An experimental test of the intermediate disturbance hypothesis: influence of two disturbance types on the structure of etablished Western Baltic fouling communities
The intermediate disturbance hypothesis (IDH) is a widely accepted concept in community ecology. It assumes disturbance to be a potent agent to override the competitive exclusion principle and to facilitate the long-term coexistence of competitively inequal species. The IDH states that diversity is maximal at intermediate levels of disturbance. The aim of this study is to verify the predictions of the concept in a eutrophic, species-poor system - as it is represented by the Western Baltic Sea - in an in situ experimental approach. In two discrete experimental series, established hard-bottom communities of two successional stages were submitted to various levels of emersion (exposure to the air) and exposure to enhanced UVB radiation. For the communities that experienced emersion treatments, the IDH was confirmed in the first year when diversity was found to peak at intermediate disturbances. However, for communities of both successional stages, diversity-disturbance relationships were U-shaped or non-significant in the second year. This ambiguous picture basically confirms the validity of the mechanisms proposed by the IDH, but shows that their forcing can be masked or reversed by fluctuations in environmental parameters, such as climatic conditions. An extension of the concept, that considers diversity enhancement under extreme conditions due to a disturbance induced change in community structure, is proposed. UVBR treatment effects were transient and did not generate a unimodal disturbance-diversity pattern. Though treatment effects were not persistent, a general tendency for green algae to increase and for red algae to decrease with increasing daily UVBR exposure length was observed.
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