Predation and habitat modification synergistically interact to control bivalve recruitment on intertidal mudflats

Bivalves are key components of coastal ecosystems because they link pelagic and benthic food webs, and shape the landscape through habitat modification. Nevertheless, many bivalve stocks have dramatically declined, and recruitment failure due to (anthropogenically-) increased predation by mesopredators and loss of facilitation mechanisms have been separately hypothesized as underlying causes. Here, we tested the interactive effects of predation and habitat modification on bivalve recruitment in a large-scale experiment in the Wadden Sea, one of the world's largest intertidal soft-sediment ecosystems. We applied anti-erosion mats to simulate biotic attachment and substrate stabilization by commonly found tubeworm beds, crossed this with addition of adult mussels, and manipulated shrimp and crab predation using exclosures within these treatments. Epibenthic mussel recruits were only found in treatments with manipulated substrates, attached to either the anti-erosion mat or adult mussels. Three out of four endobenthic species were facilitated by the mat, but were inhibited by adult mussels. In contrast, invasive surf-dwelling American razor clams were inhibited by both substrate manipulations, indicating a preference for unstable sediments. These facilitation and inhibition effects, however, only clearly emerged when predators were excluded, demonstrating strong synergistic effects between predation and habitat modification. Our findings suggest that disturbance of trophic interactions and loss of habitat modifying species interactively affect bivalve recruitment dynamics in coastal ecosystems. We conclude that conservation and restoration of bivalves should focus on protecting and restoring internal facilitation mechanisms, and should simultaneously reduce excessive mesopredator predation by restoring natural food web dynamics, including the role of top-predators.