Response of carabid beetles diversity and size distribution to the vegetation structure within differently managed field margins

Managing field margins to promote carabid diversity requires understanding the diverse responses of these insects to vegetation structure within these margins. This diversity in carabid responses could be determined by variation in species functional traits, of which body size is likely to be a key factor. In the present study, the effect of vegetation structure within differently managed field margins on species richness, activity-density and size distribution of carabids was investigated. Experimental margin plots were established in three cereal fields using a replicated block design. Carabids were sampled using pitfall traps in the margin plots, the crop edge, and the crop area of the fields. A decision tree analysis was used to classify structural variables of the vegetation according to their effect on carabids. Both a high number of carabid species and those important for effective pest control were associated with the field margins. Management influenced carabids only in the field margin. Higher plant functional diversity was identified as the primary factor promoting carabid species richness. Their activity-density was negatively correlated to the vegetation heterogeneity and positively to percentage of bare ground. Large species presented high activity-density in homogenous vegetation with high proportion of bare ground, whilst small species preferred high plant functional diversity and heterogeneous vegetation. High activity of medium sized species was associated with high but less heterogeneous vegetation. This diversity in carabid responses to the vegetation structure appears to be related not only to variation in their body size, but also in other life history traits such as diet.