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

Fig. 3. Shannon-Wiener diversity index (FT) of the grasshopper communities. They slightly changed from July till September, and the lowest diversity values were found on the meadow sites they were wandering between the sites, and mixed easily up owing to the short distances between the sites. That is why they did not show differences in their structure. Community ecologists claim that terrestrial herbivores are often predator-limited and seldom food-limited (Slobodkin et al. 1967, Hairston et al. 1960, Hairston 1989). An alternative view is that herbivore populations are food-limited (Fretwell 1977), and predators only kill prey already destined to die from food-shortages. Another possible explanation integrating prédation and food-limitation claims that the competitive allocation of food resources between the different herbivore species is modified by selective prédation (Paine 1966, 1984). Grazing could also be such a structuring factor. De Keer etal. (1989) found different horizontal distribution of the spider fauna on the intensively grazed pastures, the different pattern of these lands influences the diurnal activitiy of the spiders, too. Deltschev & Kajak (1974) determined the influence of grazing on the spider fauna and came to the conclusion that grazing destroyed almost completely the spider group associated with the vegetation zone. Duffey (1962) stressed the structural heterogeneity of the vegetation being the main factors determining the spider composition and abundance. From nature conservation point of view it seems worthwhile to try to combine both high and low vegetation - heavily grazed and lightly grazed sites - in one particular habitat. Extensive grazing has been proposed as a management practice to accomplish such combination of the two structural microhabitats (Morris 1969). Wingerden et al. (1991) established positive and neutral efffects of cattle grazing on grasshoper abundance compared to ungrazed grasslands, and intensive grazing had negative effect compared to extensive grazing. Siepel et al. (1989) made a comparison between the effects of grazing on arthropod abundance and the effects of mowing, which turned out to be unfavourable for grazing. Concerning grasshoppers the intensity with which grazing is applied, is crucial and in addition has opposite effects on the different systematic groups (Capinera & Sechrist