A comparison of ground beetle (Carabidae : coleoptera) communities in an organic and conventional potato crop

An investigation of the carabid and plant communities of a conventional and an organic potato crop was carried out in the west of Ireland from June to September 1999. At each site, carabids were sampled using 15 pitfall traps, and 2 x 2 m quadrats were used to assess weed and potato cover. A total of 17 carabid species (672 individuals) were trapped during the study with 11 species common to both sites. 82.4% of individuals were captured at the organic site compared with 17.6% at the conventional site. The reciprocal Berger-Parker diversity index was 3.4 and 2.2 for the organic and conventional sites, respectively. Pterostichus niger and P. melanarius constituted 48.3% of the total catch and were the most abundant of all species captured. However, of the catch at each site, Bembidion bruxellense (44.9%) and P. niger (29.1%) were the most abundant species at the conventional and organic sites, respectively. A total of 13 weed species were identified, 11 of which were found exclusively in the organic field. A positive relationship between weed species richness and carabid species richness was apparent with r (Spearman Rank Correlation Coefficient) = 0.679, p 0.001). The relationship was stronger between carabid species richness and weed cover (r = 0.704, p 0.001). Differences between the sites appear to be related to weed cover although other possible factors are discussed.