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

Nitrogen enrichment could affect primary consumers by increasing foliar N content (nitrogen-disease hypothesis), by increasing dominance of palatable, fast-growing plants (growth-defence trade-off) or by reducing plant diversity (resource concentration effect). These mechanisms might operate differently at different organizational levels. We tested this in a grassland experiment (PaNDiv), manipulating nitrogen enrichment, plant species richness, fast-slow functional composition, and foliar pathogens. We assessed herbivory and pathogen damage, on focal plant individuals, species, and communities. Plant community characteristics were more important drivers of consumer damage than nitrogen enrichment. Host concentration effects strongly affected pathogens but mostly at the species level. Growth-defence trade-off effects were widespread but frequently emerged from interactions between species (associational effects) or with resource concentration effects, leading to different patterns at species and community levels. Our findings suggest that key mechanisms may operate differently at different organisational levels calling for better understanding of how consumer effects scale across levels.