Niche dynamics of alien plant species in Mediterranean Europe

Aim Humans have spread plants globally for millennia, inadvertently causing ecological disruptions. However, biological invasions also provide a unique opportunity to study the process of niche dynamics, through which species adapt their niche when confronted with novel environments. Focusing on the Mediterranean Basin, we assessed 1) which traits favour niche dynamics, and 2) whether niche conservatism or niche shift promotes invasion success. Location Mediterranean Europe and the World. Methods We selected the 85 most widespread alien vascular plants in Mediterranean Europe and compiled data on their distribution in their native and invaded ranges. We then tested how a species' residence time, biogeographic origin, dispersal ability, functional traits and intraspecific trait variability (ITV) influence its niche dynamics following invasion. Using already published independent data, we finally assessed whether niche dynamics can explain different dimensions of invasion success (such as regional spread or local abundance). Results We found that niche shifts were common (71% of species) and were mostly driven by species failing to occupy all suitable environments of their invaded range (unfilling), regardless of residence time. Niche unfilling and niche expansion were more important in species with high intraspecific trait variability introduced from non-mediterranean biomes (temperate or tropical). Niche expansion was also greater in species with long-distance dispersal, bigger seeds and a narrow native niche. Interestingly, invasion success correlated more with a species’ ability to conserve its niche and residence time than with niche expansion. Main conclusions Niche shifts were better predicted by species traits than residence time. For example, high adaptive potential (inferred from high intraspecific trait variability) favoured niche shifts in general, and long-distance dispersal favoured niche expansion. Understanding how these traits relate to niche dynamics is important since a species' ability to conserve and fill its niche is in turn a good predictor of invasion success.