Habitat modification drives benthic trophic diversity in an intertidal soft-bottom ecosystem

In intertidal soft-bottom ecosystems, ecosystem engineers such as reef-building bivalves, can strongly affect the associated benthic community by providing structure and stabilizing the sediment. Although several engineering species have declined dramatically in the past centuries, the consequences of their loss for the trophic structure of intertidal benthic communities remain largely unclear. In this study, we experimentally test the hypothesis that above- and belowground habitat modifications by ecosystem engineers, facilitate distinctly different, but trophically more diverse benthic communities, using intertidal mussel and tube worm beds as model systems. We constructed a large-scale experiment at two intertidal mudflats in the Dutch Wadden Sea, with distinctly different environmental conditions. At both sites, we applied anti-erosion mats to simulate belowground structure and sediment stabilization by commonly found tube worm beds and crossed this with the addition of adult mussels to investigate effects of aboveground structure. The anti-erosion mats mainly enhanced species and trophic diversity (i.e., feeding guild richness and diversity) of the infaunal community, while the addition of mussels primarily enhanced species and trophic diversity of the epifaunal community, irrespective of location. The effect size of mussel addition was larger at the exposed site in the western Wadden Sea compared to the more sheltered eastern site, probably due to relatively stronger abiotic stress alleviation. We conclude that structure-providing and sediment-stabilizing species such as reef-building bivalves and tube worms, form the foundation for trophically diverse benthic communities. In intertidal soft-bottom ecosystems like the Wadden Sea, their conservation and restoration are therefore critical for overall ecosystem functioning.