a b s t r a c t a r t i c l e i n f oHuman exploitation of bivalve populations has changed intertidal landscapes worldwide. Many bivalves are ecosystem engineers that modify the physical environment, affecting the conditions for their survival. Here we argue that lack of recovery of overexploited intertidal bivalve beds may be partly caused by the loss of important biological feedbacks from depleted populations. In a large-scale experiment we investigated engineering effects of cockles (Cerastoderma edule L.) and lugworms (Arenicola marina L.) on juvenile cockles by adding high densities of either species to 5 × 5 m plots in areas with different hydrodynamic and sediment conditions in the intertidal flats of the Wadden Sea. We hypothesized that cockles would facilitate the new generations by increasing sediment stability, while lugworms would have negative effects on juvenile cockles through sediment disturbance. We found that in sandy areas with high wave and current energy cockles enhanced sediment accumulation and promoted local densities of young cockles, while lugworms did not have any effect on juvenile cockles. In muddy sites sheltered from the tidal currents by mussel reefs (Mytilus edulis L.), juvenile cockle densities were generally high, demonstrating the general importance of biological engineering for recruitment processes in the intertidal. We suggest that the acknowledgement of positive feedbacks between bivalves and sediment stability is essential to achieve long-term restoration goals in coastal ecosystems.

Abstract.Ecosystem engineering research has recently demonstrated the fundamental importance of non-trophic interactions for food-web structure. Particularly, by creating benign conditions in stressful environments, ecosystem engineers create hot beds of elevated levels of recruitment, growth, and survival of associated organisms; this should fuel food webs and promote production on the ecosystem scale. However, there is still limited empirical evidence of the influence of non-trophic interactions on the classical food-web processes that determine energy transfer, that is, consumer-resource interactions. On the basis of a biomanipulation experiment covering 600 m 2 of an intertidal flat, we show that ecosystem engineers influence resource uptake efficiency and the accumulation of algae following nutrient enrichment in a soft-sediment food web. Nutrient additions increased chlorophyll a concentrations in the sediment by 90%, but only in plots where we also introduced high densities (2000 per m 2 ) of a burrowing bivalve, the common cockle Cerastoderma edule. The artificial cockle beds increased the nutrient uptake efficiency of the biofilm and promoted sediment accumulation, which suggests that the cockles facilitated the sediment-living algae by increasing sediment stability. This indicates that ecological interactions, rather than the availability of nutrients per se, set the limits for production in this coastal ecosystem. Our results emphasize the need to include facilitation theory and recognize that positive interactions between species are key to understand, manage, and restore ecosystems under human influence.