Ecological communities frequently, but not always, exhibit historical contingencies, reflecting the varied importance of stochastic community assembly. Elucidating mechanisms regulating the degree of community historical contingency thus holds the key to gaining a better understanding of community assembly processes. Studying historical contingency in plant and animal communities, however, is difficult due to their long generation times. By contrast, microorganisms, such as bacteria and protozoa, have short generation times that allow their long-term community dynamics to be monitored over weeks to months. Here we summarize the findings from such microbe-based community assembly studies to identify factors and processes that influence the strength of community historical contingency.
A variety of ecological factors, such as the size of species pool, disturbance, ecosystem size, environmental productivity, and predation, can influence the strength of community historical contingency. Mechanistically, these factors influence historical contingency by influencing the strength of competition among earlier and later colonizing species. We thus propose a conceptual framework in which ecological factors modulate historical contingency by altering species niche and fitness differences, which combine to influence the outcome of species interactions and community assembly. We advocate the trait-based assembly research where species traits in relation to their niche and fitness differences are considered in the context of community assembly.