The Grass snake (Natrix natrix) has recently recolonised the Dutch part of the valley of the river Worm (situated on the border between the Netherlands and Germany) from source populations across the German border. In the 1980s they disappeared from the Dutch part of the valley due to habitat destruction as a result of the construction of a NATO goods depot. The species was rediscovered in 2005, which led to the construction of artificial breeding sites (breeding heaps) on the Dutch side of the Worm. Since then, dozens of Grass snakes have been sighted and the numbers of eggs deposited in the breeding heaps have been increasing each year. At the moment, the Grass snake uses the Dutch bank of the Worm for foraging and egg deposition, and there is no evidence that it hibernates on the Dutch side. Stimulating further colonisation of areas in Limburg by means of specific management measures requires careful consideration, in view of the importance of the province of Limburg for rare and threatened amphibians like the Tree frog (Hyla arborea), the Common spadefoot (Pelobatus fuscus) and the Midwife toad (Alytes obstetricans), which are potential preys for Grass snakes. What makes the situation interesting from a herpetogeographical point of view is that the Grass snakes in the Worm valley could have a completely different origin than the other Dutch populations, underlining the region’s reputation in terms of the national herpetofauna.
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