Beyond biodiversity: The more essential role of multi-trophic network complexity for predicting ecosystem multifunctionality under multiple stressors.

Biodiversity plays a critical role in regulating ecosystem functions in the context of global environmental change. However, current understanding remains disproportionately focused on single-trophic-level diversity and function, overlooking the importance of multi-trophic diversity and species interactions in driving multiple ecosystem functions, particularly in freshwater ecosystems. Here, we conducted a full-factorial mesocosm experiment to investigate the effects of three environmental stressors-nitrogen and phosphorus enrichment, dissolved organic carbon input, and fish disturbance-on ecosystem multifunctionality (EMF). All pairwise and three-way interactions in experimental treatments exhibited strictly additive effects on EMF. Linear regression analysis revealed that species richness and co-occurrence network complexity across multi-trophic levels (phytoplankton, zooplankton, and planktonic bacteria) have significant positive correlation with EMF. Structural equation modeling (SEM) further demonstrated that models incorporating multi-trophic biodiversity and network complexity provided the most robust explanations for the observed EMF changes. Random forest models indicated that multi-trophic biodiversity had stronger predictive power than single-taxon biodiversity. Notably, multi-trophic network complexity outperformed biodiversity alone in predicting EMF, highlighting the critical role of species interactions in determining EMF. Our results advance ecological theory by demonstrating multi-trophic network complexity involving multi-trophic richness and species connectivity as a critical determination of EMF, which provides a mechanistic framework for freshwater conservation prioritizing cross-trophic network topology rather than mere species counts.