Disentangling the mechanisms by which nitrogen enrichment affects consumer damage at different organizational levels

Nitrogen enrichment can affect primary consumers by increasing foliar nitrogen content, shifting plant communities towards fast-growing species or reducing plant diversity and increasing host concentration, but these mechanisms may operate differently across organizational levels and consumer groups. We tested this in a grassland experiment manipulating nitrogen enrichment, plant species richness, fast-slow functional composition and foliar pathogen abundance. We quantified herbivore and pathogen damage at individual, population and community scales. Nitrogen enrichment affected consumer damage primarily through indirect effects mediated by plant community characteristics rather than through direct increases in damage. Plant growth strategy emerged as a key driver of consumer pressure across scales, particularly for pathogens, while herbivory depended on interactions between focal plant strategy, neighbourhood functional composition and spatial scale, indicating strong associational effects. Resource concentration effects were present but strongly contingent on consumer type and plant growth strategy: slow-growing species often experienced increased consumer damage at high conspecific frequency, whereas fast-growing species showed little response to host concentration and instead experienced greater damage in species-rich communities. Overall, the scalability of consumer damage depends on plant functional strategy and consumer specialization. Integrating consumer specialization with trait-based approaches is, therefore, essential for predicting plant-consumer interactions under global change. This article is part of the theme issue 'Wild plant pathosystems'.