Stable isotope analysis of trophic niche in two co-occurring native and invasive terrapins, Emys orbicularis and Trachemys scripta elegans

A trophic niche overlap in native and alien turtle species can lead to competitive interactions whereby allochthonous turtles may outcompete autochthonous individuals and eventually affect viability of natural populations. The European pond turtle (Emys orbicularis) is an autochthonous species threatened by habitat encroachment and competition with the red-eared slider (Trachemys scripta elegans). The latter is an invasive species introduced in Europe from midwestern United States as a pet and now widespread in the natural habitats of E. orbicularis. The extent of trophic competition between E. orbicularis and T. s. elegans in northern Italy was assessed by nitrogen and carbon stable isotope analysis (d15N and d13C). We used blood and claw samples in order to obtain information on diet components over a short- (3–4 months) and long-term (12 months) time frame, respectively. Analysis of claw samples showed a clear separation between the diets of adults of the two species, but suggested a trophic overlap among adult invaders and young autochthonous turtles. Blood samples, on the other hand, revealed a partial overlap between niches, indicating a short-term correspondence in diet composition between species. We found that, for specific life stages and times of the year, there is potential for trophic competition, which may have important consequences for the management and conservation of E. orbicularis in Italy.