Plant biomass and macro- and micronutrients jointly drive top-heavy arthropod trophic pyramids in grassland monocultures.

Identifying the key drivers of changes in species biomass, abundance, and distribution across trophic levels in terrestrial ecosystems represents a fundamental challenge in ecology. The shape of trophic pyramids-reflecting the relative biomass or abundance of various trophic groups, such as herbivores, omnivores, predators, and parasitoids-is anticipated to vary with both the quantity and nutritional quality of plants. However, a comprehensive understanding of how these factors influence the shape of trophic pyramids in grassland ecosystems remains elusive. In this study, we utilize replicated monocultures of 15 herbaceous species in the Inner Mongolian grassland to investigate the effects of plant biomass, macronutrients (including protein, carbohydrate, and phosphorus), and micronutrients (such as sodium) on the structure of arthropod trophic pyramids and the abundances within different trophic levels. Our results demonstrated that plant biomass, leaf protein-to-carbohydrate ratio, and sodium content collectively contributed to a top-heavy structure in arthropod trophic pyramids, characterized by a relatively higher proportion of predators and parasitoids. Specifically, plant biomass enhanced this top-heaviness both directly, by increasing the abundance of predators and parasitoids, and indirectly, by increasing chewing and sucking herbivores, which in turn bolstered the abundance of predators and parasitoids. Furthermore, leaf sodium and the protein-to-carbohydrate ratio positively influenced parasitoid abundance through increasing the abundance of sucking herbivores and endophytes. Given that climate change and human activities, such as nitrogen fertilization and saline water irrigation, are altering plant biomass, nutrient composition, and sodium availability globally, our findings suggest that these changes will have significant cascading effects on arthropod trophic structures and overall ecosystem functioning.