Figure 3.

Hypothesized conceptual model linking microbial succession and assembly processes for predicting the impacts of microbial inoculation (invasion)

Community assembly processes can be measured by the β-nearest taxon index (βNTI) (Stegen et al., 2013). Ecological selection is weak in the center of the vertical axis and is stronger toward both extremes. Different lines with a number represent various trends of the community assembly process during succession through time. Depending on βNTI values, the assemblage of the resident community in a primary succession is inferred to be dominated by variable selection (VS, red lines), homogeneous selection (HS, green lines), or stochastic processes (Neutral, yellow lines). Both in the communities driven by VS (red lines) and HS (green lines), the strong selective pressure can be removed in the facilitation model as a result of damaged biotic interactions or abiotic pressure, such as the extinction of keystone microbial taxa, leading to stochasticity (lines 3, 4, 8, and 9), and these trends can be reversed when invaders impose strong selection and/or shape a more selective environment (lines 3 and 9). In the tolerance model, the increase of biotic selection and/or abiotic pressure triggered by invaders could lead to the intensive variable selection (lines 1 and 5); a constant selective pressure (lines 2 and 10), or an increase of homogeneous selection (line 11) following an invasion event is expected if the resulting environment does show a weak selection. Lines 2 and 10 can also correspond to microbial invasions having no impacts on the primary selective pressure in the inhibition model. The effects of invasions on ecologically neutral communities are difficult to predict theoretically (show as dash lines 6 and 7).