Incorporating the “Invisibles” in Ant Seed Dispersal: Microbial Mortality Agents in Myrmecochory

Ant-mediated seed dispersal, myrmecochory, is a diffuse mutualism in which ants are rewarded for seed dispersal services with food via a seed-coat derived appendage, the elaiosome. Seeds gain dispersal benefits including escape from distance- and density- dependent mortality agents, protection from seed predators, and a nutrient-rich germination site in or near ant nests. However, microbes, have been essentially overlooked in this mutualism, despite their pathogenicity to the ants and plants involved. The work presented here investigates the effects of microbial mortality agents on the risks and benefits offered to ant and plant partners in myrmecochory. First, I investigate the effects of a plant-produced chemical on the growth of a ubiquitous entomopathogen and on ant survival and foraging. Then, I investigate ant effects on the microbiomes associated with seed dispersal location and the seed coat. While a plant-produced chemical, sanguinarine, promotes the growth of an ant pathogen at certain concentrations, it did not influence ant survival or foraging patterns. These results suggest that though plant chemicals may increase pathogenic risk to ants in myrmecochory, ant social immune behaviors help counteract this threat. Ant activity influences both the ant nest and the seed coat microbiome. Increasing or stable ant activity in ant nests reduces turnover of 1) the plant pathogenic fungal community in soil substrates and 2) the myrmecochore pathogenic fungal community in log substrates. The handling of seeds by ants also alters the seed coat microbiome. Though seed species also influences the seed coat bacterial and fungal communities, ant handling and the removal of the elaiosome decreases the alpha diversity of plant pathogens on the seed coat. These results suggest that seeds are provided with additional benefits in myrmecochory in the form of pathogen defense in the ant nest environment and with ant handling. These findings, overall, suggest the importance of including microbial communities in the consideration of the risks and rewards in ant seed dispersal. Generally, this dissertation advances the field of ecological interactions by extending studies of a pairwise mutualism to include ecological community members that act as mortality agents for mutualistic partners.