Plant invader alters soil food web via changes to fungal resources.

While aboveground impacts of invasive plants are well documented, their influence on soil food webs remains less understood. Previous research has revealed that bottom-up forces are widespread in soil food webs of woodlands. Thus, an invasive plant that negatively impacts the base of the food web will likely decrease primary consumers as well as their predators. We examined how a North American plant invader, garlic mustard (Alliaria petiolata), affects arthropod primary (springtails and oribatid mites) and secondary (predaceous mites) consumers of the soil food web via changes to fungal resources. We measured the abundances of plants, soil fungi, fungivores, and predators in garlic mustard-invaded and uninvaded 1-m² plots in five Midwestern USA woodlands. We then conducted a mesocosm (0.25-m² plots) experiment to tease apart the direct and indirect effects of garlic mustard by manipulating plant identity (garlic mustard vs. native plant), soil history (invaded vs. uninvaded), and fungicide application (fungicide vs. no fungicide). Our first study revealed that plots without garlic mustard had 2.8 and 1.4 × more fungi and fungivores, respectively. Predator densities did not differ. Fungal composition and structural equation modeling (SEM) revealed the garlic mustard effects on fungivores were correlated with fungal declines. The mesocosm experiment confirmed that the impacts were indirect, as fungicide plots harbored similar fungivore densities, whereas fungivore densities differed according to plant identity and soil history in the fungicide-free plots. Our results reveal that by altering soil fungal abundance, an invasive plant can indirectly affect primary consumers in soil food webs, but this indirect effect does not influence predators.