Licciardi, Luca (2026) From structure to function: The engineering role of Mediterranean Marine Animal Forests and their vulnerability to human pressures. [Tesi di dottorato]

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Abstract

The decline of habitat-forming species represents one of the most severe threats to global marine biodiversity. In the Mediterranean Sea, the red gorgonian Paramuricea clavata acts as a key ecosystem engineer, forming extensive Marine Animal Forests (MAFs) on coralligenous reefs. These forests generate unique three-dimensional complexity, supporting high biodiversity and critical ecosystem services. Yet, in this global hotspot for climate change and anthropogenic exploitation, the mechanisms by which these forests drive ecosystem functioning and their resilience to human pressures remain largely unexplored. To address this critical knowledge gap, this thesis adopts a functional trait-based approach alongside traditional taxonomic analyses to disentangle the engineering role of P. clavata forests across multiple spatial scales and to quantify the “functional footprint” of anthropogenic pressures on these vulnerable habitats. In the first chapter, the study investigates the structural and functional role of P. clavata forests at a local scale (Gulf of Naples, Italy). The aim was to determine how the presence of the gorgonian forests modifies the taxonomic and functional structure of the associated community compared to adjacent areas where the forests are absent. Results reveal that the forest significantly increases both taxonomic and functional diversity. Trait-based analysis highlights how communities outside the forests are dominated by ephemeral, autotrophic strategies, while those within are characterized by colonial, heterotrophic species. Furthermore, the analysis of β-diversity demonstrates that the forest acts as a promoter of heterogeneity, enhancing micro-habitat availability and supporting species turnover. However, demographic assessments of P. clavata reveal populations characterized by skewed size structures dominated by small colonies, indicative of a disturbed system. In the second chapter, the research scales up to the Western-Central Mediterranean to test the consistency of these patterns. By surveying communities inside and outside P. clavata forests, along a bathymetric gradient (20-40 m) across a broad geographic scale, the study identifies a marked decoupling between taxonomic and functional diversity patterns. While species composition is strictly governed by regional biogeographic turnover, the functional identity of the understorey converges into a common profile characterized by conservative, modular traits wherever the forest is present. This demonstrates that P. clavata acts as a context-independent "biotic filter," capable of overriding both regional species pools and depth-related gradients. However, this chapter also documents widespread signs of disturbance: despite the apparent functional stability of the understorey, P. clavata populations showed a doubling of tissue necrosis rates over a single thermal season, indicating that the system's resilience is increasingly compromised by warming events. In the third chapter, the thesis examines the direct impact of Small-Scale Fisheries (SSF) on the functional integrity of coralligenous reefs. By analyzing the bycatch of trammel nets with a functional approach, the study quantifies the "functional footprint" of fishing gears on these high-biodiversity assemblages. Results show that SSF raises serious sustainability concerns as they exert a highly selective pressure on habitat formers. The catch is disproportionately composed of the same large, structural, and slow-growing species (sponges and gorgonians) shown in the previous chapters to play key roles. This pressure removes over 26% of the functional trait space, potentially pushing the system towards functional homogenization. Notably, this pattern persists regardless of protection status, indicating that gear selectivity overrides spatial protection measures, thus calling for a revision of management strategies within Marine Protected Areas (MPAs). Finally, complementing the analysis carried out on coralligenous habitats, the fourth chapter investigates the effectiveness of conservation strategies in shallow coastal seascapes. This section also integrates taxonomic and trait-based approaches to assess how MPAs interact with human pressures. The results reveal a strong context-dependency: while protection promotes the persistence of high-integrity ecosystems under low-stress conditions, high levels of human pressure tend to override the benefits of zoning. In heavily impacted areas, although protection may still drive compositional and functional changes, the system undergoes phase shifts towards degraded states (e.g., Caulerpa cylindracea and algal turf-dominated assemblages), demonstrating that spatial zoning alone is insufficient to prevent functional erosion when environmental thresholds are exceeded. Collectively, this thesis positions Marine Animal Forests not merely as biodiversity hotspots, but as key functional components capable of buffering environmental stochasticity. Yet, this work shows that the same life-history traits that underpin the stability of these habitats: longevity, three-dimensional complexity, and slow dynamics, render them disproportionately vulnerable to the synergistic impacts of ocean warming and selective fishing. These findings call for an urgent shift in conservation strategies, with approaches designed to embrace and preserve both the structural and functional complexity of these habitats. Safeguarding the physical persistence of these ecosystem engineers is crucial to reduce the risk of progressive functional degradation of Mediterranean coralligenous reefs. Additionally, given that local management cannot mitigate global changes, limiting extractive pressures represents a key management option to keep cumulative pressures below critical tipping points and support ecosystem resilience.

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