Oysters at work: How Europe's reef engineers shape species diversity

European flat oyster (Ostrea edulis) populations are collapsed, and wild habitats are now exceedingly rare. The present study is the most comprehensive species diversity study of wild Ostrea edulis habitats to date, documenting 290 species across five remnant, restored and fished oyster reefs in Scotland. By examining both epibiotic and infaunal assemblages from hand-gathered box cores, the role of native oysters in shaping benthic species diversity and functional community structure was evaluated. In oyster habitats, significantly higher epibiotic diversity and greater abundance of habitat-structuring traits (e.g. sessility and habitat creation) were observed. On average, epibiotic diversity was ∼50% higher where oysters were present and 80% higher in shallow, undisturbed habitats, compared to where they were absent, with statistical models confirming oyster presence as a significant predictor of species diversity. In contrast, infaunal assemblages showed weak and non-significant responses, with only modest functional shifts and a slight increase in species diversity. These findings provide rare empirical evidence of the importance of O. edulis as an ecosystem engineer, enhancing surface habitat complexity and supporting functionally rich and diverse epibiotic communities. Trait-based analyses further indicate that many associated epibiotic species contribute to this structure reinforcing biodiversity through positive feedback. These findings demonstrate that oyster habitats consistently deliver enhanced species diversity, highlighting the need to prioritise such habitats for restoration, whilst providing rare reference data for international restoration efforts such as those stimulated by the recent European Restoration Regulation.