Testing the directed dispersal hypothesis: are native ant mounds (Formica sp.) favorable microhabitats for an invasive plant?

Ant-mediated seed dispersal may be a form of directed dispersal if collected seeds are placed in a favorable microhabitat (e.g., in or near an ant nest) that increases plant establishment, growth, and/or reproduction relative to random locations. We investigated whether the native ant community interacts with invasive leafy spurge (Euphorbia esula) in a manner consistent with predictions of the directed dispersal hypothesis. Resident ants quickly located and dispersed 60% of experimentally offered E. esula seeds. Additionally, 40% of seeds whose final deposition site was observed were either brought inside or placed on top of an ant nest. Seed removal was 100% when seeds were placed experimentally on foraging trails of mound-building Formica obscuripes, although the deposition site of these seeds is unknown. Natural density and above-ground biomass of E. esula were greater on Formica mound edges compared to random locations. However, seedling recruitment and establishment from experimentally planted E. esula seeds was not greater on mound edges than random locations 3 m from the mound. Soil from Formica mound edges was greater in available nitrogen and available phosphorus relative to random soil locations 3 m from the mound. These results suggest Formica ant mounds are favorable microhabitats for E. esula growth following seedling establishment, a likely consequence of nutrient limitation during plant growth. The results also indicate positive species interactions may play an important role in biological invasions.