Trophic interactions between the European pond turtle (Emys orbicularis) and invasive non-native species

Biological invasions contrast with reintroduction projects, where native species are deliberately returned to their natural habitat to restore local biodiversity. Such a reintroduction project is being carried out in the Upper Rhine Valley with the European pond turtle ( Emys orbicularis ) in the transbordering region between Lauterbourg (France) and Neuburg am Rhein (Germany), hereafter referred as Neu-Woerr. Several ponds have been created to benefit the dispersal of reintroduced turtles. However, this pondscape has been recently widely invaded by the calico crayfish ( Faxonius immunis ) and partly by the zebra mussel ( Dreissena polymorpha ). These two invasive non-native species decrease the overall habitat quality due to the massive exploitation of phytoplankton by the zebra mussel as well as macrophytes and macroinvertebrates by the calico crayfish. However, these invasive non-native species are also potential novel prey for the reintroduced turtles. Here, we investigated the trophic interactions under captive conditions between the turtle and the two invasive non-native species. We conducted prey choice tests of turtles with calico crayfish of different sex, size, and shell hardness, as well as with zebra mussels of different size. Additionally, faecal samples of wild caught turtles on Neu-Woerr were sampled and analysed for calico crayfish DNA. Our results demonstrated that the turtles were capable of preying on the calico crayfish in the wild and in captivity, preferring small crayfish individuals, with no sexual preference for the less aggressive female crayfish. For the zebra mussels, turtles preyed preferably on small alive individuals, while they also preyed on larger, yet dead mussels. This study provides new insights into the feeding ecology of the European pond turtle and highlights added benefits and values of reintroduction projects when the reintroduced species additionally acts as a novel predator of invasive species.