Habitat overlap and body condition in aquatic turtles: are there additive effects between invasive and native species?

Trachemys scripta (Reptilia, Emydidae) is among the most widespread invasive reptiles in the world. In general, it is assumed that this species has negative effects on native aquatic turtles based on experiments conducted under controlled conditions. Here, we analysed a 7‒year time series (2013‒2019) of data from captures of three species of turtles (two natives and one alien) from natural populations in the northeastern Iberian Peninsula. We evaluated (i) the interspecific spatial overlap and (ii) the correlation between the scaled mass index (SMI) of the native turtles and the abundance of T. scripta, including an interaction effect between the species. The analyses revealed that T. scripta has relatively low spatial overlap with Emys orbicularis but high overlap with Mauremys leprosa. Overall, the abundance of T. scripta was not associated with significant variations in the SMI of the native species, although for the female E. orbicularis, we detected a negative trend. The analyses also indicated that the abundance of T. scripta does not have an additive effect on the SMI of other turtles, even if there is a negative effect between native species. These findings suggested that the impact of T. scripta on other turtles could be density-dependent.