De libellen van Schiermonnikoog Een eerste kennismaking

The National Park Schiermonnikoog is the northern most Wadden island in The Netherlands. Dragonflies have been recorded on Schiermonnikoog since 1950. In this paper an overview is presented of all freshwater habitats and the dragonfly species recorded so far. In the last decade of the twentieth century, Natuurmonumenten, a national association for nature preservation, created some new freshwater ponds in dune valleys. Thanks to these measures some species were able to enlarge their population on Schiermonnikoog or to settle on the island. To do so, an individual dragonfly has to fly at least 20 km across open sea water from the nearest mainland. One of the species that was able to reach the island was Erythromma viridulum. This species expanded its distribution area on the European continent in northern direction in the past decades and settled on Schiermonnikoog presumably in the early nineties. Four other new species, Coenagrion puella, Ischnura pumilio, Anax imperator and Orthetrum cancellatum occur in recent years more often and in increasing numbers. Some species only occur after exceptional weather conditions. Such drifters like Sympecma fusca, Leucorrhinia rubicunda and Sympetrum fonscolombii may be able to detect locations suitable for reproduction in the near future. Settlement on the island of Pyrrhosoma nymphula, Brachytron pratense and Cordulia aenea, all still absent on the island, may be possible in future. It is argued, that like most travelling birds, migrating dragonflies follow the coast-line of the European continent. In this perspective, the freswater wetlands on the Wadden islands are important spots to feed and rest. It is concluded that the knowledge about migration of dragonflies over large (salt-)water bodies is still poor. Therefore, the colonisation of isolated places like the island Schiermonnikoog offer an unique opportunity for further research.