De biotoop van de Kempense heidelibel (Sympetrum depressiusculum (Selys)) in Noordwest-Europa en zijn toekomst in Nederland

The habitat preferences of Sympetrum depressiusculum in the north-west of its range are discussed on the basis of the literature and personal observations. The most important characteristics are listed. An extensive zone of shallow water must be present. This zone should be vegetated, but not too densely. The water level must be stable or lowered between the time of oviposition and emergence of the larvae. The zone may even be completely dry during winter. The waters generally are not very eutrophic or acidic. The presence of a well-structured imaginal habitat is also important. The principal factor appears to be the stable or lowered water level in winter. This can be found in subalpine lakes that fill with melting snow in spring, fishponds that are emptied in autumn as well as in ponds and rice fields where a stable water level is maintained by constant in- and outflow. Most waters in the region have a raised water level in winter and are therefore unsuitable for the species. Most localities where the species occurs therefore have an artificially controlled water level. The reason why this winter drought is so important remains unclear. Freezing of the eggs, as suggested by earlier authors, appears to be of no importance as the species occurs in Mediterranean rice-fields and cooling-water ponds of factories in Belgium. The absence of dragonflies with multi-annual life cycles (that could be caused by drought or freezing) as competitors for food seems unimportant too, as they have been found with S. depressiusculum. The described hydrological factors result in high water temperatures in spring. This, and the resulting high availability of prey, might speed up larval and egg development to the level required for survival. It is suggested that the species evolved in conditions with relatively high water temperatures in spring. The high habitat specificity makes S. depressiusculum a rare and vulnerable species in the north-west of its range. Changed management and eutrophication forms a direct threat to populations, which are dependent of the maintenance of artificial conditions for their survival. Besides this vulnerability the species disperses rather weakly, as compared to its relatives. Individuals are generally not found more than 20 km away from source populations. Almost all recent Dutch records are probably the result of dispersal from populations in Belgium, not far from the border. Although reproduction was proven for the first time in The Netherlands (Rutten & Kalkman, 1999) in 1998 (in atypical habitat) it is expected that the only chance for the species would be if suitable habitat is created or restored by man.