Which factor contributes most to the invasion resistance of native plant communities under the co-invasion of two invasive plant species?

Two invasive plant species (IPS) can co-invade the same plant community. As the number of IPS increases under the co-invasion of two IPS, plant taxonomic and functional diversity, community invasibility, community stability, invasion resistance, and invasion intensity and invasiveness of IPS and their interrelationships may be altered. This study aimed to quantify the contribution of plant taxonomic and functional diversity, community invasibility, community stability, and invasion intensity and invasiveness of IPS to the invasion resistance of native plant communities under the co-invasion of the two IPS Erigeron annuus (L.) Pers. and Solidago canadensis L. in eastern China. This study also defined a method to quantify the invasion resistance of native plant communities designated the invasion resistance index. The community-weighted mean trait values of native plants and plant diversity are the factors that are the most critical to determine the invasion resistance of native plant communities. Thus, the invasion resistance of native plant communities primarily depends on the three following factors: the relative abundance of natives, the growth performance of natives, and the diversity of natives. All levels of invasion significantly decrease the invasion resistance of native plant communities. The two IPS antagonistically affect the invasion resistance of native plant communities less under co-invasion compared with their independent invasion.