The article reviews over two decades of research on how species richness of primary producers influences various ecological processes in terrestrial, marine, and freshwater ecosystems. Using meta-analyses, the authors assess the evidence for eight fundamental questions about the functional role of producer diversity, including its impact on resource use efficiency, biomass production, transfer to higher trophic levels, and decomposition. The results show that diverse polycultures generally produce more biomass, sequester more nutrients, and decompose litter more efficiently than monocultures. However, the evidence for these effects is mixed across different ecosystems and species. The authors also explore the mechanisms behind these diversity effects, finding that complementarity and selection effects play significant roles, though the specific mechanisms are often not well supported by direct statistical tests. The article concludes by highlighting the need for more targeted research to better understand the "bottom-up" impacts of producer diversity on higher trophic levels and to develop predictive models for ecosystem management and conservation.The article reviews over two decades of research on how species richness of primary producers influences various ecological processes in terrestrial, marine, and freshwater ecosystems. Using meta-analyses, the authors assess the evidence for eight fundamental questions about the functional role of producer diversity, including its impact on resource use efficiency, biomass production, transfer to higher trophic levels, and decomposition. The results show that diverse polycultures generally produce more biomass, sequester more nutrients, and decompose litter more efficiently than monocultures. However, the evidence for these effects is mixed across different ecosystems and species. The authors also explore the mechanisms behind these diversity effects, finding that complementarity and selection effects play significant roles, though the specific mechanisms are often not well supported by direct statistical tests. The article concludes by highlighting the need for more targeted research to better understand the "bottom-up" impacts of producer diversity on higher trophic levels and to develop predictive models for ecosystem management and conservation.