Simulated ungulate herbivory affects differently two herbivorous arthropod guilds in bilberry

Dense ungulate populations, both domestic and wild, accompanied by high grazing intensities, are known to have effects on the abundance and diversity of the community of arthropods hosted by plants. However, research combining ungulate herbivory with nutrient addition effects is scarce. By means of a nitrogen (N) addition and clipping experiment, we investigated the effects of both factors on the diversity, abundance and trophic structure of the arthropod community hosted by the bilberry, Vaccinium myrtillus, focusing on the response of herbivorous arthropods by considering leaf-chewers and sap-feeders separately. In addition, we assessed altitudinal variation on the arthropod community and also specifically on herbivores. Neither of the variables estimated for the arthropod community (total abundance, richness and biomass) were affected by simulated ungulate herbivory. However, clipping reduced the abundance of sap-feeders although had no significant effect on leaf-chewer insects. We therefore suggest that ungulates can compete with sap-feeders by reducing suitable food resources, or via changing plant nutritional state. Moreover, the lack of response on leaf-chewers could correspond with the absence of chemical defence in bilberry after repeated damage. Despite this effect on sap-feeder abundance, clipping did not cause a cascade effect on the trophic structure of the arthropod community associated with bilberry, at least in the short term. In turn, N fertilization at levels close to atmospheric N deposition did not cause any significant effect on the invertebrate community. Finally, the number of arthropod morphospecies and biomass decreased with altitude.