Consequences of interaction between the native ant community and an invasive plant (Euphorbia esula)

Mutualisms, or mutually beneficial species interactions (+/+), play a key role in ecological community functioning, influencing community stability and biological diversity. Despite their importance in ecological processes, the study of mutualism has limitations. First, seemingly positive interactions are often considered mutualisms without proper evaluation of the interaction consequences to both partners involved. Second, studies on mutualism often focus on a single life history stage, yet organisms often engage in multiple forms of mutualism during their lifetime (e.g., many plants rely on pollinators and seed dispersers). My research addressed these limitations in mutualism research by investigating (1) the outcomes of interaction between an invasive plant (Euphorbia esula L., Euphorbiaceae) and the native ant community during both the seed and flowering plant stages, and (2) how both the ant and plant benefit from myrmecochory (ant-mediated seed dispersal). My research had three main objectives: (1) examine the role native ants play in invasive E. esula seed dispersal, with an emphasis placed on whether Formica ants act as directed dispersers and provide a favorable microhabitat for E. esula seedlings and/or established plants, (2) test if the lipid-rich elaiosome reward provided by E. esula to seed-dispersing ants has a positive influence on ant colony demography and reproduction, and (3) determine how ant visitation to E. esula nectar glands influences plant reproduction and herbivory. My research showed several native ant species collect invasive E. esula seed, often depositing the seeds in their nests. While Formica ant mounds did not increase seedling recruitment or establishment of experimentally planted seeds, density and above-ground biomass of naturally growing E. esula were greater along mound edges than random locations. This finding is likely a consequence of the elevated available nitrogen and phosphorus found in ant mound soils relative to surrounding soils. Diet supplementation of E. esula seeds to ant colonies had no effect on growth or reproductive output of lab-reared Aphaenogaster rudis, a common seed-dispersing North American ant. Lastly, the exclusion of ants to E. esula stems with active nectar glands had no effect on plant reproduction or incidence of herbivory, indicating that ants are not food-for-protection mutualists with E. esula during the flowering stage. This research highlights the importance of testing the consequences of seemingly positive interactions rather than making assumptions about interaction benefits. Additionally, this research provides an example of local-scale facilitation of an invasive species, as native Formica mounds provide favorable habitat for established E. esula plants. Research on species interactions at the local scale provides a necessary foundation for future study on the role positive species interactions play in biological invasions at community- and landscape-levels.