High temperature frequently increases facilitation between aquatic foundation species: A global meta-analysis of interaction experiments between angiosperms, seaweeds and bivalves

1. Many studies have quantified the ecological impacts of individual foundation species (FS). However, emerging data suggest that FS often co--occur, potentially inhibiting or facilitating one another, thereby causing indirect, cascading effects on surrounding communities. Furthermore, global warming is accelerating, but little is known about how interactions between co-occurring FS vary with temperature. 2. Shallow aquatic sedimentary systems are often dominated by three types of FS: slower-growing clonal angiosperms, faster-growing solitary seaweeds and shell-forming filter-and deposit-feeding bivalves. Here, we tested the impacts of one FS on another by analyzing manipulative interaction experiments from 148 papers with a global meta-analysis. 3. We calculated 1942 (non-independent) Hedges' g effect sizes, f rom 11,652 extracted values over performance responses, such as abundances, growth or survival of FS and their associated standard deviations and replication levels. Standard aggregation procedures generated 511 independent Hedges' g that were classified into six types of reciprocal impacts between FS. 4. We found that (i) seaweeds had consistent negative impacts on angiosperms across performance responses, organismal sizes, experimental approaches and ecosystem types; (ii) angiosperms and bivalves generally had positive impacts on each other (e.g. positive effects of angiosperms on bivalves were consistent across organismal sizes and experimental approaches, but angiosperm effect on bivalve growth and bivalve effect on angiosperm abundance were not significant); (iii) bivalves positively affected seaweeds (particularly on growth responses); (iv) there were generally no net effects of seaweeds on bivalves (except for the positive effect on growth) or angiosperms on seaweeds (except for the positive effect on 'other processes') and (v) bivalve interactions with other FS were typically more positive at higher temperatures, but angiosperm-seaweed interactions were not moderated by temperature. 5. Synthesis. Despite variations in experimental and spatiotemporal conditions, the stronger positive interactions at higher temperatures suggest that facilitation, particularly involving bivalves, may become more important in a future warmer world. Importantly, addressing research gaps, such as the scarcity of FS interaction experiments from tropical and freshwater systems and for less studied species, as well as testing for density-dependent effects, could better inform aquatic ecosystem conservation and restoration efforts and broaden our knowledge of FS interactions in the Anthropocene.