|Beschreibung:||Dispersal and disturbance are major factors regulating the number of coexisting species. However, the relationship between species diversity and ecosystem processes has mainly been analysed for communities closed to dispersal, i.e. without looking at the mechanisms of coexistence. I experimentally investigated how initial local diversity, dispersal and disturbance rates affect local diversity and biomass production in metacommunities with marine benthic microalgae and in natural rock pool assemblages. Final local species richness and local biomass production were strongly influenced by dispersal frequency and by disturbance but not by initial local diversity (chapters 1 and 2). Both final local richness and final local biomass showed a hump-shaped pattern with increasing dispersal frequency and also with increasing suucessional time, with a maximum at intermediate dispersal frequencies and successional time. Consequently, final local biomass increased linearly with increasing final richness. In the rockpools system, diversity effects occurred in the absence of dispersal and depended on two effective grazers in the community (chapter 3). I conclude that the general relationship between richness and ecosystem functioning remains valid in open systems, but the maintenance of ecosystem processes significantly depends on the mechanism of species coexistence.