The context-depency of biological invasions makes it difficult to understand why some species become succesfull invaders and others not. Such understanding requires studying closely-related invasive and non-invasive alien taxa sharing the same introduction history in the same environment. We identified this unusual situation in Kenya where the individuals that founded invasive Prosopis juliflora and non-invasive P. pallida populations are still present in original plantations. We evaluated field-measured traits, conducted greenhouse experiments simulating different nitrogen and water availability treatments, and did reciprocal transplants to compare functional traits and plasticity between the founders of both species (i.e. ‘invasive-non-invasive congeners comparison), and between P. juliflora individuals from plantations and invaded sites (i.e. testing for rapid evolution during invasion). We found that planted individuals of P. julifora and P. pallida differed in a number of key traits related to performance and spread (root:shoot ratio, number of stems and susceptibility to seed damage) as well as in levels of phenotypic plasticity in growth responses to resource availability, which may explain their differential invasiveness at species level. Offspring of invasive P. juliflora individuals had higher seed mass and production, germination, survival, produced more stems, matured earlier and had higher plasticity compared with those of founder individuals, indicative of rapid post-introduction evolution. By using this exceptional study system, we show that differences in values of only a few key traits, increased phenotypic plasticity, and post-introduction evolution have all contributed to the success of P. juliflora as an invasive species in Kenya.

Reciprocal transplant in the field and manipulative glasshouse experiments where performed on Prosopis juliflora and P. pallida in Kenya. These data were used to infer the contributions of phenotypic plasticity and rapid evolution to the invasiveness of these species. Please see Castillo et al. 2021 (Ecography) for full details of data collection and analyses.