Several hypotheses have been proposed to explain the advantage of ant-mediated seed dispersal (myrmecochory). In particular, the directed dispersal hypothesis, which suggests ants place seeds in a favorable microhabitat (typically the ant nest or refuse pile) that increases plant fitness relative to a random location, has received considerable attention. Because ant nests differ in soil quality from the surrounding soils, ant nests may provide plants with greater amounts of necessary resources, such as nitrogen and phosphorus, compared to random soil locations. We tested whether Formica ant mounds serve as a favorable microhabitat for an invasive plant, Euphorbia esula (leafy spurge), in a sand oak savanna in Wisconsin. We compared soil quality (including available nitrogen and phosphorus content), total leafy spurge density (measured as the total number of ramets (stems in a clonal plant) per 0.5m²), mature leafy spurge density (measured as the number of mature ramets per 0.5m²), and percent successful seedling recruitment (emergence of a seedling with cotyledons) and establishment (seedling survival through August) for experimentally planted seeds in paired locations on and off ant mounds.

Results/Conclusions

Results from 2009 found no difference in seedling recruitment or establishment between seeds planted on and off Formica mounds. In addition, we found no difference in total ramet density comparisons on and off ant mounds. However, there was a trend for greater density of mature (flowering) ramets on mounds compared to areas off mounds. We also found Formica mounds have elevated levels of available nitrogen and phosphorus. Our results suggest Formica mounds may serve as a favorable microhabitat for invasive leafy spurge by providing an increase in essential nutrients; however, this advantage may only be realized for established leafy spurge plants and does not affect leafy spurge success during the seed and seedling stages.