Responses of non-native earthworms to experimental eradication of garlic mustard and implications for native vegetation

Recent studies in invasion biology suggest that positive feedback among two or more introduced organisms facilitate establishment within a new range and drive changes in native plant communities. Here, we experimentally tested for relationships between native plants and two non‐native organisms invading forest habitats in North America: garlic mustard (Alliaria petiolata, Brassicaceae) and earthworms. In two forested sites, we compared understory vegetation and earthworm biomass in plots where garlic mustard was removed for three years, plots without garlic mustard invasion, and plots invaded by garlic mustard that was not removed. Earthworm biomass was highest in the plots with garlic mustard, and long‐term eradication of garlic mustard reduced earthworm biomass to levels similar to those observed in the uninvaded control plots. Invasion treatment, and the interactions between earthworm biomass and treatment, explained most of the variation in plant community composition and diversity—suggesting that earthworms alone do not necessarily drive forest understory floristic patterns. In contrast to broader geographic patterns indicating earthworms as the main driver of vegetation change in the presence of non‐native plants, we show that garlic mustard solely, or in conjunction with earthworm biomass, drives changes in native plant composition and diversity at the scale of individual forests. From a local management perspective, our data suggest that garlic mustard eradication can directly assist in the conservation of native plant communities and simultaneously reduce earthworm biomass