Background/Question/Methods   The idea that alien species with close indigenous relatives in the introduced range may have reduced chances of success (Darwin's naturalization hypothesis) has a long history in ecology, and overlaps conceptually with modern ideas of niche theory and phylogenetic structuring of community organization. Bringing together a leaf, height and seed traits data database spanning 7108 species (5286 with measures in the native, 1045 in the introduced and 518 on both ranges) co-occurring over 404 sites, we evaluated how successful introductions are influenced by scale, niche overlap and taxonomic relatedness. For this, traits of co-occurring native and alien congeneric and confamilials were compared using both uni- and a multi-dimensional approaches (SMA regressions and convex hulls derived metrics respectively).

Results/Conclusions   We show how the level of niche similarity increased with both increased scale and relatedness. This pattern was consistent across plant habits, growth forms and scales. Ours results indicate that closely related species may share traits that pre-adapt them to the new environment, or may increase mutualistic or facilitative interactions, contradicting Darwin's naturalization hypothesis predictions. We believe that using taxonomic or phylogenetic relatedness of aliens to natives has a large potential to identify possible invasive plant which might threat native communities. Specifically, we suggest that particular attention should be paid to newly introduced species for which there are close relatives with similar ecological attributes in the recipient native community.