Non-additive effects of dispersal and selective stress on structure, evenness, and biovolume production in marine diatom communities

Changes in environmental conditions can impose stress that alters the structure and function of communities. However, ecologists are only starting to explore how stress can interact with dispersal. In this study, we tested how dispersal affects the structure, diversity (evenness), and function (productivity) of marine diatom communities (Bacillariophyceae) exposed to herbicide stress using a mainland-island framework. In a microcosm experiment, we manipulated the sequence (5 levels) and speed (two dispersal levels) of species arrival under no-stress conditions and two levels of stress. When stress was absent or low, priority effects regulated community dynamics, keeping the densities of new arrivers low. Consequently, evenness was lower in dispersed than in non-dispersed communities. Moreover, because of strong local interactions, dispersal decreased productivity under no-stress conditions and low stress. Under high stress, the selection for tolerant species regulated community dynamics. This generated a decrease in evenness but buffered productivity by compensating for the loss of sensitive species. Our results show that (1) dispersal reduced evenness, but that underlying mechanisms depend on the stress-level, and (2) dispersal can function as a spatial insurance against local changes in environmental conditions. Accounting for regional processes is therefore essential for estimating the consequences of environmental changes for ecosystem functions.