Veranderingen in de niet-marine molluskenfauna van Vlieland

The isle of Vlieland is located in-between the much larger isles of Texel and Terschelling. Within an area of less than 34 km2, it offers a wide range of terrestrial, freshwater and brackish habitats. The island was visited by different persons on various occasions between 1994 and 2003, and the present paper gives an overview of the molluscs found. In 1994-2003, 57 species were recorded, which is a 40% increase in species diversity since 1974. Of approx. 40 species known up to 1973, six could not been re-found during the last decade, the most significant loss being Catinella arenaria, in spite of the fact that suitable habitat for this species is still locally available. Also Vertigo pygmaea, in 1974 reported to be fairly common, could not be found living on the island. Oxychilus cellarius seems to have completely been replaced by O. draparnaudi. Significant new records include Stagnicola fuscus (first Dutch record confirmed), Assiminea grayana, and Deroceras agreste (second Dutch record). The increase in the number of species may partly be due to undercollecting in the past, but a substantial proportion of the previously unknown species seems to be genuinely new, consisting of common freshwater snails and anthropochorous land snails, a feature currently apparent on all Dutch Wadden Sea isles.