Biodiversity loss can reduce the capacity of ecosystems to provide the many functions and services on which humans depend. However, most evidence for this comes from experiments, which ignore the complexity of natural ecosystems and exclude other important drivers of ecosystem functioning such as the abiotic environment or land use. Studies in real landscape are becoming more common but they largely focus on the effects of the diversity of single trophic groups or on individual ecosystem functions. We therefore have very limited understanding of how different trophic groups might vary in their effects on functioning and how many groups drive particular functions. We measured the species richness and abundance of nine trophic groups (including plants, herbivorous and predatory invertebrates, bacteria, mycorrhizae and protists), which together contained 4,600 taxa, in 150 grasslands. We then analysed the effect of these 9 groups on 14 individual ecosystem functions and on multifunctionality (the simultaneous provision of all functions).
We found that all trophic groups were functionally important: the richness and/or abundance of each group were important drivers of at least one function and three trophic groups, on average, influenced each ecosystem function. Therefore, models including multiple trophic groups explained three times more variation in both individual functions and in multifunctionality than plant species richness alone, the most widely used metric of biodiversity. We were also able to identify plants, herbivorous invertebrates and bacteria as the most functionally important trophic groups. Furthermore, biotic variables (the abundance and diversity of the trophic groups) were as important drivers of the functioning of these real-world ecosystems as abiotic conditions (e.g., soil type, climate) and land-use intensity. These findings indicate that a previous focus on single groups may have underestimated the importance of biodiversity for the functioning of real world systems.