Species pool, local assembly processes: Disentangling the mechanisms determining bacterial α‐ and β‐diversity during forest secondary succession

Across ecology, and particularly within microbial ecology, there is limited understanding how the generation and maintenance of diversity. Although recent work has shown that both local assembly processes and species pools are important in structuring microbial communities, the relative contributions of these mechanisms remain an important question. Moreover, the roles of local assembly processes and species pools are drastically different when explicitly considering the potential for saturation or unsaturation, yet this issue is rarely addressed. Thus, we established a conceptual model that incorporated saturation theory into the microbiological domain to advance the understanding of mechanisms controlling soil bacterial diversity during forest secondary succession. Conceptual model hypotheses were tested by coupling soil bacterial diversity, local assembly processes and species pools using six different forest successional chronosequences distributed across multiple climate zones. Consistent with the unsaturated case proposed in our conceptual framework, we found that species pool consistently affected α‐diversity, even while local assembly processes on local richness operate. In contrast, the effects of species pool on β‐diversity disappeared once local assembly processes were taken into account, and changes in environmental conditions during secondary succession led to shifts in β‐diversity through mediation of the strength of heterogeneous selection. Overall, this study represents one of the first to demonstrate that most local bacterial communities might be unsaturated, where the effect of species pool on α‐diversity is robust to the consideration of multiple environmental influences, but β‐diversity is constrained by environmental selection.