Patterns of phylogenetic diversity are linked to invasion impacts, not invasion resistance, in a native grassland

Question: There are often more invasive species in communities that are less phylogenetically diverse or distantly related to the invaders. This is thought to indicate reduced biotic resistance, but recent theory predicts that phylogenetic relationships have more influence on competitive outcomes when interactions are more pair-wise than diffuse. Therefore, phylogenetic relationships should change when the invader becomes dominant and interactions are more pair-wise, rather than alter biotic resistance, which is the outcome of diffuse interactions with the resident community; however both processes can produce similar phylogenetic structures within communities. We ask whether phylogenetic structure is more associated with biotic resistance or invasion impacts following Bromus inermis (brome) invasion and identify the mechanisms behind changes to phylogenetic structure. Location: Native grassland in Alberta, Canada. Methods: We tested whether phylogenetic structure affected biotic resistance by transplanting brome seedlings into intact vegetation and quantified invasion impacts on community structure by surveying across multiple invasion edges. Additionally, we tested whether relatedness, rarity, average patch size, evolutionary distinctiveness or environmental tolerances determined species' response to brome invasion. Results: Neither phylogenetic diversity, nor relatedness to brome, influenced the strength of biotic resistance; resource availability was the strongest determinant of resistance. However, communities did become less diverse and phylogenetically over-dispersed following brome invasion, but not because of the loss of related species. Brome invasion was associated with declines in common species from common lineages and increases in shade-tolerant species and rare species from species-poor lineages. Conclusions: Our results suggest that invasion is more likely to affect the phylogenetic structure of the community than the phylogenetic structure of the community will affect invasion. However, they also suggest that the degree of relatedness between the invader and the resident community is unlikely to drive these effects on phylogenetic community structure. Consistent with previous studies, invasion effects were stronger for common species as they have reduced shade tolerance and cannot persist in a subordinate role. This suggests that invasion effects on phylogenetic community structure will depend on which species exhibit traits that enable persistence with the invader and how these traits are distributed across the phylogeny.