S. Mahunka szerk.: Folia Entomologica Hungarica 56. (Budapest, 1995)

The effects of sheep grazing on the community structure of grasshoppers (Orthoptera) T. Kisbenedek The effects of sheep grazing on the communtiy structure of grasshoppers (Orthoptera) - Community structure parameters of three grasshopper communities were compared to each other on three neighbouring sites near Orfű in the Mecsek Mts, Hungary. Two of the study sites were lightly grazed by sheep twice a day and one site was undisturbed. Two hypotheses were tested whether (1) the species number and density of grasshoppers decrease with regularly grazing, (2) some grasshopper species prefer the disturbed sites and their species number and abundance increase there. In spite of the different vegetation structure of the study fields, the statistical analyses showed no significant differences in species compositions between the disturbed and the undisturbed sites. But in abundance and life-time population dynamics of the grasshopper communities significant differences were found. On the disturbed sites these parameters were similar to each other and they differed from the on undisturbed site. INTRODUCTION Several studies have emphasized the relationship between the structure of grasshopper communities and plant communities (Quinn et al. 1991û, Joern & Lawlor 1981, Kemp et al. 1990, Joern 1979) and also the relationships between the species richness and diversity of grasshopper communities and plant species richness and diversity (Quinn & Walgenbach 1990, Kemp 1992, etc.). It has also been proved that some of the insect groups are determined by plant species composition, while others by plant architecture (Southwood et al. 1979).The main habitat factors, which seem to play important role in structuring the grasshopper communities, are vegetation morphology, percentage of plant cover, composition of plant species, spatial patterns and soil composition (Kemp 1992, Quinn et al. 1991a, 19916). Joern (1979) emphasized that the morphological characteristics of plant specimens and the nutritional quality of plant species also play important roles in structuring grasshopper communities. So it can be seen that the morphology of the individual plants and other parameters of the vegetation are among those habitat characteristics, which are responsible for the structuring of the grasshopper community. One of the most frequently occurring disturbances, which is able to alter the community structure of invertebrates via changing the vegetation structure, is usually the grazing of livestock, e.g. cattle or sheep on different grasslands and pastures. Grazing can change the above mentioned factors, but the reare indirect effects which are not always evident. Welch etal. (1991) compared the densities and the species composition of grasshoppers between lightly and heavily grazed sites for almost twenty years. They did not find significant differences in species compositions, but detected significant changes in den-