Road-side verges : habitat and corridor for carabid beetles of poor sandy and open areas

This research deals with the question whether or not heathy strips, like poor sandy and open road-side verges, can direct dispersal movements of stenotopic carabid beetles from one heathland fragment towards another, and under what conditions.First by placing pitfall traps at ten heathy road-side verges in the north-eastern part of the Netherlands it was established that beetles of poor sandy and open habitats do not avoid road verges. A relation was found for the species occurring at the verge with both distance to a heathland fragment and width of the heathy strip at the verge. In a heathy verge connected to a fragment of heathland more heathland species and in higher numbers were present close to the fragment than farther away. At broad verges the occurrence of these species persisted at longer distances.Mark/recapture experiments with three different carabid species showed that dispersal takes place randomly. Despite of these random movements, entrance of habitat areas of another kind, like dense grass vegetations or forest, surrounding the heathy corridor hardly occurred. This means that beetles stay in their corridor by the avoidance of the bordering areas. These experiments showed also that most individuals disperse not more than 50 m per year at verges of a width of 12 m, and at more narrow verges even less. Only occasionally a few individuals bridged distances of 150 m or more. Therefore, to bridge distances longer than 100 m a verge needs to be habitat as well, so that a new generation can continue dispersal along the corridor.Distances walked per day in different kinds of habitat and chances of entering different kinds of habitat were accumulated in a simulation programme. With this programme it was possible to calculate the effects of differently sized corridors on the dispersal distances of the three beetle species tested for a time period of more than ten years. From these results recommendation for the creation and management of dispersal corridors, like heathy road-side verges, could be formulated.