Co-occurring invasive plant and propagule pressure accelerate invasion, but only multiple species invasion changes ecosystem multifunctionality

The understanding of the interactions between individual invasive plant species in biological invasion is currently limited. Specifically, the impact of both invasive plant richness (interaction between invasive species) and abundance (propagule pressure) is likely to be crucial in determining invasion outcomes and their potential effects on ecosystem functions and multifunctionality. Here, we conducted a mesocosm experiment to assess the effects of different levels of invasive plant richness and abundance on invasion success, community properties, ecosystem functions, and multifunctionality. We show that both invasive plant richness and abundance promoted invasion success, primarily due to the allelopathic effects on native plants. We also observed invasive plant richness had different effects on ecosystem functions, and an initial increase followed by a subsequent decrease in ecosystem multifunctionality as invasive plant richness increased, with no significant impact from invasive plant abundance. This suggests that the interaction between invasive plants plays a more important role in the impacts on ecosystem functions and multifunctionality than propagule pressure. Furthermore, plant invasion indirectly affected multifunctionality by impacting species diversity indices and environmental factors. These results highlight the potential for aggravated biological invasion and intensified ecosystem damage in the future as multiple invasive species co-invade.