Marine foundation species facilitate biodiversity: systematic and quantitative syntheses identify trait‐based standards to understand seascape functionality

Marine foundation species play a critical role in shaping seascapes and supporting biodiversity through their physical structure and biological activity. However, global changes can alter foundation species and subsequently induce profound changes in ecosystem functions and integrity. To understand better how foundation species influence biodiversity, we first conducted a systematic review of 311 articles that either compared biodiversity metrics associated with foundation species or examined relationships between biodiversity and foundation species traits (at the individual or habitat scale). A subsequent meta-analysis on a subset of suitable articles showed that foundation species generally enhance species richness by an average of 72%, although the strength and nature of this effect vary across scales and taxa. To understand these variations better, we synthesised the range of traits associated with foundation species and how these relate to various diversity metrics. While many traits are measured at both individual and habitat scales, inconsistencies across studies and foundation species groups prevent a formal assessment of functional trade-offs across marine foundation species. Despite high variability in trait-richness relationships, both common (e.g. density) and more specific (e.g. interstitial space) traits can serve as reliable proxies for species richness when capturing facilitative processes at the relevant scale for the response community. To reconcile this diversity of traits, we classify foundation species traits into 26 standardised categories, which constitutes a valuable framework to consolidate the interoperability of future research on the effects of foundation species on biodiversity. This classification and the associated database on foundation species-biodiversity relationships set a baseline for a comprehensive assessment of how multiple foundation species shape heterogeneous seascapes, which is essential for predicting the effects of habitat alteration or restoration on coastal biodiversity.