Grazing impacts on upland grassland arthropods in Scotland with particular reference to moths

Acid grassland covers 9% of Scotland and hosts many plant and animal species of conservation interest as well as being important for livestock-rearing. Invertebrates are a key component of the biodiversity of such areas for their role in nutrient cycling and as food for upland birds. We investigated interactions between grazing livestock and arthropods in terms of abundance and, for Lepidoptera, species composition. A replicated ramdomized block experiment was set up with four different grazing treatments. Three years after establishment the biomass of foliar arthropods in the ungrazed treatment was approximately twice that in the commercial stocking treatment. The numbers of spiders, bugs, beetles, craneflies and caterpillars were related to sheep stocking density or an interaction of sheep with year since the start of the experiment. We further investigated nocturnal adult Lepidoptera assemblages by light trapping in year 5. Abundance and species richness were higher in less-grazed plots with, in particular, species feeding on graminoids and over-wintering as larvae being well-represented in the ungrazed treatment. However the commercial stocking treatment hosted a higher proportion of moth species that are in severe decline in the UK. This underlines the importance, for nature conservation, of grazing aimed at producing structural heterogeneity within the vegetation and, in particular, the risks associated with abandonment of grazing in the Scottish uplands.