Organic cultivation reduces barrier effect of arable fields on species diversity

Spatial and temporal changes in ground beetle populations of a northern German farm during the conversion from conventional to organic methods were studied between 2001 and 2007. The conversion of the total arable area was partially performed beginning in autumn 2001 and completed in spring 2004. During the conventional period, species richness and diversity of ground beetles were negatively correlated with distance from field margin toward field centre, reaching a minimum at 60–240 m. Species diversity increased to match levels found in set-aside areas outside of arable fields 4 years after conversion. Species richness, however, remained poor at the centres of arable fields. Pterostichus melanarius was the most abundant species, averaging more than 50% dominance. At the beginning of organic farming in 2002, the abundance of nine species typical for arable fields and grassland increased, whereas the dominance of P. melanarius decreased. Species affected positively by organic farming were found in the conventional period outside the arable fields in high numbers.