Shading out species richness: edge effect of a pine plantation on the Orthoptera (Tettigoniidae and Acrididae) assemblage of an adjacent dry grassland

Edge effects are increasingly recognised as a threat to interior species, but the influence of edges on open habitats has rarely been addressed so far. We investigated the diversity pattern of the Orthoptera fauna across a habitat edge between a pine plantation and a natural steppe ecosystem. Thirty pitfall traps were positioned along a transect line from 80 m in the forest to 208 m in the grassland at 10 fixed distances from the edge. The orthopteran assemblage of the pine plantation was depauperate in species and individuals and, with a few exceptions, lacked steppe species. We found no increase in species richness at the forest-grassland edge. In the grassland, Orthoptera species numbers increased steadily with increasing distance from the forest edge. Two-phase regression analysis revealed a traceable edge effect up to about 30 m into the grassland. This effect was exclusively caused by an increase in acridid species numbers, while tettigoniid species numbers showed no clear relation to edge distance. Correlation analysis of the habitat variables suggested that this pattern is a result of irradiance and soil temperature sums. Since acridids show species-specific temperature requirements for their development, shading of the soil surface prevents a number of species from completing their life cycle. We suggest that present trends of shrub encroachment and eutrophication of dry grassland habitats in eastern Austria are serious threats for a large part of the orthopteran fauna.