The impact of a biological invasion on native communities is expected to be uneven across invaded landscapes due to differences in local abiotic conditions, invader abundance, and traits and composition of the native community. One way to improve predictive ability about the impact of an invasive species given variable conditions is to exploit known mechanisms driving invasive species’ success. Invasive plants frequently exhibit allelopathic traits, which can be directly toxic to plants or indirectly impact them via disruption of root symbionts, including mycorrhizal fungi. The indirect mechanism – mutualism disruption – is predicted to impact plants that rely on mycorrhizas but not affect non-mycorrhizal plant species. To assess whether invader-driven mutualism disruption explains observed changes in native plant communities, we analyzed long-term (1998–2018) plant cover data from forest plots across the state of Illinois. We evaluated native plant communities experiencing a range of abundance of invasive allelopathic garlic mustard (Alliaria petiolata) and varying environmental conditions. Consistent with the mutualism disruption hypothesis, we showed that as garlic mustard abundance increased over time in 0.25 m² sampling quadrats, the abundance of mycorrhizal plant species decreased, but non-mycorrhizal plant species did not. Over space and time, garlic mustard abundance predicted plant abundances and diversity at the quadrat level, but this relationship was not present at a larger scale when quadrats were aggregated within sites. Garlic mustard’s impact on the plant community was highly localized, yet it was as important as abiotic variables for predicting local plant diversity. We showed that garlic mustard abundance was a key predictor of patterns of plant diversity across invasion intensity and environmental heterogeneity in a way that is consistent with mutualism disruption. Our work indicates that the mutualism disruption hypothesis can provide generalizable predictions of the impacts of allelopathic invasive plants that are evident at a broad spatial scale.

These data were collected by the Illinois Critical Trends Assessment Program (CTAP) of the Illinois Natural History Survey (Carroll et al. 2002). This program was established to quantify the conditions and ecological trends of three terrestrial plant habitat types (forests, wetlands, grasslands) in Illinois. We used plant survey data from the CTAP forest ground cover sites collected from 1998 to 2018. Sites were resampled on a 5-year rotation, meaning that ~1/5 of the plots were surveyed each year. If a site no longer met the CTAP criteria for inclusion in the monitoring program when resurveyed, a new, unassociated site elsewhere in the township was established to replace it. Because many plots are located on private land, latitude and longitude were rounded to 2 decimal points for privacy. Original dataset available by request from https://publish.illinois.edu/ctap/inhs/.

We used the species-specific ground cover data (<1m in height) for this study. At each site, ground cover sampling occurred at 30, 0.25 m² quadrats distributed every five meters along three permanently marked 50-m transects (10 quadrats per transect), that radiated out from a randomly selected center point in randomly selected, nonoverlapping directions. Ordinal plant cover classes (<1%, 1–5%, 5–25%, 25–50%, 50–75%, 75–95% and 95–100%) were recorded for each vascular plant species in each quadrat. Methods are further detailed in Carroll et al. (2002). We used the midpoint of cover classes for our analyses.

We included absences only for those species that had occurred in a site or quadrat in at least one other sampling time point to account for losses or additions of species over time. Since mycorrhizal association types are often consistent within plant families, we used established plant family mycorrhizal associations (Brundrett and Tedersoo 2019), to determine mycorrhizal status (i.e., whether a plant species forms mycorrhizal associations, and if so, with what type of mycorrhizal fungi) of all plant species in the CTAP ground cover dataset. We assigned these plant species to one of four mycorrhizal categories following Brundrett and Tedersoo (2019): arbuscular mycorrhizal (AM), facultative arbuscular mycorrhizal (NM-AM), ectomycorrhizal (EM), and non-mycorrhizal (NM).