Centenary shipwrecks reveal the limits of artificial habitats in protecting regional reef fish diversity.
The sinking of artificial structures has become increasingly common around the world, but whether the artificial structures favour or disfavour fish diversity remain under debate. Sinking may empty the nearby natural reefs locally and regionally by attracting their biota. Conversely, it may improve environmental conditions for species survival and reproduction, acting as source of diversity at the local and regional levels. We tested these contrasting hypotheses by assessing the taxonomic, functional and phylogenetic diversity of 12 fish communities in Northeast Brazil: four ageing (>100-year-old) shipwrecks and eight surrounding natural reefs at comparable depths and distances. We partitioned the gamma diversity of artificial and natural reef communities into independent alpha and beta components, accounted for species' abundance and assessed whether beta patterns were mostly driven by spatial turnover or nestedness. We recorded 6,335 individuals distributed in 88 fish species and 38 families. While artificial and natural reefs shared 50 species (57%), 21 species (24%) were exclusive to the artificial reefs, suggesting that the nearby natural reefs-the most likely original source of these exclusive species-do not harbour them anymore. Alpha diversity of typical and dominant species did not significantly differ between the reef types, but alpha diversity of rare species was taxonomically, functionally and phylogenetically higher in artificial reefs, indicating positive effects of the structures at the local scale. By contrast, regional beta diversity was higher in natural reefs in terms of taxonomic and functional diversity, regardless of species abundance. Pairwise beta diversity indicated that turnover had a large effect on the compositional dissimilarity in both reef types, whereas nestedness was almost irrelevant in artificial reefs. Synthesis and applications. Artificial structures such as shipwrecks may promote the co-occurrence of rare species, but they are unable to produce the beta diversity patterns that natural reefs do, even following many decades of colonization. Although artificial habitats host a significant portion of the regional reef fish diversity, they may have also contributed to the degradation of nearby natural reefs. We recommend the establishment of regulated diving spots, fishing grounds and no-take areas as a strategy to conserve regional fish diversity.