ant-dispersed plants. Fire ants destroyed 86% of Viola rotundifolia seeds recovered from trash piles. In addition, seed handling by fire ants reduced germination in Viola pedata. Seeds that escape predation might be discarded in unsuitable locations for seed germination or seedling establishment. Nests of the native seed-dispersing ant, Aphaenogaster spp., are constructed of leaf litter, whereas fire ant mounds occur in open disturbed areas with bare soil. I found most (68%) seeds recovered from fire ant mounds on the soil surface, whereas, 83% of seeds recovered from native ant nests were within the leaf litter. Once ants discard seeds, both pathogenic and nonpathogenic fungi in the soil can hinder seedling recruitment by lowering seed viability. It is unknown how differences in fungal populations in ant-occupied soil affect seedling recruitment in ant-dispersed plants. I found that fungal populations in native ant nests differed substantially from those found in fire ant mounds. Overall, as fire ant populations increase, they can replace native ant species and act as ineffective seed dispersers, which can ultimately result in reduced seedling recruitment and thereby endanger communities of ant-dispersed plants.