High invader density alone drives invasive plant dominance, but its impacts on native community biomass and diversity depend on nutrients and microplastics

Freshwater ecosystems, which are increasingly exposed to multiple global change drivers such as nutrient enrichment and emerging pollutants, which can influence biological invasions. While high invader density is known to increase the likelihood of invasion success, the consequences of this increased invader density for the diversity and productivity of native plant communities under these interacting stressors remain insufficiently explored. We conducted a factorial mesocosm experiment using four invasive aquatic macrophytes and five native plant communities to assess the individual and interactive effects of invader density (one or two individuals), nutrient enrichment (enriched or unenriched), and microplastic pollution (presence or absence) on invasion success, as well as on native community biomass and diversity. High invader density substantially increased both the absolute and relative biomass of invasive plants (+75.9% and +32.1%, respectively), and this effect was consistent across nutrient and microplastic treatments. In contrast, responses of native communities varied: under enrichment without microplastics, high invader density enhanced both biomass and diversity, whereas under low nutrients it reduced biomass without affecting diversity. When enrichment and microplastics co-occurred, high invader density decreased diversity while leaving biomass unchanged, while under low nutrients with microplastics, it increased diversity but had no effect on biomass. These findings show that modest increases in initial invader density can strongly influence invasion success, whereas the consequences for native communities arise from the interplay of multiple biotic and abiotic factors, underscoring the multi-factorial nature of plant invasions.