Spiders are not less diverse in small and isolated grasslands, but less diverse in overgrazed grasslands: A field study (East Hungary, Nyírség)

The effects of size, isolation and grazing intensity on spider assemblages were studied in the fragments of sandy grasslands (East Hungary, Nyírség). Spiders were sampled by sweep-netting at eight different-sized grassland fragments between 2001 and 2003 from April to October fortnightly. The following hypotheses were tested: (i) the rules of classical island biogeography are assessed for grassland fragments: the number of species increases with the size and decreases with the isolation of the fragment. (ii) Species richness of spiders decreases by the intensity of grazing. (iii) Grazing may have a negative influence on the large, web-builder spiders and on the diurnal hunters associated to the vegetation. During the 3-year study period, 3842 spider specimens belonging to 90 species were collected from the eight sandy grassland fragments. We found no significant relationship for the size and isolation of grassland fragments neither with the number of species nor the number of vegetation-dwelling species, species associated with sandy soils, and with the vegetation-dwelling species specific to sandy area. Our result suggests that even the small fragments had a relatively large species pool. There was a positive correlation of the average height of vegetation as a measure of grazing intensity with the total number of spider species, as well as with the number of vegetation-dwelling species, species associated with sandy soils, and also with the number of vegetation-dwelling species specific to sandy area. By indicator species analysis (IndVal) we found that the grazing had a negative influence on the large, web-builder spiders and also those diurnal hunters, which were associated to the vegetation.