Functional and phylogenetic similarity of alien plants to co-occurring natives.

Over the years, several arguments have been proposed to explain the invasibility of a given community based on the properties of the recipient community. Here, I assessed whether the balance between native species' phylogenetic and functional variability determines vulnerability to invasion. I explored this hypothesis using a consensus phylogenetic tree and a database of leaf, height, and seed traits of alien and native species co-occurring over 83 sites worldwide. An analysis of contrasts between aliens and natives indicates that aliens are as phylogenetically close to the incumbent native community as natives are among themselves (aliens are nested within the native community phylogeny), but functionally distinct to the native community (aliens are more functionally distant to the community of native taxa than natives are among themselves). These contrasting trends are consistent for different comparison criteria (comparisons to all natives or to the nearest native) and comparisons both within and across communities, habitats, and continents. Furthermore, aliens are more functionally divergent than the native community and the closest native relative in both phylogenetically poor and rich communities. The phylogenetic similarity and functional distinctiveness of aliens with respect to the incumbent native community may explain why certain species succeed in some communities and not others. This is a step forward in resolving the long-standing debate on the role diversity--both phylogenetic and functional--plays in determining the success of introduced plants.