The invasion of alien species has major ecological, social and economic consequences, and the ability to predict potential invaders and their ecological impacts has become a central challenge. Darwin's naturalization hypothesis, as one of the earliest hypotheses on biological invasions, suggests that invaders should be less successful in communities that contain their close relatives. The rationale is that the invaders and their closely related residents often share similar traits, occupy similar niche, and compete more strongly, reducing the success of invaders. Recently, this classic niche-based scenario has been challenged by modern species coexistence theory, which suggests the outcome of invasion depends on both niche difference and relative fitness difference between the invader and resident species. However, how niche and fitness differences relate to species invasion, and their association with phylogenetic patterns, have not yet been explicitly tested. To address this knowledge gap, we conducted a bacterial microcosm experiment. We established resident communities using eight resident species, and then challenged them by three invading species, to assess the effect of phylogenetic distance, niche and relative fitness differences on invasion success and impact.
We found that invaders could establish better and become more successful in communities with larger niche differences, whereas invaders tend to have stronger impacts on the resident communities which showed larger fitness disadvantages to the invaders. Therefore, invasion success mainly depended on niche difference, while invasion impact mainly depends on fitness advantage of the invader. Further, if the niche and fitness differences of the invader relative to the resident species showed significant phylogenetic signal, phylogenetic relatedness is a good predictor of invasion success and impact. In contrast, if the niche and fitness differences was not related to phylogenetic relatedness, invasion success and impact did not show clear phylogenetic patterns. Together, our results reveal that the merging of phylogenetic ecology with modern coexistence theory could help us to gain a more mechanistic understanding of species invasion and its consequence for the invaded ecosystems.