The effects of varied grazing management on epigeal spiders, harvestmen and pseudoscorpions of Nardus stricta grassland in upland Scotland

A hypothesis that epigeal arachnid assemblages benefit more from greater ve getation structure than botanical species composition in upland grasslands was tested. The test was carried out within a grazing experiment, initiated in 1991, to investigate vegetation dynamics in response to stocking with m ixed livestock at varied rates. The experimental treatments comprised: no l ivestock, sheep only or sheep with cattle. Livestock treatments were grazed to maintain either 4.5 or 6.5 cm average sward heights between tussocks. T wo replicates of each treatment were used and allocated to 10 plots across 22 ha of Nardus stricta-dominated grassland. The effects on epigeal arachnids (excluding acarines) of the botanical and structural differences of the grassland between treatments during April-Oct ober 1993 and 1994 were assessed. Epigeal arachnid species composition was estimated using continuous pitfall trapping acid the densities of mainly mo ney spiders (Araneae: Linyphiidae) were estimated from monthly suction samp ling and visual counts of spider webs in micro-habitats. These data were la ter compared with stocking rate, botanical species composition and vegetati on structure. Forty of the 84 sampled species occurred in all experimental treatments. Th ere was a significant effect of treatment on the number of arachnid species in suction but not in pitfall samples. There was also a significant effect of treatment on the relative abundance of 26% of these arachnid species. F or most species of spider, harvestmen and pseudoscorpion, abundance was gre ater in the ungrazed and taller, grazed swards although a few species were captured in greater numbers in the treatments with shorter swards. Botanica l composition, mean vegetation height and grazing intensity accounted for 4 8.5-53.2% of the variability in the species composition/relative abundance of these arachnids, calculated by direct gradient analysis. Almost half of the species were randomly distributed across the experimental treatments an d are recorded as widespread in upland heathland or grassland habitats and lowland grassland. More spider webs were counted during July-September 1993 -1994, with greater numbers (dominated by the linyphiid species, Lepthyphan tes mengii) counted in tall, ungrazed swards compared with taller grazed sw ards created by sheep alone or sheep with cattle. In the treatments with fe wer webs, these were occupied by more linyphiid species. Suction sampling d etected greater diversity of arachnids in the ungrazed N. stricta, This was related to increased plant litter below the leaf stratum where webs were c ounted. Vegetation structure and not botanical species composition within the N. st ricter plant community determined arachnid species composition and abundanc e. Furthermore, no single grazing treatment supported the total number of a rachnid species represented across the entire grazing experiment. It is con cluded that varied grazing management, including some temporary ungrazed ar eas, is necessary to maintain the structural variability of grassland patch es so as to maintain a spatial mosaic that favours the optimum arachnid fau na of upland grasslands. (C) 2001 Elsevier Science B.V. All rights reserved .