Aim: Understanding how local biotic filters imposed by habitat-forming species interact with regional species pools across broad biogeographic gradients is a central challenge in community ecology and biogeography. We evaluated the engineering role of the red gorgonian Paramuricea clavata, testing whether its biotic filtering drives functional convergence in coralligenous understorey assemblages, thereby overriding regional biogeographic variability, while concurrently assessing its thermal vulnerability under ongoing climate stress.
Location: Western-Central Mediterranean Sea (Italy), spanning a depth gradient of 20-40 m.
Methods: We assessed the benthic assemblages associated with P. clavata forests and adjacent unforested areas through photographic sampling (N = 1300 quadrats). Taxonomic structure and functional identity (via Community-Weighted Mean) were analysed using a mixed multifactorial PERMANOVA design across regions, locations, forest condition (Inside vs. Outside), depth layers, and sampling times (T0: Summer; T1: Autumn). Eight functional traits capturing resource acquisition, life-history strategies, and space occupation were assigned to all taxa. Multivariate dispersion (PERMDISP) was used to assess beta-diversity patterns. Tissue necrosis of P. clavata was quantified through fixed video transects at T0 and T1.
Results: Taxonomic structure exhibited strong regional and depth-related turnover. In contrast, functional identity converged consistently within forests regardless of region, location, depth, or time, with Condition emerging as the primary driver. Assemblages inside the forest were dominated by heterotrophic, long-lived, and colonial organisms, while outside by opportunistic autotrophs. Beta-diversity was significantly lower inside the forest at the among-site scale. Concurrently, P. clavata tissue necrosis nearly doubled over a single summer.
Main Conclusions: Paramuricea clavata acts as a "functional engine" that decouples species composition from ecosystem functioning, overriding biogeographic variability across regions and depth. However, this cross-scale buffering capacity is threatened by accelerating thermal stress. Our findings provide strong empirical support for prioritising habitat-formers in conservation frameworks aimed at preserving ecosystem functioning across broad biogeographic gradients.