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.