Facilitation by ecosystem engineers enhances nutrient effects in an intertidal system

Ecosystem engineering research has recently demonstrated the fundamental importance ofnon-trophic interactions for food-web structure. Particularly, by creating benign conditions in stressfulenvironments, ecosystem engineers create hot beds of elevated levels of recruitment, growth, and survivalof 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 theclassical food-web processes that determine energy transfer, that is, consumer–resource interactions. Onthe basis of a biomanipulation experiment covering 600 m2 of an intertidal flat, we show that ecosystemengineers influence resource uptake efficiency and the accumulation of algae following nutrientenrichment in a soft-sediment food web. Nutrient additions increased chlorophyll a concentrations in thesediment by 90%, but only in plots where we also introduced high densities (2000 per m2) of a burrowingbivalve, the common cockle Cerastoderma edule. The artificial cockle beds increased the nutrient uptakeefficiency of the biofilm and promoted sediment accumulation, which suggests that the cockles facilitatedthe sediment-living algae by increasing sediment stability. This indicates that ecological interactions, ratherthan the availability of nutrients per se, set the limits for production in this coastal ecosystem. Our resultsemphasize the need to include facilitation theory and recognize that positive interactions between speciesare key to understand, manage, and restore ecosystems under human influence