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In Switzerland all eight snake species are threatened and have been added to the Swiss Red List with dif- ferent levels of concern. The most threatened is the viperine snake, Natrix maura, a semi-aquatic snake, which is considered as “Critically Endangered” (CR) in Switzerland. Only one population is presently living on the shores of Lake Geneva, mainly located between St-Saphorin and Treytorrens (Canton Vaud). But the population size estima- tion trends show a drastic reduction of this species. Alteration of the lake shore structure and the introduction of the ecologically very similar dice snake, Natrix tessellata, since the 1920s could be the causes of the observed decline of the viperine snake in the last 15 years. A higher fecundity and a larger body size render the alien species probably more competitive and possibly have a key role on the population decrease of the native species. In order to test the hypothesis of competition between both species, their habitats between St-Saphorin and Treytorrens were described in detail in order to detect differential interspecific use. In total 23 environmental variables were measured at the study area and data were subsequently analysed using tests of proportion. Results indicate that the dice snake prefers slopes with limited vegetation height (0.5–1 m), but inhabits also slopes with light vegetation cover and relatively nar- row littoral zones. In contrast, the habitat use of the viperine snake relates to littoral zones with less steep slopes, but abundant vegetation. In addition, wide littoral zones seem to be preferred by the native species, such as the region near Treytorrens where the viperine snake was observed to be most numerous. Results obtained in this study were used to suggest particular shore management action, in particular to promote the native species.
