De libellen van de Grafelijkheidsduinen: de invloed van het waterpeil op de libellenfauna

In the period 1999-2006, the dragonfly fauna of the Grafelijkheidsduinen, the northernmost dune area of the Dutch mainland was studied quantitatively. The dragonflies of two dune lakes were counted along fixed transects. The dragonflies of most other ponds in the area were recorded during several visits each year. Changes in water level were monitored by water level gauges. The period 1999-2002 was extremely wet, whereas 2003 was extremely dry. In late-summer of the latter year, some ponds dried out completely, while the water quality of others deteriorated. In following years the water levels recovered again. Water levels proved to affect the abundance of several dragonfly species strongly. The Zygoptera showed the strongest reaction on the declining water levels in 2003: the population size of most species dropped dramatically in 2004. The response of the Anisoptera was less clear. The species-poor dragonfly fauna of the Grafelijkheidsduinen (where a total of 19 species was recorded), is probably caused by the rather isolated location of the dune area and the dominant unfavourable climatic conditions.