The order and timing of species arrival during community assembly can have long term effects on community structure due to priority effects. The importance of such processes in complex microbial communities where dispersal involves mixing (coalescence) of entire communities instead of sequential arrival of single species is, however, currently not known. To investigate this, we used a transplant experiment with two complex bacterioplankton communities of different origin (freshwater and brackish). Sterile medium of each origin was initially inoculated with a bacterial community of different (“alien”) origin, followed by dispersal of the respective “home” community at different time points (0, 6, 12, 24, 48, 72, and 96 hours) after initial inoculation.
We found that the later the dispersal with the home community occurred, the smaller was its effect on the community composition at the end of the experiment (2 weeks after initial inoculation). This suggests that priority effects by the initially inoculated alien community dampen the establishment success of taxa from the later arriving community and that this effect depends on dispersal timing. Our study therefore shows that the timing of coalescence events can have a pronounced influence on community structure due to priority effects and stresses the importance of a historical perspective on microbial community assembly.