Marine Animal Forests (MAFs), such as those formed by gorgonians, play a vital role in enhancing local biodiversity by creating complex three-dimensional habitats, and provide a wide range of ecosystem services, including carbon sequestration, nutrient cycling, and constitute nursery grounds for species of commercial interest. Despite their ecological importance, MAFs face significant threats including climate change, overfishing, and pollution, resulting in the loss of biodiversity and ecosystem functions. In the Mediterranean Sea, the red gorgonian Paramuricea clavata contributes to the three-dimensional complexity of rocky benthic habitats. While previous studies have established that benthic biodiversity is generally higher within gorgonian forests than in no forest/absence of forest? areas, the relationship between forest density and associated benthic diversity remains poorly understood. Determining optimal gorgonian density who maximize diversity is crucial for designing effective restoration plans that promote both self- and other species’ recruitment, sustainability and ecosystem services.

In this study, we evaluated what is the P. clavata forest density that maximize affects alpha and beta diversity of understory benthic communities. Sampling was conducted within the Réserve Naturelle Marine de Cerbère-Banyuls (NW Mediterranean Sea). In thirty 1 m² quadrats all P. clavata colonies were counted and measured to characterize the demographic structure of the forest. Within each quadrat, an understory sample (22.5 ´ 22.5 cm) was then scraped and metabarcoding analyses using COI and 18S markers were performed.

The density of gorgonians emerged as a key driver of both alpha and beta diversity, affecting biodiversity indices, composition and structure of the understory, and abundance of taxa (i.e Porifera, Bryozoa increase with the P. clavata density). These findings suggest that determining the optimal gorgonian densities should be a priority before implementing conservation and restoration efforts, to preserve the beneficial effects of MAF and maintain the ecological complexity of these habitats.