Competitive ability and genetic diversity are factors frequently consulted to explain plant invasion success. However, their relative importance and interaction have rarely been addressed experimentally. Testing explicitly for main and interactive effects of genetic diversity in plant establishment on a population level is especially challenging, as it is either necessary to use single defined genotypes or manipulate genetic diversity experimentally by explicitly composing mixtures of populations based on genetic distances among them.
Making use of a next-generation sequencing method (ddRADseq) we determined the pairwise genetic distances within 22 invasive (Pacific Northwest) and 24 native (Central Europe) populations of Jacobaea vulgaris GAERTN. on a population level. Based on this information, we created 15 mixtures following a gradient of increasing levels of genetic diversity each per region of origin.
In a common garden experiment, we assessed the performance of mixtures in three different environments of increasing interspecific competition as induced by a matrix organism. In a total set of 498 pots, we repeatedly monitored number and size of individuals. We expected to find significant positive effects of genetic diversity on the mean establishment success and tested for significant interactions indicating more or less pronounced effects for invasive genotypes if compared to natives.
Results/Conclusions
Overall, population mixtures of invasive origin had a higher establishment success, whereas native mixtures had more larger individuals early on in the experiment. At the end of the growing seasons, mixtures of invasive origin experienced a faster decline in individual numbers, approaching the levels of native mixtures.
Higher levels of interspecific competition increasingly impaired the successful establishment of individuals both in numbers and in size. We found no significant difference between mixtures of native or invasive populations, while the role of interspecific competition might become more important over time.
Overall, genetic diversity in the invaded range was reduced if compared to the populations sampled in the native range. In the native mixtures, increasing genetic diversity resulted in higher numbers of established individuals, whereas no such effect was observed for the mixtures of populations of invasive origin. However, invasive mixtures had overall higher numbers of established individuals than the native mixtures independent of genetic diversity.
Our results suggest that invasive populations might have undergone selection processes during exotic range expansion favoring higher performing genotypes. While in native population, apparently, genetic diversity may foster plant colonization success, in the invasive range, the genetic diversity is of minor importance for successful establishment.