Origin matters: Dissecting impact of plant invaders in the home vs. introduced range
Knowledge from basic plant ecology suggests that impact of one plant species on another is either due to competition for the same limiting resources, and/or by unique plant traits. Potential drivers underlying these two processes might be context specific, explaining a differential impact of exotic plant invaders in their home vs. introduced range. To further our knowledge on such drivers, we focused on the effects of neighboring plant species and community, soil biota and invader genotype as these biotic/genetic factors have been repeatedly put forward to explain invasion success. With the help of a conceptual framework, we aimed at identifying the impact type of the invasive spotted knapweed Centaurea stoebe and assessing relative importance of those drivers in explaining the high impact of C. stoebe during the invasion of new sites in the introduced range. We conducted several greenhouse and mesocosm competition experiments using European (old) and North American (new) neighboring species growing with European (EU) and North American (NA) C. stoebe in either EU vs. NA soil.
Our greenhouse experiments showed a significantly higher impact of C. stoebe on new than old neighbors. Interestingly, biomass of C. stoebe explained a substantial amount of variation in biomass of co-evolved neighbors, but not of new “naïve” neighbors. Thus, the impact at home appears to be driven by competition for the same resources, but by other factors in the introduced range, e.g. exploitation of resources that are not used by new neighbors and/or interference competition. Soil origin and C. stoebe origin had no effect on the knapweed’s impact. Soil sterilization increased growth of both C. stoebe and neighbors, but had only moderate effects on impact of C. stoebe. Our results suggest that during colonization of new sites in NA grasslands, impact of C. stoebe is strongly driven by inherently different mechanisms underlying competitive interactions with its new than old neighbors, while altered biotic soil conditions in the introduced range and post-introduction evolutionary changes in invader are of less importance. These findings, well in line with results from a mesocosm experiment under semi-natural conditions, have important consequences for management, as in our study, ecosystem recovery in the introduced range is less likely after simple biomass reduction.