Thesis title: Functional diversity, resilience and compensatory dynamics of Mediterranean fish communities
This PhD thesis explores how Mediterranean fish communities respond to the combined pressures of climate change and fishing with a particular focus on the role of biodiversity in enhancing resilience. Building on standardized abundance data from the Mediterranean International Trawl Survey (MEDITS) and employing trait-based methods, the research identifies key patterns in demersal fish communities across a broad spatial and temporal range, providing a critical foundation for understanding their responses to anthropogenic stressors.
The central theme of the thesis is one of the fundamental topics in ecological studies, namely, the relationship between diversity and stability, which is examined through four primary analyses. Initially, the spatial, temporal, and functional attributes of fish communities are characterized, highlighting the distribution of species and their traits across different depth strata and macro-regions. Shelf communities (0–200 m) consistently harbor more coastal, fast-growing species, while slope communities (200–800 m) include larger, longer-lived taxa associated with colder waters. Species richness peaks in areas like the Adriatic and the Central Mediterranean shelves, yet functional redundancy remains relatively low throughout, indicating that many newly recorded species occupy novel niches rather than overlapping with resident taxa. Functional diversity indices are then linked to community-wide stability metrics to evaluate how asynchrony among species can buffer total biomass from fluctuations. The results point to an overall positive influence of trait diversity and species asynchrony on stability, although the degree to which they mitigate disturbances varies according to fish communities and regional conditions.
Subsequent chapters focus on recent climate-driven changes, including shifts in mean bottom temperature and the composition of fish communities. Analyses indicate that thermophilic species are expanding, while colder-water taxa face potential declines. A case study from the western coast of Italy illustrates localized warming patterns that appear to outpace fish communities’ capacity to adapt, underscoring how high fishing pressure can compound warming effects. Notably, some Mediterranean regions (e.g. the Aegean, the Ionian and the Western Mediterranean) have recently experienced a reduction in fishing effort, leading to an overall increase in total fish biomass. This trend may offer partial mitigation for traditional cold-water species, which have historically been targeted by fisheries, by reducing overall harvest rates and allowing these populations to maintain or recover some of their prior abundance in the face of rapid environmental change. To address the need for precise information about species’ environmental tolerances, the final chapter presents a dataset of thermal preferences tailored to demersal and benthic macrofauna in the Mediterranean, incorporating specific bottom temperature records with survey-based biomass data.
Together, these findings reinforce the importance of both preserving functional breadth and managing resource use in order to maintain resilient marine ecosystems under intensifying climate impacts. By uniting trait-based approaches, stability frameworks, and explicit thermal data, the thesis provides a basis for future management strategies aimed at safeguarding the ecological integrity of Mediterranean fish communities.