W2A-024 Grazing and beetles-0.txt
EFFECTS OF GRAZING INTENSITY ON HEATHLAND VEGETATION AND GROUND BEETLE ASSEMBLAGES OF THE UPLANDS OF COUNTRY ANTRIM, NORTH-EAST IRELAND

D.M. McFerran, W.I. Montgomery and J.H. McAdam

ABSTRACT

Grazing pressures have increased on the uplands of the British Isles. This is particularly evident on the Antrim Plateau of north-east Ireland. The effects of grazing pressure on heathland vegetation and ground beetle assemblages was investigated experimentally. Between June and September in 1988 and 1989, enclosures (0.64ha) on three types of heathland community — low, medium and high density of Calluna — were grazed at one of four intensities equivalent to 0-4.5 Scottish Blackface sheep/ha. The effects of grazing intensity on above-ground biomass, individual species cover, botanical composition, sward structure and associated ground beetle assemblages were assessed using standard methods. At higher grazing intensities, percentage composition of green heather, live Gramineae species, live non-Gramineae species and other dead material declined. Live Calluna was reduced by increased grazing intensity once more nutritious species were depleted. Ground beetle assemblages differed with respect to heathland community, grazing intensity and trapping dates. Trapping success of the most abundant species, Nebria salina, increased with increasing grazing intensity. The effects of management changes on the ecosystem of the Antrim Plateau are discussed.

INTRODUCTION

Economic pressures over the last four decades have increased livestock numbers in hill areas (Eadie 1984). The increased grazing pressure has resulted in significant changes in the natural vegetation of these areas. Ball et al. (1982) described the loss of plant communities with a high conservation value, such as those dominated by heather, and the increase in less valuable grasses and mosses.

Little is known of the effects of management on the wildlife of upland heaths, but recent changes in management practices such as moor drainage, reduction in burning and more intensive grazing have prompted concerns over the loss or decline in abundance of some species (Gimingham 1985); Hudson 1986). Welch (1984a, 1984b) suggested that heather declined when stocking rates were greater than 0.2 cattle and 2.7 sheep/ha. Subsequent studies by Grant, Barthram et al. (1978) and Grant, Milne et al. (1982) indicate that under'severe' grazing pressure, height, cover, standing crop, shoot production and flowering of the heather stand are reduced. Under 'moderate' grazing pressure, however, productivity may be unaffected and accumulation of biomass slowed down, resulting in reduced necessity to burn (Hudson 1986).

Since entry to the EC and the adoption of the Less Favoured Areas directive in 1975, stocking densities of sheep and cattle have increased on Northern Ireland hill farms (McAdam and Chance 1988; McAdam 1992). The effects of these increases on the plant and animal communities of the uplands of north-east Ireland are poorly documented (Meharg and Montgomery 1988). This paper reports experimental investigations carried out to determine the response of indigenous heath vegetation and associated assemblages of ground beetles to a range of grazing intensities corresponding to 0, 0.5, 1.0 and 1.5 times the stocking levels characteristic of hill farms in County Antrim. The results are discussed with respect to the future conservation of wildlife and current management practices prevailing in the Antrim Plateau.

MATERIALS AND METHODS

The Greenmount Agricultural and Horticultural College Hill Farm, Department of Agriculture (NI), Glenwherry, Co. Antrim (Irish Grid Reference D279000), lies between 250m and 400m above sea-level. The topography, land use and history of Glenwherry are described in detail elsewhere (McAdam and Chance 1988; Meharg 1988; McFerran 1991). Replicate blocks (0.64ha) of three vegetation types (overgrazed short Calluna (5-10% cover) with Eriophorum and Scirpus, here termed low-density heath; Calluna (40-50% cover) with Deschampsia and Eriohorum, here termed medium-density heath; and mature, dense Calluna (85-95% cover), constituting high-density heath) were selected and fenced on the Point and Creeve hills, approximately 1km apart. Within each block, four adjacent plots were subject to one of four grazing regimes (none, and approximately 1.5, 3.0 and 4.5 mature Scottish Blackface wethers per hectare) from June to September in 1988 and 1989. All grazed plots were 80m • 80m and the ungrazed control was 80m • 20m. Normal grazing intensity at Glenwherry is approximately 3 sheep/ha, Grazing plots were located randomly in each of the heath density blocks. Two replicate blocks (260m • 80m) of low- and high-density heath were utilised in the present study. Given the time available and the quantity of material collected, only one block of medium-density heath was investigated.

Species composition, biomass and plant community structure were measured in each plot in June and September 1988 and 1989. A modified Hill Farm Research Organisation sward stick was used to determine canopy height (mean of 320 and 80 readings in each grazed and ungrazed plot respectively). Above-ground biomass and percentage species composition by dry weight of the vegetation was determined from five randomly located 0.25m quadrats clipped to ground level. Sward structure was determined using an inclined point quadrat (IPQ) with approximately 200 hits per plot. Cover and botanical composition were assessed using a point quadrat (200 hits of a vertical needle per plot). Baseline data from June 1988 were used for comparison with results obtained in September 1989 to determine the overall impact of grazing. Although samples were taken at different times of the year, the dominance of herbaceous species in the stands and the detailed nature of the sampling techniques seem likely to permit a valid comparison of vegetation data from the beginning and end of the experiment. Plant nomenclature follows Tutin et al. 1964-80.

