Canopy-forming species mediate the effects of disturbance on macroalgal assemblages on Portuguese rocky shores

Understanding the relationship between biodiversity and stability is a central issue in ecology. This is particularly needed under current scenarios of biodiversity loss due to multiple anthropogenic stressors. In this study, we experimentally examined the combined effects of the loss of key functional species (canopy-forming macroalgae) and mechanical disturbance on macroalgal intertidal assemblages at 2 sites along the rocky coast of northern Portugal. We tested the model that the canopy may buffer the effects of disturbance on associated organisms, leading to the logical hypothesis that effects of mechanical disturbance would be lower where the canopy was left intact compared to patches where it was experimentally removed. The proposed model was supported by multivariate and univariate results, as both differences in the structure of whole assemblages and in the abundance of individual taxa between disturbed and undisturbed assemblages were reduced or prevented by the presence of the canopy, independently of the examined site. Specifically, only in the absence of the canopy did disturbance determine a decrease in the abundance of the red alga Ahnfeltiopsis devoniensis and an increase in articulated coralline algae of the genus Corallina. Present findings have relevant implications for understanding and predicting the consequences of multiple factors responsible for the erosion of biodiversity occurring globally in coastal areas, as current rates of loss of canopy-forming species in urban areas may be expected to exacerbate the effects of predicted climate change, including modifications in intensity and temporal patterns of storms.