Invaders among locals: Alien species decrease phylogenetic and functional diversity while increasing dissimilarity among native community members

Abstract

Plant invasions can drastically change the structure of native communities, but it is not fully understood whether alien species occupy phylogenetic and functional space within the range occupied by natives, or provide a novel set of evolutionary origins and traits to the invaded communities. Here, we evaluated this open question with data on a large number of plant communities from different terrestrial habitats. We used ~27,000 vegetation plots from 26 terrestrial habitats in the Czech Republic and compared phylogenetic and functional diversity (PD and FD, respectively) and community trait means in invaded and non-invaded plots. We tested for differences (1) between invaded vs. non-invaded plots, (2) among natives in invaded vs. non-invaded plots, and (3) in invaded plots only, with and without aliens. To minimize habitat filtering effects on PD and FD, we ran these tests within the habitat-specific species pools of the 26 vegetation types. In general, PD, FD and trait mean values changed with invasion, with changes being rather consistent across the habitats considered. Invaded plots were less phylogenetically, but more functionally diverse than non-invaded plots. The greater FD in invaded plots, compared to non-invaded ones, was due to greater dissimilarity between natives. In fact, native species in invaded plots showed higher PD and FD than native species in non-invaded plots, while alien species reduced PD and FD in invaded plots. Changes in the trait means with invasion were due to differences in native species in invaded and non-invaded plots, rather than to an effect of alien species. Within most habitats, the trait means and variance of all aliens were similar to those of all natives, while in some habitats, the variability in traits was greater between aliens that belonged to phylogenetically closer clades. Synthesis. Our results suggest that alien species more often occupy a phylogenetic and functional space within the range formed by the native species in a community. They do so either by filling empty gaps or by excluding natives from the existing phylogenetic and functional space, rather than occupying or creating a phylogenetic and trait space outside of it.