Regional conditions determine sea urchin pressure on macroalgal forests and modify the buffer capacity of the alternative statesResistance, Resilience and Phase Shifts

Teresa Alcoverro¹, Laura Tamburello^2,3, Candela Marco-Méndez¹, Giulia Ceccherelli⁴, Simone Farina², Jordi Boada¹, Bernat Hereu⁵, Antonio Di Franco^2,3, Antonio Calò^2,3, Ivan Guala⁴, Anaïs Barrera¹, Federico Pinna⁴, Manfredi Di Lorenzo^2,3, Matteo Sinerchia⁶, Stefano Piraino⁷, Francesca Necci⁷, Andrea Toso⁷, Alessandra Martines⁷, Luigi Piazzi⁴, Gabriella La Manna⁸, Arianna Pansini⁴, Patrizia Stipcich⁹, Riccardo Vargiu², Alessia Crobu⁴, Alessandra Puccini⁴, Mariangela Moro Merella⁴, Marta Conesa¹, Pol Ramis¹, Aina Alemany¹, Lucia Rodriguez-Arias¹, Jordi Pagès¹, Martin Marzloff¹¹, Giovanni Romagnoni¹², Mario Minguito-Frutos^11,1

¹Centre d’Estudis Avançats de Blanes (CEAB-CSIC), Spain - ²Stazione Zoologica Anton Dohrn Sede Interdipartimentale della Sicilia, Italy - ³National Biodiversity Future Center, Italy - ⁴University of Sassari, Italy - ⁵Universitat de Barcelona, Spain - ⁶CNR-IAS - Institute for the Study of Anthropic Impact and Sustainability in the Marine Environment, Italy - ⁷Università del Salento, Italy - ⁸MareTerra Onlus-Environmental Research and Conservation, Italy - ⁹University of Naples Federico II, Italy - ¹⁰University of Cagliary, Italy - ¹¹Ifremer, France - ¹²University, Kiel, Germany

Macroalgal communities dominating shallow rocky reefs in the Mediterranean are characterised by non-linear dynamics, with sea urchin overgrazing triggering transitions between alternative states either dominated by algal forests or barren communities. Unpacking the mechanisms underlying these transitions can help considerably reduce the unpredictability of those changes and inform effective management. At the basis of these transitions there is the relationship between sea urchin consumption and primary production. This relationship is based on two factors: the density of urchin and the strength of the interaction between herbivory and algal production. Using field observations across the Western Mediterranean we investigated how differences in context-specific conditions – such as nutrient regimes, temperature, protection level, and species composition- mediate the vulnerability of macrophyte communities. Sea urchin densities and their pressure differ substantially between regions determining differential transitions. These differences can be attributed to context-specific conditions including nutrient availability and temperature. Marine protected areas also play a role, although minor, for sea urchin densities and herbivory impact with clear distinct thresholds. Our study demonstrates that by mediating the underlying drivers, regional conditions can strongly influence the buffer capacity of nonlinear systems. Effective management will require regional-specific plans that consider the unique interplay between protection levels, nutrient availability, and other local factors.