De molluskenfauna van Ameland

The malacofauna of the island of Ameland was studied during a fortnight’s stay in July 1962 by means of a qualitative census. The results are compared with the results of former investigations (VAN BENTHEM JUTTING, 1956). Quite a number of molluscs, viz. Carychium minimum, Segmentina nitida, Catinella arenaria, Punctum pygmaeum, Zonitoides nitidus and Trichia hispida, could not be rediscovered, whereas other mollusc species could be recorded for the first time. The new records are Pisidium obtusale, P. milium, Nesovitrea petronella, Arion circumscriptus, Euconulus fulvus and Potamopyrgus jenkinsi. The island consists of two nuclei with two villages each. These nuclei were semi-islands in former times (ca. 1800), separated at high tides by the water of the sea and connected at low tides by a sandy plain. Both nuclei have dunes protecting the inhabited sites from the sea in the north. A third dune complex, the „Oerd”, was situated further eastward as another semi-islet. An artificial dune dike was built in 1851 connecting the dune areas of the inhabited nuclei. This dune dike brought a new dune area the „Zwanenwaterduinen” into existence. The „Oerd” dune area was connected with the inhabited complex in 1900 by a similar artificial dune dike. The western part of the present island is protected from the Waddenzee in the South by a dike built in 1915. The dike protecting the existing centre part was built in 1930. The eastern and third part of the island, a „kwelder” called „Nieuwlandsrijd”, can be invaded by the water of the Waddenzee from the South. Since 1890 four sites in the dune area north of the villages Nes and Buren were planted with Pinus mixed with some Quercus robur and Acer pseudoplatanus. Forest molluscs are very scarce ( Euconulus fulvus, Trichia hispida, Nesovitrea petronella, Cepaea nemoralis, Punctum pygmaeum) or not present ( Discus rotundatus) on the island. Arion rufus is the only mollusc common to the fir plantations. Grass land mollucs ( Deroceras reticulatum, Vallonia excentrica, V. pulchella, Vertigo pygmaea, Vitrina pellucida, Pupilla muscorum, Cochlicopa lubrica) are well represented on the slopes of the dikes, however, scarce in the dune area. As well represented are molluscs of wet grass lands ( Succinea elegans, Galba truncatula and Deroceras laeve). On the other hand Carychium minimum, Zonitoides nitidus and Vertigo antivertigo are scarce as is the case with Catinella arenaria from the wet dune valleys. The freshwater fauna of the dune marshes in the valleys consists of Radix peregra, Anisus leucostoma and Armiger crista, a community in which Pisidium obtusale was also found and of which Segmentina nitida formerly recorded from ditches, should also take part. The fauna of the bodies of permanent freshwater is represented by Physa fontinalis, Armiger crista, Gyraulus laevis, Pisidium nitidum, P. milium and Sphaerium lacustre and Pisidium subtruncatum in very small numbers only. Potamopyrgus jenkinsi and Radix peregra penetrate far into the brackish zone. Radix peregra in Ameland is represented by a form with high spira only. Arion rufus is present in the dark brown (nearly black) variety only, with a dark reddish band bordering the foot. Nesovitrea petronella, originally a forest mollusc, occurs on the dikes in the South only. Trichia hispida and Zonitoides nitidus were recorded from near inhabited sites only. Catinella arenaria and Gyraulus laevis are molluscs which are very rarely found in the Netherlands. It is suggested that their presence in the biotopes of the island prevents the problem of a fauna poor in species being solved by an isolation mechanism through which species cannot reach the isolated site. The suggestion is supported by the fact that molluscs which are very common on the mainland e.g. Anisus vortex and Planorbis planorbis apparently failed to settle. Forest molluscs living abundantly in deforested areas in the mainland e.g. Discus rotundatus, Trichia hispida a.o. are very scarce or fail to appear in this island fauna. Isolation may result in the development of „foreign” or strange environments the influences of which only few molluscan species can tolerate and to which only very few species are complety adapted and as a consequence feel „at home”. These particular biotopes must be very instable and hard to maintain if man reaches the site and neutralizes the isolation or if nature itself changes the milieu suddenly for a short time now and then. Attention is also drawn to the natural environment of Galba truncatula. In the undisturbed western dune area where sand banks grow to dunes separating a new valley from the sea, the valleys then changing into freshwater marshes, the liver fluke snail is present in very small numbers only. An outburst of Galba truncatula lasts only one season at most. However, if a dune dike is built and man keeps changing the milieu every now and then without giving nature time for recuperation Galba truncatula numbers keep at high levels. It must be taken into consideration that subsequent censuses of isolated sites within larger periods tend to result in the discovery of species disappearing and in the recording of species „new” to the area investigated. It is clear from the above that it is not necessary to accuse investigators of not having searched the isolated areas thoroughly in order to explain discrepancies with former investigations.