Competitive exclusion from native plants and co-occurring exotic plants was the most important factor influencing plant invasion in freshwater ecosystems

Few studies have evaluated the relative importance of various factors in the invasion process of exotic plants in freshwater ecosystems due to the difficulty of observing numerous factors simultaneously. In this study, to explore how various biotic and abiotic factors determine the overall invasion extent of all exotic plants and the invasion extent of different life-form exotic plants as well as assess their relative importance, we surveyed 236 exotic aquatic plant communities using 2267 fine quadrats in China's freshwater ecosystems. We found that competition from native plants was the most vital factor determining the mean biomass of all exotic plants and biomass of submerged plant Cabomba caroliniana, while competition from co-occurring exotics was the most important for biomass of emergent plant Alternanthera philoxeroides and free-floating plant Eichhornia crassipes. The population biomass of different exotic species responded differently to climate change. Water eutrophication could accelerate the invasion of exotic plants by directly favoring them and indirectly weakening the resistance of native plants. Water depth, habitat size, herbivory, and anthropogenic disturbance had relatively weak impacts on the biomass of exotic plants. Moreover, some factors have different modes of influence on different exotic plants. Our study suggested interspecific competition plays a more important role in the population spread of exotic plants than climate and abiotic environment once the plants have successfully established, implying that biodiversity conservation and vegetation restoration were the fundamental methods to control invasion. In addition, our study highlights the importance of studying the overall invasion extent of all exotic plants and interactions among invaders in multi-invader communities.