The functional role of producer diversity in ecosystems is a key area of research in biodiversity and ecosystem functioning (BEF). This review summarizes over two decades of experiments examining how species richness of primary producers influences ecological processes in terrestrial, marine, and freshwater ecosystems. Using formal meta-analyses, the authors assess the balance of evidence for eight fundamental questions about the functional role of producer diversity. These include how primary producer diversity influences resource use efficiency and biomass production, how it affects biomass transfer to higher trophic levels, and the number of species and spatial/temporal scales at which diversity effects are most apparent. The review finds strong evidence that producer diversity enhances the efficiency of resource use and biomass production, with diverse polycultures generally outperforming monocultures. However, the results are not universal, and there is considerable variation across ecosystems. The study also highlights that diversity effects on decomposition are generally negative, with diverse polycultures showing lower standing stocks of detritus. The mechanisms behind diversity effects are complex, with complementarity and selection effects playing significant roles. While complementarity effects are more prominent in aquatic ecosystems, selection effects are more significant in terrestrial systems. The review also addresses the question of whether diverse communities outperform their most efficient or productive species. While some studies show transgressive overyielding, the majority do not, suggesting that diversity does not always lead to higher productivity. The authors conclude that the field of BEF needs to move beyond qualitative reasoning and post hoc statistical attempts to interpret mechanisms. Instead, experiments should directly test the mechanisms they hypothesize are responsible for greater biomass at higher species richness. The study emphasizes the importance of understanding the functional role of producer diversity in ecosystems and the need for further research to develop predictive science for ecosystem conservation and management.The functional role of producer diversity in ecosystems is a key area of research in biodiversity and ecosystem functioning (BEF). This review summarizes over two decades of experiments examining how species richness of primary producers influences ecological processes in terrestrial, marine, and freshwater ecosystems. Using formal meta-analyses, the authors assess the balance of evidence for eight fundamental questions about the functional role of producer diversity. These include how primary producer diversity influences resource use efficiency and biomass production, how it affects biomass transfer to higher trophic levels, and the number of species and spatial/temporal scales at which diversity effects are most apparent. The review finds strong evidence that producer diversity enhances the efficiency of resource use and biomass production, with diverse polycultures generally outperforming monocultures. However, the results are not universal, and there is considerable variation across ecosystems. The study also highlights that diversity effects on decomposition are generally negative, with diverse polycultures showing lower standing stocks of detritus. The mechanisms behind diversity effects are complex, with complementarity and selection effects playing significant roles. While complementarity effects are more prominent in aquatic ecosystems, selection effects are more significant in terrestrial systems. The review also addresses the question of whether diverse communities outperform their most efficient or productive species. While some studies show transgressive overyielding, the majority do not, suggesting that diversity does not always lead to higher productivity. The authors conclude that the field of BEF needs to move beyond qualitative reasoning and post hoc statistical attempts to interpret mechanisms. Instead, experiments should directly test the mechanisms they hypothesize are responsible for greater biomass at higher species richness. The study emphasizes the importance of understanding the functional role of producer diversity in ecosystems and the need for further research to develop predictive science for ecosystem conservation and management.