Impact of native community diversity and plant-soil feedbacks on invasiveness in experimental temperate grasslands varies among invasive species.

Plant-soil feedbacks (PSFs) are a key mechanism regulating species dynamics in plant communities, typically negative due to the buildup of species-specific pathogens. Species-rich communities experience less negative PSFs than species-poor ones due to pathogen dilution, making PSFs a proposed mechanism behind the positive biodiversity-productivity relationship. PSFs may also benefit invasive species through the loss of their pathogens enhancing competitiveness according to the enemy release hypothesis (ERH). We hypothesized that invasive species would benefit from enemy release in species-poor communities, but less in species-rich ones where the advantage of enemy release disappears due to pathogen dilution. Accordingly, we expected that in sterilized soils, community diversity would not affect invasion success. We conducted a greenhouse experiment measuring the invasion success of three invasive forbs (Solidago gigantea, Avena sterilis, and Lupinus polyphyllus) in plant communities of varying richness (1, 2, 4, 8, and 12 species) in both unsterilized and sterilized soils. In unsterilized soils species diversity had a significant positive impact on native community biomass, while S. gigantea showed a clear negative effect from this increased competition at higher species diversities, as predicted. In sterilized soils these effects were strongly reduced. Growth of the two other invasive species was not affected by species diversity of the native community, showing that this mechanism may be species-specific. These findings support the ERH but only when exotic plants invade species-poor communities. Furthermore, our results suggest that the greater resistance of diverse plant communities to invasion may be at least partly be ascribed to increased competitive strength of the native community due to pathogen dilution.