Unpredictable assembly of arbuscular mycorrhizal fungal communities

•A recent global survey suggested that AM fungal community composition is largely determined by local environmental conditions.•Re-analysis of these data suggest that, within similar environments, these communities exhibit patterns consistent with neutral or highly stochastic assembly.•Greater unpredictability was associated with dicot hosts than with monocot hosts.•Assembly outcomes were more unpredictable at higher elevations and lower latitudes.•These results suggest future research opportunities to understand mycorrhizal fungal community assembly processes.

In their analysis of arbuscular mycorrhizal fungal communities sampled from roots collected around the globe, Davison et al. (2015, Science 349: 970) demonstrated that these fungi exhibit low endemism and suggested that their biogeography is largely determined by local environmental conditions. Here, we show that, within similar environments, these communities are largely unpredictable based on environmental conditions and exhibit patterns that are either consistent with neutral models of community assembly or are more stochastic than what models based on environmental filtering, niche-based assembly, or neutrality would predict. We also show that the degree of unpredictability is related to geography and the characteristics of the host plant, suggesting opportunities for future research to understand assembly processes in mycorrhizal fungal communities.