A glitch in the Natrix: cryptic presence of alien grass snakes in Switzerland

The difficulty to detect invasive species depends of numerous factors, such as morphological similarities with local taxa (Dubey et al., 2014), low densities (Fitzpatrick et al., 2009), cryptic behaviours (Mazzotti et al., 2015), or remote distribution ranges (Lass et al., 2002). In some circumstances, alien taxa hybridize with local ones resulting in individuals with intermediate phenotypes. Their challenging detection requires sophisticated tools such as genetic analyses (e.g. Dufresnes et al., 2017). Even large size alien taxa can thus remain unnoticed for decades, which might have important ecological consequences and prevents their efficient eradication in early stages (e.g. Dufresnes et al., 2016). In this respect, about 20-30 alien striped grass snakes [Natrix n. persa (Pallas, 1814)] from Western Croatia (Rijeka) escaped from the outdoor enclosures of a reptile park in the 1970s in Lausanne, Western Switzerland (J. Garzoni, pers. comm.), but were never detected by local authorities or specialists the following years. In the region, only the barred grass snake [N. n. helvetica (Lacépède, 1798)] is naturally present (Fig. 1), where this species is considered threatened (Status “Vulnerable” in the Swiss red List of reptiles; Monney and Meyer, 2005). These two large colubrids (>1.20m) differs in colour patterns: N. n persa is characterized by two white or yellow stripes running down the back (additional dark spots can also be present) with highly variable lateral patterns, whereas N. n helvetica presents dorsal dark spots only (stripes are absent) with lateral patterns of one row or more of dark bars, which can be fragmented in spots (Kreiner, 2007). In order to assess the presence of alien grass snakes in this area, Herpetology Notes, volume 10: 205-208 (2017) (published online on 25 April 2017)