When protection backfires: Contrasting states of macroalgal forests and sea urchin populations in a Mediterranean MPA

Marine forests play a vital role in shaping seascape complexity worldwide. In the Mediterranean Sea, they are mainly formed by macroalgal species belonging to the Cystoseira sensu lato group (i.e. Cystoseira , Ericaria and Gongolaria ). These species are declining throughout Mediterranean coastlines due to multiple, often synergistic stressors. Among the principal biotic drivers of this regression is grazing by herbivores (i.e. the sea urchins Paracentrotus lividus and Arbacia lixula and the teleost Sarpa salpa ), which can transform luxuriant marine forests into urchin barrens—ecosystems of reduced complexity, lower species richness and limited ecological function. A widely used management tool to preserve marine ecosystems is the implementation of Marine Protected Areas (MPAs), often including no‐take zones (NTZs).

In this context, we assessed the effectiveness of MPAs and NTZs in preserving Ericaria brachycarpa forests. To this end, two large‐scale monitoring campaigns were conducted in the Nature Reserve of the Strait of Bonifacio (Corsica, France), the first in 2011 and the second in 2025. Sea urchin populations, algal cover and fish communities (in 2025 only) were surveyed using SCUBA diving.

Our results revealed a drastic decline in E. brachycarpa cover, accompanied by a steady increase in urchin barrens and sea urchin abundance in NTZs between the two campaigns. Conversely, in fishing zones, E. brachycarpa populations remained stable while both urchin barrens and sea urchin abundance decreased.

Synthesis and applications . This study demonstrates that NTZs can be counterproductive, as they may exacerbate the decline of marine forests by protecting sea urchins that are not effectively controlled by predators. We therefore call for a reassessment of management policies, including the targeted harvesting of herbivores where natural top‐down control is absent.