Impacts of the invasive herb garlic mustard (Alliaria petiolata) on local ant (Hymenoptera: Formicidae) communities in northern temperate forests

Garlic mustard, an invasive shade-tolerant species introduced to North America from Eurasia in the late 1860s, now is widely distributed throughout the US and Canada. The presence of garlic mustard results in displacement of understory species and subsequent decline in native plant diversity. By displacing native plants, garlic mustard can affect resource availability and habitat quality, and thereby affect animals from different trophic levels. Although hypothesized, these impacts have been infrequently documented. Our study focused on the small-scale effects of garlic mustard invasion on an abundant and important group of forest-floor arthropods. We evaluated the effects of garlic mustard on ground-dwelling ant assemblages in invaded and non-invaded areas of two mesophytic forests of northeastern Ohio. Ants were collected from May to August 2008 via Winkler litter extraction in five invaded and five non-invaded plots for each site. Compared to uninvaded plots, plots invaded by garlic mustard showed significant reduction in leaf litter depth, and an increased abundance of nonnative earthworms. Sample-based rarefaction and similarity analyses revealed that the absence or presence of garlic mustard, and the associated decrease in the leaf litter, had no detectable effect on the observed and expected ant species richness and community composition. Rank-abundance distributions were also largely unchanged by the presence of garlic mustard. Our results suggest that regional sylvan ant communities are unaffected by the generally presumed negative effects of garlic mustard invasions, or that these effects may be more subtle or confounded by other dominating factors.