Resource-use complementarity of producer species is often invoked to
explain their generally positive diversity-productivity relationships.
Additionally, multi-trophic interactions that link processes across
trophic levels have received increasing attention as a possible key
driver. Given that both are integral to natural ecosystems, their
interactive effect should be evident but has remained hidden. We address
this issue by analyzing diversity-productivity relationships in a
simulation experiment of primary producer communities nested within
complex food-webs, manipulating resource-use complementarity and
multi-trophic animal richness. We show that both mechanisms' joint
contribution to positive diversity-productivity relationships generally
exceeds their individual effects, as both interactively create diverse
communities of complementary producer species. Specifically,
multi-trophic interactions in animal-rich ecosystems increase
complementarity the most when resource-use complementarity is low. The
interdependence of top-down and bottom-up forces in creating
biodiversity-productivity relationships highlights the importance to
adopt a more multi-trophic perspective on biodiversity-ecosystem
functioning relationships.