Abstract: The evolution of increased competitive ability (EICA) hypothesis posits that invasive species in their introduced ranges exhibit reduced defense but enhanced growth and competitive ability. However, these three core predictions are rarely evaluated simultaneously, and the divergent responses of introduced versus native populations to competition and pathogens remain poorly understood. To address these gaps, we conducted a common garden experiment using 10 populations of Phytolacca americana from its introduced (China) and native (North America) ranges to assess trait differences and phenotypic plasticity in response to intraspecific and interspecific competition and pathogen attack. Our results revealed that, compared to native populations, introduced ones prioritized reproductive allocation over total biomass and exhibited enhanced competitive tolerance rather than suppression. Introduced populations displayed a markedly higher disease index than natives under no competition, while no significant differences between them were observed under inter- or intraspecific competition. Furthermore, introduced populations exhibited reduced plasticity in response to competition but enhanced plasticity in response to pathogen attack. In conclusion, our study demonstrates that P. americana inherently possessed strong growth ability; during invasion, its competitive tolerance and reproductive ability were enhanced, whereas its defense against pathogens declined, offering a new perspective on EICA. This study also highlights the importance of phenotypic plasticity in response to competitors and pathogens.

Key words: enemy release, pathogen accumulation, phenotypic plasticity, competitive ability, biological invasion