Trait interactions help explain plant invasion success in the German flora

1. The search for characteristics that promote invasion success constitutes one of the most challenging tasks in invasion ecology. So far the main focus in multispecies studies of plant invasion success has been on single traits. Only few generalizations have emerged from this work and single traits yielded very limited explanatory power for invasion success. Here we hypothesize that the consideration of ecological strategies, determined by different combinations of traits, will improve explanatory power. 2. To test this hypothesis we analysed the relative importance of 40 traits for species' invasion success in the German neophytic flora. Success was expressed as map grid cell frequency. After quantifying the relevance of single traits, we quantified the importance of different trait combinations for invasion success by calculating a multiple trait model, with explicit consideration of trait interactions. In all analyses we considered the effects of phylogeny. 3. In general, neither single traits nor phylogenetic relatedness held much explanatory power. In contrast, the amount of variation explained in the multiple trait model was distinctly higher, mainly due to the incorporation of trait interactions. Thus, particular combinations of trait attributes rather than distinctive attributes per se appear to be associated with invasion success. 4. In single trait analysis, traits associated with flowering and reproductive biology, and with ecological tolerance and the residence time of the species were significantly associated with invasion success. Multiple trait analysis revealed that the relationship between the length of flowering season and invasion success was contingent upon pollination modes. Moreover, the success of polyploids and of species with certain vegetative reproductive trait attributes depended on the species' flowering phenology. 5. Synthesis. Our results indicate that different ecological strategies, determined by particular combinations of traits, can facilitate plant invasion success. Our findings highlight the importance of incorporating trait interactions when testing for characteristics that promote plant invasion success. Improved explanatory power of traits suggests that our new approach can provide an important step forward in the risk assessment and management of new arrivals in regional floras.