Food web complexity underlies biodiversity effects on ecosystem functioning.

Biodiversity change has elicited widespread concern over the consequences for functions and services provided by ecosystems. Despite extensive evidence for a positive effect of biodiversity on ecosystem functioning within a single trophic level, how this biodiversity effect varies with multi-trophic food web structure remains unresolved even though most ecosystems contain two to six trophic levels. We investigate how food web complexity modulates biodiversity-ecosystem functioning relationships in nature by quantifying energy fluxes as proxies for two principal ecosystem functions-primary consumption and predation-in 318 highly resolved, complex food webs from marine, lake, stream and soil ecosystems. Ecosystem functioning increased consistently with taxon richness across all trophic levels and ecosystems, which arose from greater vertical diversity (that is, maximum trophic level) and trophic complementarity of predators in more taxonomically diverse food webs. Furthermore, predator trophic complementarity increased predation fluxes in all freshwater ecosystem types. These findings highlight the threat of trophic downgrading to critical ecosystem functions (for example, biological control and maintenance of biodiversity and ecosystem stability) provided by predators, which are typically most vulnerable to anthropogenic disturbances. Our study demonstrates that the consequences of biodiversity change are deeply entangled within the web of life, emphasizing the need to conserve the trophic complexity underlying biodiversity-ecosystem function relationships.