Unveiling hidden relationships between biodiversity and ecosystem functioning in Mediterranean rocky reefs HI-BEF project

4 - Research Project Abstract In the Mediterranean, overfishing of large predatory fish, especially of Diplodus spp., can cause severe sea urchin outbreaks and promote a shift in benthic communities. The variation in sea urchin grazing intensity may drive switches between one complex state, dominated by a stratified assemblage of several erect algae including those of Cystoseira genus (Erected Macroalgae Assemblage, EMA), to a simpler one, the barren, dominated by few encrusting algae (Encrusting Corallines algae Assemblages, ECA). ECA state is geographically widespread and can paradoxically persist for long periods also in marine protected areas (MPAs). Theoretically, the ban of extractive human activities usually leads to the recovery of key predators of sea urchins (fishes, lobsters, sea stars), which control urchins populations and allow the restoration of the erect macroalgal-dominated states. However, large barren areas can be found along the oldest Mediterranean MPAs. In these cases, recovery of the top predators is too slow, due to their initial over-exploitation, and sea urchins abundance may explode, due to the combined effect of lack of natural predators and human exploitation. EMA states and associated organisms are known to be highly productive compared to structurally simpler states, such as ECA. EMAs transfer vast quantities of carbon, nitrogen and phosphorus to coastal food webs either through direct transfer of animal biomass (i.e. predation, movements of individuals) or out-welling of dissolved and particulate organic matter. On the contrary, ECA systems are assumed to be characterized by low diversity and productivity. There is indirect evidence that deforestation can negatively affect the productivity of individual species or groups of species, with effects that are likely to propagate along food chains. However, recent studies conducted in the North Pacific indicated that, although ECA exhibits a low structural complexity, it can provide a high number of microhabitats available for a wide variety of cryptic invertebrate species. ECA state areas are widespread in the Mediterranean, but the information on the associated biodiversity and functioning are still very scarce. The aims of this project are: (1) Assessing mechanisms involved in the transitions between EMA and ECA states; (2) Assessing the benthic biodiversity in EMA and ECA; (3) Assessing ecosystem functioning in EMA and ECA. A multidisciplinary approach (manipulative, descriptive, isotopic and of mass balance) will be adopted to achieve these aims. For (1) assessing mechanisms involved in the transitions between EMA and ECA states, a manipulative approach will be adopted. Our intention is to study, with a rigorous, manipulative approach, the factors (abiotic and biotic) responsible for the creation, stability and transition between EMA and ECA present on the upper infralittoral rocky shores of the Mediterranean Sea. Manipulative experiments will be employed to assess the relative role of total density and composition of herbivore species, refuges availability for herbivores, disturbance and switches in herbivore feeding mode in triggering the shift from EMA to ECA and maintaining ECA. For (2) Assessing the benthic biodiversity in EMA and ECA, and (3) Assessing ecosystem functioning in EMA and ECA, community structure and functional diversity of benthic assemblage, energy flow and trophic interactions will be defined in EMA and ECA states and compared at a mesoscale level. The structure of the food webs will be determined through the study of trophic guilds and stable isotopes (13C/12C and 15N/14N). The functioning of the trophic model will be developed by using both mixing models and ECOPATH and ECOSIM software. Moreover, the null hypothesis of no relationships between biodiversity (eg, species richness, rarefaction indices and functional diversity) and ecosystem functioning (eg, biomass, production, number of trophic l