Figure 1. The predicted relative importance of ecological and
evolutionary factors that drive the distributions of host-associated and
free-living microorganisms. Here, the hypothesized strength of the
factor is illustrated by the size of the arrow, with larger arrows
representing stronger effects. Examples of host-associated
microorganisms include bacterial microbiomes of beetle and deer
digestive tracts, arbuscular mycorrhizal fungi, the absorptive hyphae of
ectomycorrhizal fungi, and root nodule-forming bacteria (Rhizobium
spp. ). Examples of free-living microorganisms include soil and aquatic
aggregates of bacteria, fungi, and archaea and protists such as diatoms.