The response of epigeal beetles (Col.: Carabidae, Staphylinidae) to varied grazing regimes on upland Nardus stricta grasslands.
Abstract
The effect of different livestock grazing regimes on the insect fauna of an upland, semi-natural grassland was measured in 1993 and 1994 in the UK by a survey of the epigeal Carabidae and Staphylinidae within an experiment established in 1991. Grazing by sheep, or sheep and cattle, to achieve two different inter-tussock sward heights, provided four treatments. In addition, a further treatment was ungrazed from 1992 to test the impact on beetles of a short-term cessation of grazing, trampling and dung inputs. Pitfall traps sampled Carabidae and Staphylinidae within the N. stricta-dominated grassland of the experiment. Data on these epigeal Coleoptera were collected from April to October in 1993 and 1994 (years three and four of the experiment). The epigeal Coleoptera species were ranked by decreasing abundance in traps, where the captures in traps were accumulated for both seasons. The responses to the grazing regimes were analysed using ANOVA, applied to the most abundant species (that together represented 99% of the two seasons' catch). There were significant experimental effects of grazing regime on 5 of the 32 species of Coleoptera, namely Carabus violaceus, Othius angustus, Pterostichus strenuus, Xantholinus linearis and Olophrum piceum. The ordination technique, canonical correspondence analysis (CCA), was applied to the data on the Coleoptera assemblage. Variables measured to represent the experimental treatments (mean vegetation height, stocking rate and botanical diversity) and environmental covariables (altitude and aspect) were entered in the direct gradient analysis procedure of CCA. This application of CCA removed the effects of altitude and aspect of each plot and revealed the significant effects of vegetation structure, botanical species composition and stocking density on a larger number of Coleoptera species than suggested from ANOVA. Twenty-four of the 32 most abundant Coleoptera species correlated with the effects of different grazing regimens imposed on Nardus grassland. Greater abundances of C. violaceus, Othius angustus, X. linearis and T. corticinus were indicative of the typical upland grassland and heathland Coleoptera assemblage. These species could be monitored to balance the impact of grazing management on arthropod biodiversity with the need to restrict the dominance of N. stricta in drier upland grasslands, achieved in this instance, by summer grazing sheep and cattle to maintain an average between-tussock sward height of 6-7 cm. However, the results from the direct gradient analysis suggested that the grazing regimes should be varied in rotation over time to achieve a mosaic of structurally different grassland patches (0.70-4.73 ha), because this encourages a larger overall number of beetle species.