Microplastics as habitat-dependent ecological filters: facilitating plant invasion in water while reinforcing biotic resistance on land

Background and aims Global plastic pollution and biological invasions are two defining features of the Anthropocene, yet their interactive effects across different ecosystems remain poorly understood. We investigated whether microplastics act as context-dependent ecological filters that differentially influence the performance of the invasive Alternanthera philoxeroides and its native congener A. sessilis across aquatic and terrestrial habitats. Methods In a fully crossed factorial experiment, both species were grown independently and exposed to three polymer types—polyethylene (PE), nylon (PA6), and biodegradable polylactic acid (PLA)—across two particle sizes (micro and macro) under both aquatic and terrestrial conditions. These treatments were applied to evaluate habitat-specific responses. Growth performance was quantified using biomass accumulation and relative effect sizes. Results A striking habitat-switch in growth performance patterns was uncovered. In aquatic environments, microplastics acted as an "invasion catalyst": they significantly stimulated the biomass accumulation of the invader A. philoxeroides —particularly under macro-PE and micro-biodegradable treatments—while suppressing the native A. sessilis, especially under nylon exposure. In sharp contrast, terrestrial soil pollution reinforced biotic resistance: microplastics generally inhibited the invader's growth while unexpectedly facilitating the native species, effectively reversing the performance hierarchy. Conclusions Our findings indicate that microplastics function as habitat-specific modifiers of plant growth rather than uniform environmental stressors. By differentially influencing invasive and native plant species in aquatic versus terrestrial systems, plastic pollution may alter vegetation dynamics across ecosystems. These results highlight the importance of considering habitat context and plastic characteristics when evaluating ecological impacts of microplastics in invaded environments.