OOS 35-5 - Linking theory and global data to evaluate microbial control over plant coexistence

Background/Question/Methods
Soil microorganisms play a major role in shaping plant diversity, not only through their direct effects as pathogens, mutualists, and decomposers, but also by altering the outcome of plant interactions. In particular, previous research has shown that the soil community often generates frequency-dependent feedback loops between plants and their soil microbial communities that can stabilize or destabilize plant coexistence. However, recent theoretical advances incorporate modern coexistence theory to show that soil microbes affect plant coexistence not only through these stabilizing or destabilizing feedbacks, but also through generating frequency-independent competitive fitness differences. While microbially mediated fitness differences have rarely been quantified in the literature, many previous experiments nevertheless have generated the data required to calculate these effects post hoc. Utilizing the theoretical advance and relevant data, we conducted a multivariate meta-analysis to quantify and compare microbially mediated (de)stabilization and fitness differences. We further evaluated how the balance between the two metrics predict competitive outcomes among plant species pairs in the classic plant-soil feedback model.

Results/Conclusions
We quantified the microbially mediated (de)stabilization and fitness differences in 518 plant species pairs from 50 qualified studies. Across all species pairs, we found that soil microbes generated both substantial (de)stabilization and fitness differences (all mean effect sizes > 0 with p < 0.001), but that the fitness differences dominated (p < 0.002). Consequently, if plants are otherwise equivalent competitors, the larger microbially mediated fitness difference than (de)stabilization is predicted to drive plant species exclusion much more frequently than coexistence or priority effects. The above results are robust to variation in experimental treatments or analytical methods, highlighting the prevalence of this previously overlooked effect of microbes, and its dominating role in shaping plant coexistence. Our work calls for further empirical evaluation of microbially mediated fitness differences, especially in more realistic settings involving multiple species and ecological processes, for a more comprehensive understanding of the mechanisms that maintain plant diversity.