Truly invasive or simply non-native? Insights from an artificial crested newt hybrid zone

Introductions of non-native species can pose serious threats to native populations and ecosystems. However, the impact of introduced species depends on intrinsic characteristics, local habitat conditions, and the interaction with native species. Case-specific management strategies may therefore be required. Using phenotypic characters and molecular markers for species identification, we provide insights into an artificial hybrid zone between two closely related newt species, the native Triturus cristatus and the introduced T. carnifex, near Tubingen, south-west Germany. Our analyses revealed a central Italian origin of the non-native T. carnifex and suggested their sustained presence in the study area for at least six years, probably much longer. In some ponds, extensive hybridization with native T. cristatus was detected. However, we found no evidence for a displacement of the native species by its non-native congener. The gradient from pure T. carnifex to pure T. cristatus currently extends over 7 km. A future expansion of the hybrid zone and swamping of a neighboring T. cristatus meta-population appears unlikely under the local configuration of breeding ponds. We propose to monitor the hybrid zone using genetic markers for evaluating the direction and speed of gene flow, complemented by capture-recapture studies to reveal trends in species-specific population sizes. To protect the native T. cristatus, we recommend practitioners to maintain their habitats, for example, by preventing illegal release of gold fish, by counteracting early drying of the breeding ponds, and by regularly cutting back trees and shrubs along the shoreline.