Arbuscular mycorrhizal fungi (AMF) influence plant growth through nutrient cycling processes, but we know little about the forces influencing AMF species abundances. There are relatively few identified AMF 'species', but many AMF virtual taxa (VT; an established proxy for species) are globally distributed and may be stochastically distributed rather than in relation to specific habitat preferences based on traits. If so, than AMF assembly should fit a neutral null model framework. Additionally, AMF VT should have similar realized niches (i.e. minimal niche partitioning) because they colonize similar habitats. To test this, we first constructed neutral null-models that differentiate between niche and neutral assembly. Next, we constructed joint species distribution models to characterize the environmental properties and intraspecific interactions associated with relative abundances of AMF VT. This approach is akin to characterizing the realized niches of AMF. We analyzed AMF from two experiments: [1] a global survey of AMF biodiversity, and [2] a local survey in a global change experiment simulating climate warming, nitrogen deposition, and biotic invasion by Alliaria petiolata.
Results/Conclusions
At the global scale, variation in AMF communities was not due to niche processes but random variation in species migration, extinction, and speciation (neutrality). At the local scale, there was minimal variation in AMF communities due to small scale neutral processes. This may be a result of greater competition between AMF at the local scale, as supported by strong negative co-variation between AMF taxa at the local scale but not at the global scale. In particular, the two most abundant AMF in the local study negatively co-varied with the other AMF, while the most common AMF at the global scale tended to positively co-vary with each other. The environmental variables influencing AMF relative abundances were also distinct for the two dominant AMF at the local scale, but similar for most VT at the global scale and characteristic of a "generalist" ecology. Co-existence among similar 'generalist' AMF at the global scale is likely due to neutral forces. Less common AMF at the local scale exhibited similar patterns to AMF at the global scale, and similar neutral processes may promote co-existence of the rarer AMF biosphere at local scales. In conclusion, we show that neutrality was most important for AMF assembly at the global scale and for rare AMF at the local scale, while niche processes were most important for determining local species dominance.