The effects of livestock grazing on foliar arthropods associated with bird diet in upland grasslands of Scotland.
Upland biotopes have conservation importance for their typical plant and animal species. Recently, the condition of upland habitats has deteriorated with associated declines in many upland birds. Grazing by increased densities of sheep has been implicated in these changes. Studies in lowland agricultural land have shown a link between declines in bird populations and the availability of arthropod prey. We studied the effect of three grazing regimes and an ungrazed control, each replicated six times in a total of 24 3.3-ha plots, on the numbers and overall biomass of foliar arthropods in upland grassland in the Southern Highlands. The three grazed treatments of the experiment were stocked with the commercial stocking density of sheep, one-third of the commercial stocking density by sheep only and one-third of the commercial stocking density by sheep and cattle. Arthropod groups recognized as components of the diet of moorland birds were sampled by motorized suction sampler and sweep net. Arthropod numbers were unaffected by grazing treatments 6 months after grazing commenced. Significant grazing treatment effects on spiders, bugs and beetles were observed in years 2 and 3, with higher numbers in the less-grazed treatments, but no such effect on brachyceran flies, caterpillars and craneflies. A residual maximum likelihood (REML) analysis related the numbers of spiders, bugs, beetles, craneflies and caterpillars to either the stocking density of sheep or an interaction of sheep with year. The analysis related bugs and brachyceran flies to an interaction between cattle stocking density and year. Estimated total biomass of foliar arthropods increased significantly with decreasing grazing intensity in years 2 and 3 and biomass in the ungrazed treatment was approximately twice that in the commercially grazed treatment. The REML analysis related biomass to the stocking density of sheep and both the stocking density of sheep and of cattle in an interaction with year. Synthesis and applications. We demonstrated that the stocking density and inclusion of cattle grazing affects the numbers and biomass of particular foliar arthropods in the uplands of Scotland. Grazing management is important not only for the conservation of arthropods per se but also as food for insectivorous birds of conservation concern.