Bioturbators enhance ecosystem function through complex biogeochemical interactions

Predicting the consequences of species loss is critically important, given present threats to biological diversity such as habitat destruction, overharvesting and climate change1. Several empirical studies have reported decreased ecosystem performance (for example, primary productivity) coincident with decreased biodiversity2,3,4, although the relative influence of biotic effects and confounding abiotic factors has been vigorously debated5,6,7. Whereas several investigations focused on single trophic levels (for example, grassland plants)8,9, studies of whole systems have revealed multiple layers of feedbacks, hidden drivers and emergent properties10,11, making the consequences of species loss more difficult to predict12. Here we report functionally important organisms and considerable biocomplexity in a sedimentary seafloor habitat, one of Earth's most widespread ecosystems. Experimental field measurements demonstrate how the abundance of spatangoid urchins—infaunal (in seafloor sediment) grazers / deposit feeders—is positively related to primary production, as their activities change nutrient fluxes and improve conditions for production by microphytobenthos (sedimentatry microbes and unicellular algae). Declines of spatangoid urchins after trawling are well documented13,14, and our research linking these bioturbators to important benthic–pelagic fluxes highlights potential ramifications for productivity in coastal oceans.