Rhizobia mutualists contribute to phylogenetic clustering and legume community assembly globally

Mutualistic interactions are increasingly recognized as playing an important role in community assembly. We hypothesize that mutualisms can influence the amount of phylogenetic diversity within plant communities during community assembly. We test this hypothesis by comparing phylogenetic diversity among clades in the Fabaceae (i.e. legumes), which has repeatedly gained and lost the ability to form mutualistic symbiosis with nitrogen-fixing bacteria (rhizobia). Here, we analysed global distribution data for both native and introduced legumes and quantified the impact of gains and losses of the rhizobia symbiosis trait on the phylogenetic structure of regional legume assemblages. Generally, we find that legume assemblages exhibit evidence of phylogenetic clustering-lower phylogenetic diversity than expected, suggesting that species assembly is generally constrained by conservation of traits related to assembly. More importantly, by leveraging naturally existing plant trait variation in the ability to form rhizobial symbiosis, we were able to estimate the contribution of rhizobia mutualism to global patterns of phylogenetic clustering. Specifically, we found relatively higher phylogenetic diversity in regional native legume assemblages that contain higher proportions of non-rhizobial native legumes. Moreover, the probability of plant species being recorded as introduced species in a new range is negatively related to phylogenetic distance to its nearest native relative in the new range, but importantly, this effect is significantly weaker for species that are not symbiotic with rhizobia. Together, these results suggest that rhizobia mutualism contributes to (1) the patterns of phylogenetic clustering for native legume communities and (2) the assembly of communities of legumes through the introduction and establishment of alien species. Synthesis. Our data support the hypothesis that the probability of mutualistic plant species establishing in a community is more strongly constrained by the phylogenetic conservation of compatible locally available mutualistic partners than is true for related species that do not form mutualistic symbiosis. In addition, this constraint occurs both in the native and introduced plant ranges. Our findings highlight the significant role mutualism (in the form of bacterial symbiosis with nitrogen-fixing rhizobia) can play in constraining phylogenetic diversity in legume communities across the globe. Our study advances community assembly theory and underscores the importance of considering mutualism in the conservation and restoration of phylogenetic diversity.