Plant nutritional and structural diversity shape multitrophic arthropod communities and grassland productivity

Arthropod communities, comprising diverse trophic groups such as herbivores, predators, and parasitoids, are intricately linked to plant traits that provide food and habitat. While it is well-established that changes in plant functional diversity (e.g., trait identity and diversity) can significantly alter arthropod diversity across trophic levels, the cascading effects on ecosystem functions remain less understood. Particularly, the role of multitrophic arthropod diversity in mediating the relationship between plant functional diversity and grassland productivity presents a critical knowledge gap in ecosystem ecology. We employed a long-term plant removal experiment in the Inner Mongolian grassland to systematically investigate how variations in the community-weight mean and diversity of multiple plant traits influence the diversity (measured by taxon richness and abundance) of herbivores and their natural enemies. Furthermore, we explored how trophic interactions between herbivores and their natural enemies influence plant community productivity. Our findings indicate that high diversity in plant nutritional traits (e.g., nitrogen, phosphorus, and sodium contents) negatively impacts plant productivity through both direct and indirect pathways. The adverse effect was mediated by an increase in the richness of sucking and chewing herbivores, which exploited high resource complementarity yet collectively suppressed plant productivity. In contrast, higher community-weighted means of plant structural traits (e.g., vegetative height and leaf lateral spread) were associated with greater plant productivity. This positive effect appears to arise from enhanced top-down control, whereby predators—particularly spiders—reduced both the richness and abundance of herbivores. Synthesis. Our study reveals that herbivores and their natural enemies respond distinctly to the variation in the composition and diversity of plant nutritional and structural traits. We show that cross-trophic interactions—specifically, diversity within herbivore and predator guilds—constitute a primary pathway through which plant functional diversity influences grassland productivity. By disentangling the links between plant trait spectra, arthropod community structure, and ecosystem functioning, our findings provide key insights for biodiversity conservation and the design of ecosystem management strategies in grasslands.