Vertebrate grazing can mitigateimpacts of nutrient addition on plant diversity and insect abundance in a semi‐natural grassland

Human‐induced nutrient eutrophication is a major threat to grassland biodiversity, because it promotes the dominance of fast‐growing plants. Negative impacts of fertilization on plant biodiversity may be offset by grazing by large vertebrate herbivores. However, whether grazers also mitigate impacts of nutrient addition on insects is less well understood. We use a field experiment to test how plant communities and abundances of pollinators and grasshoppers respond to nutrient addition and grazing by different assemblages of large herbivores, i.e. access by all herbivores (including cattle and horses), access by wild herbivores only (wild boar and deer), no access by large herbivores. Plant biomass increased, plant diversity decreased and community composition shifted towards lower forb cover in response to fertilization, but only in the absence of all herbivores. Flower visitation by Hymenoptera (bees and wasps), i.e. the most abundant pollinator group, was reduced by nutrient addition only in the absence of all herbivores and was positively related to flowering plant richness. In contrast, flower visitation by Diptera (e.g. hoverflies) was enhanced by fertilization, but not affected by grazing. Orthoptera (grasshopper) abundance was reduced by grazing and enhanced by nutrient addition, with positive impacts of fertilization tending to be stronger in plots with only wild or no herbivores. The abundance of grasshoppers was positively related to grass biomass. We conclude that vertebrate herbivores can offset impacts of fertilization on both plant and insect communities, making grazing by large mammals an essential tool to protect insects, particularly pollinators. Most responses to nutrient addition were only apparent in plots without any large herbivores, suggesting that wild herbivores alone could already mitigate nutrient impacts. We also show that insects with contrasting feeding guilds may be favoured by fertilized, ungrazed conditions. Therefore, creating a mosaic of patches grazed at different intensities will enhance overall insect biodiversity.