An Established Plant Invader May Still Benefit From Increasing Genetic Diversity—Insights From Artificial Populations in a Common Garden Experiment

Genetic diversity and competitive ability, though extensively studied in the context of biological invasions, are still poorly understood in their relative importance, especially when shifting the perspective from an individual plant's phenotype to overall population performance. Most approaches addressing the role of genetic diversity involve the comparison of standing genetic variation in field populations combined with experimental treatments on individual plants. Composing predefined mixtures of populations to manipulate genetic diversity would be an experimental approach to test for direct effects on population performance. We determined pairwise genetic distances among 16 invasive and 22 native populations of <i>Jacobaea vulgaris</i> GAERTN. using single nucleotide polymorphisms (SNP). Based on this information, we created each 15 de-novo populations with different levels of genetic diversity for both origins. These de-novo populations were subjected to three levels of decreasing microsite availability by using a matrix of either 0, 5, or 10 individuals of <i>Festuca rubra</i>. We monitored population performance continuously throughout two growing seasons to study effects of interactions between origin, microsite availability, and genetic diversity with (generalized) linear effects models. This allowed us to uncover whether the relative importance of those factors varies with the life-stage of this biennial species. We found no ambiguous patterns on the hypothesized beneficial effect of genetic diversity for <i>J. vulgaris</i> populations. Native populations tended to respond negatively to increasing genetic diversity, especially under more favorable site conditions, but this was not a persistent pattern and was only evident through continuous monitoring. Invasive populations could benefit from increasing genetic diversity during early establishment, but not in interaction with restricted microsite availability. Our results do suggest that genetic variation supports population establishment and performance under certain environmental conditions. Therefore, for recommendations in nature conservation, efforts should still aim at limiting propagule addition in already invaded areas, even in well-established invasive species.