Sharks and rays (elasmobranchs) play crucial roles in marine ecosystems, yet face significant threats. Conservation efforts have primarily focused on taxonomic diversity and evolutionary distinctiveness, overlooking the critical aspect of functional diversity—the range of ecological roles species perform. This study addresses this gap by comprehensively assessing the functional diversity of elasmobranchs at a global scale, identifying regions with high functional diversity, evaluating the overlap with human impacts and protected areas, and informing conservation prioritization.

Previous research has highlighted hotspots of evolutionary distinctiveness and endemism for elasmobranchs. However, these studies have largely ignored functional diversity. Existing studies often focus on a limited number of species, specific diversity metrics, or regional scales. The need for a global-scale analysis incorporating multiple facets of biodiversity, including functional diversity, is highlighted by the increasing recognition of the importance of considering multiple dimensions of biodiversity for effective conservation.

This study compiled trait data for over 1000 elasmobranch species (representing a significant portion of the total), using FishBase as a primary data source and supplemented with published literature. Seven functional traits were assigned: body size, habitat, terrestriality, vertical position, diet, feeding mechanism, and thermoregulation. Functional diversity was quantified using functional richness (FRic), functional uniqueness (FUn), and functional specialization (FSp). The Functionally Unique, Specialised, and Endangered (FUSE) metric was used to prioritize species for conservation, combining functional uniqueness and endangerment status. Spatial analyses were conducted using grid cells to identify global hotspots of functional diversity. The congruence between these hotspots and existing marine protected areas (MPAs) and fishing pressure indices was also assessed using various threshold values to account for potential arbitrary selection of thresholds. The study also incorporated phylogenetic diversity and evolutionary distinctiveness (EDGE) metrics for comparison and to assess the degree of congruence between functional and evolutionary metrics.

Threatened elasmobranch species disproportionately contribute to functional diversity, occupying almost the entire extent of the functional space. Several unique elasmobranch functional diversity hotspots were identified, not always congruent with hotspots of other diversity facets, notably in oceanic islands and high seas. There was limited spatial overlap between functional diversity hotspots and existing MPAs, indicating a significant gap in protection. A noteworthy mismatch was observed between functional diversity hotspots and existing protected areas, highlighting that current conservation efforts might not adequately safeguard crucial functional roles within these ecosystems. The analysis of FUn revealed an opposite pattern compared to FRic, concentrating in high latitudes with lower values on continental shelves. This suggests low redundancy in species-poor assemblages, with the loss of individual species having a large impact on functional space. The top FUSE species spanned various orders, indicating that conservation priorities must consider a wide range of taxa. Analyses showed that there was limited congruence between functional metrics and evolutionary distinctiveness.

The findings highlight the importance of incorporating functional diversity into elasmobranch conservation planning. The identified hotspots of functional diversity, especially those not adequately represented in existing MPAs, should become conservation priorities. The limited congruence between functional and phylogenetic diversity suggests that relying solely on phylogenetic methods for conservation prioritization may be inadequate. The results emphasize the need for a comprehensive approach, integrating multiple facets of biodiversity to effectively conserve marine ecosystems. Future research should explore more refined trait classifications and consider incorporating additional traits, such as swim speed and thermal tolerance, to improve the accuracy of the analysis and conservation prioritization.

This study provides a global assessment of elasmobranch functional diversity, highlighting the disproportionate contribution of threatened species and identifying unprotected hotspots. The limited congruence between functional and phylogenetic diversity underscores the importance of integrating functional considerations into conservation efforts. Future work should focus on finer spatial resolution and regional-based analyses to refine conservation strategies.

The study's reliance on FishBase data, while extensive, acknowledges potential data gaps and inconsistencies. The broad categories used for some traits, such as vertical position, may have limited the resolution of functional differences. The arbitrary selection of thresholds for defining biodiversity hotspots might impact the interpretation of results. The study focused exclusively on elasmobranchs, potentially neglecting potential functional redundancy from other taxonomic groups.