The relative contribution of non-selection and selection processes in marine benthic assemblages.

We tested the hypothesis that the ubiquity of marine meiofaunal nematodes and their indiscriminate passive dispersal create assemblages that are less limited by its environment; whereas the relatively smaller population sizes of macrofauna, associated with their ability to track environmental conditions before settlement, renders their distribution more environmentally-restricted. We compared the empirical distribution of macrofauna and nematode species with that of communities simulated under different assumptions of selection (e.g. environmental filtering) and non-selection (e.g. dispersal limitation) processes. Selection processes were the prime driver of both meio- and macrofauna assemblages, with rare species strongly contributing to this component. The total number of species explained by non-selection processes was 27% higher in nematodes than in macrofauna. Our results underline the importance of a species-level approach to determine the contribution of selection and non-selection assembly processes. Moreover, they highlight the important yet overlooked role of dispersal and stochastic processes in determining species dynamics.