Testing the Optimal Foraging Theory in a Generalist Feeder: The Case of Reintroduced European Pond Turtles and Its Impact on Macroinvertebrates Communities

In the context of biodiversity erosion, species reintroductions are considered a promising strategy for limiting species extinction. However, the impacts of introducing a formerly extinct species in current ecosystems are poorly reported in terms of ecosystem functioning and trophic ecology, especially in the case of reintroduced generalist feeders. Based on the optimal foraging theory, we tested the prediction that the generalist European pond turtle (<i>Emys orbicularis</i>) feeds on a wide range of prey, yet optimises its energy intake by targeting larger and/or softer prey. We characterised the diet of captive-bred turtles once released on the Woerr site, Upper Rhine Valley, North East of France. eDNA metabarcoding was implemented on faecal samples from 15 subadult individuals, with a focus on consumed macroinvertebrates (MI) that are reported to be the major prey of the species. Furthermore, we investigated the temporal trends of the MI community throughout successive turtle releases over 5 years, in order to assess the consequences of the turtle reintroduction on ecosystem functioning. Faecal eDNA analyses revealed that after their release turtles exhibited a highly diversified diet (insects, gastropods, but also plants and amphibians). Importantly, turtles showed a preference for prey with relatively large potential body size and high longevity (Odonata, Coleoptera and Hemiptera). Yet, the successive releases of turtles did not impact the MI community over time. We conclude that reintroduced European pond turtles operate as a new top predator in the system, with larger turtles potentially feeding on larger prey, as predicted by their opportunistic generalist feeding ecology, yet without changing the overall community of MI, most likely due to the low predation pressure they exert on the prey community. This study highlights the relevance of conservation initiatives such as species reintroductions benefiting local biodiversity without jeopardising existing ecosystem functioning.