A multivariate analysis of cropping effects on Irish ground beetle assemblages (Coleoptera: Carabidae) in mixed arable and grass farmland

The assemblage of carabid species trapped over a 3 yr period in eight separate but contiguous fields was analysed by Canonical Correspondence Analysis (CCA). Seven of the fields were subject to arable crop rotations involving a mixture of autumn and early spring-sown cereals (wheat or barley), late spring-sown crops (potatoes, sugar and fodder beet and maize) and short-term uncultivated grass leys. The eighth field was an established 'permanent' grass pasture of at least 50 yrs standing. Pooled samples collected in the first half of the growing season (April-June) showed clear evidence of soil cultivation effects on community structure. Systematic exclusion of samples and re-analysis distinguished the fauna of firstly, old pasture samples, samples from ley pastures of increasing age and finally samples from fields with different times of soil cultivation. In the latter analysis, the main effects of soil cultivation were related to differences in ground cover over the winter and a direct effect of spring soil cultivation on autumn breeding populations. The ordination of pooled catches for the second half of the growing season (July-September) could not be related to known year, field or crop cultivation variables. The underlying species ordination suggested that later in the summer the effect of crop cover on soil microclimate may mask cultivation effects by influencing the post-emergence dispersal of autumn-breeding populations and the reproductive success of spring-breeders. The combination of earlier soil cultivation effects, and probably microclimatic influences later in the season, resulted in the strong distinction of whole season carabid catches from individual fields. It is concluded that the uniqueness of individual field histories may provide a mechanism to promote the co-existence of ecologically similar species within the farmed landscape and enhance the abundance and bio-diversity of species in the face of routine soil cultivation.