Patterns of spatial variability between contrasting substrata: a boulder-field study

International audienceBoulder fields are complex habitats in which many species coexist and are important contributors to coastal biodiversity. These habitats experience important natural disturbances due to wave action, over which anthropogenic stressors, such as hand-fishing and urban sprawl, can be added. Despite their particularity and vulnerability, there have been few attempts to disentangle the processes that actually structure boulder-field communities at different scales. In order to help direct future research in boulder-field systems, diversity and community structure, as well as associated variability patterns, were compared between boulder and bedrock habitats at a hierarchy of spatial scales (from 100s of cm to 10s of km) along approximately 100 km of shoreline in NW Brittany. Specifically, it was hypothesised that (1) differences in physical structure would produce greater variability on boulder than on bedrock substrate at small scales and (2) contrasting patterns of variability would emerge with increasing scales, as the processes operating at a large scale on the bedrock would be dampened on boulders undergoing physical disturbance. Overall, both hypotheses were rejected with regards to variability patterns in diversity, community structure and abundances of most functional groups, except for ephemerals in both mid- and low-shore heights and limpets in the low-shore. Variability was generally concentrated at the smallest spatial scale, but bedrock showed greater patchiness than boulders for most of the response variables. With increasing spatial scales, the variability patterns were consistent overall between habitats. Among potential mechanisms, the interplay between grazing and physical disturbance over several spatial scales deserves further experimental scrutiny