Testing the intermediate disturbance hypothesis: response of fouling communities to various levels of emersion intensity

Connell’s intermediate disturbance hypothesis (IDH) postulates that diversity peaks at an intermediate level of disturbance frequency or intensity. To test the validity of this concept for the species-poor marine hard-bottom community of the Western Baltic, we chose an in situ experimental approach. Undisturbed fouling communities of 2 different successional stages, 3 and 12 mo old, were submitted to various levels of emersion intensities, defined as time spans of continuous exposure to the air d–1. Disturbance levels ranged from 0.25 h up to 12 h of daily exposure. The study on 3 mo old communities was repeated in 2 subsequent years, 1999 and 2000. Species richness, evenness and diversity (Shannon index) were recorded to measure the effect of intensity treatments on community structure. 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 not 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 by fluctuations in environmental parameters, such as climatic conditions. An extension of the model is proposed, that considers diversity enhancement under extreme conditions due to a disturbanceinduced change in community structure. Furthermore we discuss a conceptual linkage of the IDH to the multiple stable-state hypothesis. Finally, we found community stability not to be positively correlated with community age and complexity.