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The intermediate disturbance hypothesis (IDH) predicts maximal diversity at intermedi- ate levels of disturbance, but the validity of this hypothesis is controversially discussed. In this study, results of a field experiment, which was conducted on the northern-central Chilean coast, are pre- sented. Fouling communities on artificial settlement substrata were studied. A total of 7 disturbance frequencies were applied to previously established communities, and a single disturbance event resulted in a removal of ~20% of the biomass. Species cover was estimated at the end of the experi- mental period, and it was found that diversity was strongly affected by disturbance frequency. With high disturbance frequencies the composition of the community was changed, with a decrease in the solitary ascidian Pyura chilensis (Molina 1782). The decrease of P. chilensis resulted in an increase of the colonial ascidian Diplosoma sp. A unimodal relationship between disturbance frequency and spe- cies richness was found, supporting the IDH. The results suggest that disturbance sustains diversity by reducing the abundance of the dominant species (e.g. P. chilensis), preventing competitive exclu- sion of the subordinate species, thus allowing subordinate species to re-emerge when competition is alleviated by disturbance. The results also suggest that these species show a trade-off between com- petitive and colonizing abilities, pointing to the existence of a competitive hierarchy. Therefore, the presence of competitive exclusion and disturbance-induced suppression of the dominant species remains a crucial mechanism, permitting species coexistence in the context of the IDH in the system studied.

Effects of disturbance on the diversity of hard-bottom macrobenthic communities on the coast of Chile