Between May and October 1989, ten pitfall traps (polythene containers 9cm in diameter and 20cm deep) were set at 10m intervals along the south-west to north-east diagonal of each plot. Approximately 250ml of 5% formalin solution, with a few drops of detergent, was added to prevent the escape and deterioration of specimens before collection. Traps were emptied 2-4-week intervals throughout the study period. Material obtained was frozen at -5 ̊ C until sorted. All adult ground beetles (Coleoptera: Carabidae) were separated and stored in 70% alcohol and later identified to species level. Nomenclature follows Lindroth 1974. Identifications were confirmed by Dr R. Anderson (Agricultural Chemistry Research Division, DANI).

DATA ANALYSIS

The effect of heathland community and grazing intensity on vegetation and ground beetle data in replicate blocks of low- and high-density heath was assessed using analyses of variance carried out on logarithmically transformed data (Sokal and Rholf 1981). A data matrix listing the presence or absence of each ground beetle species in each of the twenty plots was constructed. The total numbers of individuals of each species caught were not used in ordination and classification analyses since abundance data are influenced by factors other than population density, such as the activity patterns of individual species (Rushton et al. 1989). Detrended correspondence analysis (DECORANA; Hill 1979a) was used to ordinate the ground beetle species and samples from the twenty plots. The non-parametric Spearman rank correlation (Zar 1984) was used to determine the relationship between Axes scores and botanical variables. This technique was used in preference to canonical correspondence analysis (Ter Braak 1987) as the number of samples involved and the number of environmental variables measured were limited. The plots were also classified on the basis of species presence and absence using two-way species indicator analysis (TWINSPAN; Hill 1979b). The ordination and classification methods employed have been used widely in analysis of invertebrate communities (for example Fishpool and Usher 1989; Luff et al. 1992).

RESULTS

PLANTS

Biomass and species composition

Percentage species composition by dry weight varied with both heathland community and grazing intensity (Table 1). In September 1989, canopy heigth was significantly greater on high-density heath () than on low density heath, and increasing grazing intensity significantly reduced canopy height (). Increased grazing intensity also resulted in a significant increase and reduction in the percentage of dead heather () and other dead material () respectively.

Botanical composition-species cover

In September 1989, the percentage cover of Eriophorum vaginatum () and Scirpus caespitosus () were significantly greater and lower respectively on the high-density heath than on the low-density heath. By September 1989, increased grazing intensity significantly increased the proportions of live Calluna vulgaris () and Erica cinerea () and decreased the proportion of Eriophorum vaginatum (). Over time there was a significant increase in the proportion of moss species associated with the high-density heathland community (Table 2). Between 1988 and 1989 there was also a significant increase in the proportions of Calluna vulgaris and Erica cinerea on plots grazed at 3 and 4.5 sheep/ha. There was also a significant increase in the proportion of Eriophorum vaginatum () on the two plots subject to the lowest grazing intensities (Table 2; Fig. 1).

Vegetation structure

The botanical composition and species distribution as determined by IPQ changed over time and with grazing intensity. There was a greater density and proportion of Calluna vulgaris and Eriophorum vaginatum in the upper canopy horizons (20-30cm) of the high-density heath plots than in the low-density heath (Table 3; Fig. 2). Conversely, Eriophorum vaginatum and Scirpus caespitosus had a significantly greater abundance within the lower canopy horizons of the low-density heath than in the high-density heath. Comparison of sward structure between ungrazed plots and plots grazed at the highest intensity suggests that sward structure was independent of grazing pressure (Table 3; Fig. 3). However, increased grazing intensity resulted in a significant decline of dead Eriophorum vaginatum and Eriophorum angustifolium in comparison with control plots, in which dead material generally accumulated. The proportion of live Calluna vulgaris increased significantly with intensity of grazing in the 15-20cm horizon.

GROUND BEETLE ASSEMBLAGES

A total of 5510 carabid beetles of eighteen species were collected between May and September 1989 (Table 4). Three species, Pterostichus niger, Nebria salina and Carabus granulatus, comprised over 63% of the total catch. Catches of individuals and species on control plots were corrected to number per week for each of six sampling dates. Temporal changes in total numbers of individuals and species were similar, rising from May to reach a peak in August. There was a decline in individual abundance and number of species in September. Adult Pterostichus niger and Nebria salina made a major contribution to these patterns of abundance.
Total numbers of individuals trapped did not differ significantly between heath type or with grazing intensity. However, in comparison with low-density heath, significantly greater numbers of species were recorded on the high-density heathland community (). Numbers of species trapped showed a significant increase with increasing grazing intensity (). Results for alpha diversity index and the Berger-Parker dominance index suggest that diversity differed significantly between grazing intensities (equation inserted) while dominance varied significantly with heath density (). The high-density heathland community had the highest alpha index and the second highest Berger-Parker index. The highest Berger-Parker index was recorded on the low-density heath. Total numbers of the four most abundant species, Pterostichus niger, Nebria salina, Carabus granulatus and Carabus problematicus, were collated for each heathland community and associated grazing intensity. Numbers trapped on replicate plots of high- and low-density heathland communities were meaned to allow for comparison with the medium-density heath type. The most abundant species, Nebria salina, was trapped most frequently on low-density heath, numbers increasing progressively with increasing grazing intensity. There was increased trapping success for Carabus granulatus and Pterostichus niger with increased heath density. There was no clear heath density or grazing effect on numbers of Carabus problematicus. Grazing significantly affected the numbers of Carabus arvensis trapped (), with greater numbers of individuals trapped on the ungrazed control plots.
Investigation of ground beetles obtained from low- and high-density heathland communities and associated grazing intensities for six sampling dates between May and September 1989 suggested that total numbers of individuals and species varied significantly between sampling dates (respectively) and heathland community (respectively). Greatest numbers of individuals and species were recorded in June and least in September. Greater numbers of individuals and species were recorded on the high-density heath than on the low-density heathland community. Numbers of individuals also changed significantly over time in the different heathland communities ().
