Fungi disproportionately influence global carbon and nutrient cycling processes as decomposers and mycorrhizal symbionts, but the assembly processes that give rise to variation in fungal communities are still largely unknown. We analyzed two global datasets of fungal taxonomic relative abundances to determine where across the globe fungi assemble in relation to environmental filtering or stochastically due to random dispersal, recruitment, and drift. We created null models of random fungal community composition based on taxonomic presence or absence as well as relative abundance. We then split the fungal community into functional guilds and generated guild specific null models. We measured the deviation of actual community composition from the null models as an effect size. Effect size represents a continuum of assembly processes with purely stochastic assembly at 1 and absolute niche partioning at -1. Our primary statistical analysis regressed effect size against a large suite of global climatic, edaphic, and floral predictor variables.
Results/Conclusions
Effect size was positive in 91% of the plots across the globe, indicating overwhelmingly stochastic community assembly. Positive effect sizes were not correlated to any of the environmental predictor variables. Across fungal functional guilds, arbuscular mycorrhizal fungi (AMF) and yeasts had the most idiosyncratic compositions while ectomycorrhizal fungi (EMF) had the least idiosyncratic. Just a small number of AMF host plants harbored AMF communities with predictable compositions. For the 9% of sites where effect size was negative, there was extensive variation along environmental gradients. These sites were all in northern latitude biomes dominated by EMF. Negative effect sizes were also correlated to overall variation in environmental conditions and to net primary productivity (NPP), with the most predictable fungal communities found in sites with homogenous environmental conditions and low NPP. High NPP and environmental variation can increase the range of taxa that can theoretically inhabit a site, which can increase stochasticity in communities via priority effects and drift. In conclusion, our study shows that fungal communities assemble differently across the globe, with most communities primarily structured by stochastic processes except in northern latitude ecosystems where niche processes dominate.