The mixed effects of experimental ant removal on seedling distribution, belowground invertebrates, and soil nutrients

Ants are ubiquitous members of most forest communities, where they disperse seeds, prey on other species, and influence the flow of nutrients. Their effects are often described as substantial, but few studies to date have simultaneously examined how the presence of ants affects both above and belowground processes. In this study, we experimentally reduced ant abundance in a suite of deciduous forest plots in northern Georgia, USA to assess the effects of ants on the spatial distribution of a common understory plant species, Hexastlylis arifolia, the structure of soil mesofaunal communities, and soil nitrogen dynamics. Over the course of several years, the removal of ants led to significant spatial aggregation of H. arifolia seedlings near the parent plant, most likely due to the absence of the keystone seed dispersal species, Aphaenogaster rudis. Seedling emergence was higher in ant removal plots, but seedling aggregation did not affect first or second year seedling mortality. Ammonium concentrations were 103 higher in ant removal plots relative to control plots where ants were present in the first year of the study, but this increase disappeared in the second and third years of the study. The effects of ant removal on the soil mesofauna were mixed: removal of ants apparently did not affect the abundance of Collembola, but the abundance of oribatid mites was significantly higher in ant removal plots by year two of the study. Taken together, these results provide some of the first experimental evidence of the diverse direct and indirect effects of ants on both above and belowground processes in forest ecosystems and demonstrate the potential consequences of losing an important seed dispersing ant species for the plants they disperse.