Artificial reefs as functional substitutes? insights from a nine-year reef fish monitoring in Madeira

Abstract

Artificial reefs are widely deployed to mitigate natural reef degradation, yet their capacity to replicate the functional structure of natural reefs remains poorly understood. Using long-term underwater visual census data (2016-2024) from Madeira and Porto Santo Islands (NE Atlantic), we compared reef fish assemblages on shipwreck artificial reefs and natural rocky reefs through a functional trait-based framework. We quantified multiple dimensions of functional diversity, including functional richness, dispersion, specialization, redundancy, vulnerability, and functional beta diversity, and assessed temporal trajectories across reef types. Natural reefs consistently occupied a larger proportion of functional trait space than artificial reefs, reflecting broader representation of ecological roles. Functional beta diversity was dominated by nestedness, with artificial reefs generally supporting subsets of the regional functional pool. Temporal analysis revealed no evidence of progressive convergence of artificial reefs toward the functional structure of natural reefs over nearly a decade. Island context strongly modulated outcomes: in species-rich Madeira, functional structure was relatively homogeneous across reef types, whereas in Porto Santo, artificial reefs supported higher functional diversity than a heavily fished natural reef. Functional redundancy was low and functional vulnerability high across all assemblages, indicating limited buffering capacity against species loss. These results demonstrate that shipwrecks act as context-dependent complements rather than functional substitutes for natural reefs and highlight the value of functional approaches for evaluating artificial reef performance under ongoing reef degradation.