Variation in the structure of subtidal landscapes in the NW Mediterranean Sea

Compounded effects of climate change and local human activities are threatening marine biodiversity worldwide. At a regional scale (10s to 100s km), comparisons among areas characterized by the prevalence of different human activities provide an insight into the effects of anthropogenic disturbances at multiple levels of ecological organization (i.e. from landscapes to assemblages). At the landscape scale (1000s m), we hypothesized that patchiness in habitat distribution and proportion of degraded assemblages would increase with increasing levels of disturbance, as a result of the decline of habitat-forming species. In addition, we hypothesized that prevailing human influences would affect the structure and variability of rocky benthic assemblages at smaller spatial scales (10s cm to 10s m). An extensive survey encompassing areas subjected to different human influences (i.e. from urbanized to protected areas) was carried out along the coasts of Tuscany (NW Mediterranean Sea). Seagrass beds and macroalgal canopy stands were the dominant habitats in relatively pristine areas, while macroalgal turfs and dead rhizomes ofPosidonia oceanicawere the most extended habitats in urbanized areas. In general, habitat fragmentation did not vary among areas subjected to different human influences. At a smaller scale (10s cm to 10s m), urbanization favored dominance by opportunistic species and promoted biotic homogenization. Our study shows that regional variations in the composition of landscapes and assemblages can be predicted on the basis of prevailing human activities. Our results also suggest that variations in landscape composition could be an effective descriptor of the effects of multiple human stressors in marine environments.