1. Although invasive plants increasingly contribute to the current biodiversity crisis, the mechanisms through which they impact native communities are still poorly understood. Community ecology theory has emphasized direct competitive displacement over common resources, but invasion-driven ecosystem modifications, such as altered soil pH, might also have consequences for plant diversity. However, the relative importance of ecosystem modification compared to direct resource competition has rarely been tested.

2. Here we studied the invasive vine Vincetoxicum rossicum across invaded meadows in southern Ontario, Canada. In each meadow site, we quantified: 1) the strength of impact on the resident plant community, 2) the potential for competition with resident species (as the degree of niche-dissimilarity and competitive superiority to the residents based on their functional traits), and 3) the amount of ecosystem modification related to invasion.

3. We found that impacts on plant biodiversity were more negative where the invader had greater potential to competitively displace species (because it had a similar niche as the residents or was competitively superior), but also where it strongly altered soil N pools, moisture and pH.

4. Synthesis. Our case study suggests that, while competition is undoubtedly an important driver of invasion impact, ecosystem modifications can have cascading effects on plant communities, thereby magnifying the impacts of biological invasions.