Abstract
A hypothesis that epigeal arachnid assemblages benefit more from greater vegetation 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 mixed livestock at varied rates. The experimental treatments comprised: no livestock, sheep only or sheep with cattle. Livestock treatments were grazed to maintain either 4.5 or 6.5 cm average sward heights between tussocks. Two 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–October 1993 and 1994 were assessed. Epigeal arachnid species composition was estimated using continuous pitfall trapping and the densities of mainly money spiders (Araneae: Linyphiidae) were estimated from monthly suction sampling and visual counts of spider webs in micro-habitats. These data were later compared with stocking rate, botanical species composition and vegetation structure. Forty of the 84 sampled species occurred in all experimental treatments. There 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. For most species of spider, harvestmen and pseudoscorpion, abundance was greater in the ungrazed and taller, grazed swards although a few species were captured in greater numbers in the treatments with shorter swards. Botanical composition, mean vegetation height and grazing intensity accounted for 48.5–53.2% 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 and 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, Lepthyphantes mengii) counted in tall, ungrazed swards compared with taller grazed swards created by sheep alone or sheep with cattle. In the treatments with fewer webs, these were occupied by more linyphiid species. Suction sampling detected greater diversity of arachnids in the ungrazed N. stricta. This was related to increased plant litter below the leaf stratum where webs were counted. Vegetation structure and not botanical species composition within the N. stricta plant community determined arachnid species composition and abundance. Furthermore, no single grazing treatment supported the total number of arachnid species represented across the entire grazing experiment. It is concluded that varied grazing management, including some temporary ungrazed areas, is necessary to maintain the structural variability of grassland patches so as to maintain a spatial mosaic that favours the optimum arachnid fauna of upland grasslands.
Published Version
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