Dispersal timing determines the importance of priority effects in bacterial communities.

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 bacterial communities where dispersal involves mixing of entire communities is currently not known. Here we used a transplant experiment with two 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 after initial inoculation. We found that the later the dispersal with the 'home' community occurred the smaller the effect on the final community composition. This suggests that priority effects by the initially inoculated community reduce the establishment success of taxa from the later arriving community and that this effect depends on dispersal timing.