Eco-evolutionary experience outweighs soil effects in shaping Senecio inaequidens invasion success

Introduced species encounter novel biotic and abiotic conditions that influence their success in new environments. Their advantages often arise from reduced competition with species lacking eco-evolutionary experience and from their capacity to preempt or efficiently use resources. Once established, their success can also be shaped by interactions with soil microbial communities. Understanding how these factors influence invasion success can provide valuable insights into identifying the mechanisms that enable certain species to become dominant in a plant community. In this study, we examined how eco-evolutionary experience and invaded-range soil bacterial communities modulate the performance of the invasive subshrub Senecio inaequidens DC. We conducted a fully factorial additive experiment in growth chambers, with S. inaequidens individuals growing at the center of each pot. The design included two factors: (1) competitor community identity with three levels (i.e., plant species from the native range (South Africa), species from the invaded range (Italy), and a control with no competitor species); and (2) soil biotic conditions with two levels (i.e., autoclaved soil with reduced microbial load and non-autoclaved soil). Our results showed that plant community identity had the strongest effect on S. inaequidens growth (height and lateral spread), with the smallest individuals occurring in competition with species from the native range. Growing on autoclaved soil had no major impact on plant height, suggesting that soil microbial communities played a minor role modulating competitive interactions, which were the dominant driver of plant growth responses. Suppression was strongest when competitors were phylogenetically closer to S. inaequidens , especially those from its native range. Soil bacterial communities were influenced by both competitor community identity and soil treatment, with lower bacterial diversity found in pots with better Senecio plant growth. These results indicate that promoting competition with closely related natives and maintaining diverse soil microbiota may limit invasion success.