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

Freshwater ecosystems are increasingly exposed to multiple global change drivers such as nutrient enrichment and emerging pollutants, which significantly influence biological invasions. Although high invader density is known to enhance invasion success, its consequences for native community biomass and diversity under 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 vs. two individuals), nutrient enrichment (enriched vs. unenriched), and microplastic pollution (presence vs. absence) on invasion success and native community biomass and diversity. High invader density substantially increased both the absolute and relative biomass of invasive plants (by 75.9% and 32.1%, respectively), and this effect was consistent across nutrient and microplastic treatments. In contrast, native community responses were highly context-dependent. Under nutrient enrichment without microplastics, high invader density enhanced both native biomass and diversity, whereas under low nutrient conditions it reduced native biomass without affecting diversity. When nutrient enrichment and microplastics co-occurred, high invader density decreased native diversity while leaving biomass unchanged; under low nutrient conditions with microplastics, it increased diversity but had no effect on biomass. These findings demonstrate that modest increases in initial invader density can strongly influence invasion success, whereas consequences for native communities depend on the interplay of multiple biotic and abiotic factors, underscoring the multi-factorial nature of plant invasions in changing environments.