The environmental context and traits of habitat-forming bivalves influence the magnitude of their ecosystem engineering

The effective use of ecosystem engineers in biodiversity conservation is contingent on an understanding of those factors that influence the magnitude and direction of their effects. At patch scales, effects of ecosystem engineers on associated communities can range from positive to negative according to how the ecosystem engineer modifies environmental conditions. In a metaanalysis of 68 empirical studies, we assessed how, for a widespread group of ecosystem engineers- the marine habitat-forming bivalves-bivalve taxon, density, habitat, tidal elevation and latitude, as well as habit, or lifestyle, of associated taxa, influences the magnitude and direction of their effect on associated invertebrates. Overall, marine bivalves had a positive effect on both species abundance and species density, but effect sizes varied considerably according to bivalve traits and environmental setting. Oysters enhanced invertebrate abundance to a greater extent than either mussels or pinnids, perhaps because of the greater habitat heterogeneity they provide. Nevertheless, the effect of mussels on associated communities was generally more responsive to spatial variation in engineer traits and environmental context than the effect of oysters or pinnids. Positive effects of mussels on associated species abundance decreased at high mussel densities, were greater at subtidal than mid-low intertidal elevations and differed among faunal habits depending on habitat setting. Knowledge of those conditions under which positive effects of bivalves on associated biodiversity is greatest will help in identifying which species of ecosystem engineer, at which sites, should be prioritised for conservation and restoration, where the goal is enhancement of biodiversity.