Effect of buffer strips along small watercourses on farmland spiders (Araneae) and ground beetles (Carabidae)

Buffer strips along small waterways that are adjacent to arable land are important for improving water quality and are common measures in agri-environmental schemes. To assess their contribution to arthropod species richness (alpha and gamma diversity) and differences in assemblages (beta diversity) we used pitfall traps to catch arachnids and ground beetles at 40 fields in four regions across Bavaria, Germany, during two or three one-week sampling periods in summer. A permanent vegetated buffer strip was present on 25 of the study fields, 15 were cropped to the field border adjacent to the waterway. Trapping was conducted in the riparian field border, the buffer zone (with or without an established buffer strip), the field edge about 15 m distant to the field border and in the field centre in 80 m distance. Results indicated that alpha and gamma diversity were lowest in the field centre, and the riparian field border had the highest species richness of arachnids. Alpha diversity of ground beetles and spiders was not enhanced in fields with a buffer strip and the buffer strip did not have significantly higher species richness than cropped fields at the same field position. In contrast for ground beetles a higher species richness was observed in the unbuffered field border. An indicator species analysis showed that most of this effect was due to spillover of eurytopic arable species from the neighbouring field. For ground beetle assemblages buffered riparian field borders showed a higher dissimilarity to the other sampled field positions than riparian field borders without an adjacent buffer strip. We conclude that the establishment of buffer strips altered the faunal composition within the buffered riparian field border habitat in summer. We discuss ecological consequences, such as increased beta-diversity and changes in competition, which make buffer strips an important component of the preservation of biodiversity in agricultural landscapes.