Living with aliens: Suboptimal ecological condition in semiaquatic snakes inhabiting a hot spot of allodiversity

The presence of alien species can embody a form of disturbance for natural communities and the concomitant presence of alien species at different levels within the trophic chain may compromise ecosystem functionality. We studied the ecology of two species of snake (Natrix tessellata and N. natrix) in a system of five ponds with a high number of alien species at a Mediterranean area in central Italy. We evaluated the potential perturbations to ecological traits of snakes due to the presence of alien species, including their body size, population density, and food habits. We compared the studied populations' body size to that estimated in conspecific populations studied elsewhere, including populations at close distance from the study site. Distance sampling and Capture-Mark-Recapture techniques were used to calculate the population density. Diet was estimated using stomach contents and stable isotope ratios (δ 15 N and δ 13 C), using Bayesian stable isotope mixing models to estimate the contribution of food sources and species' isotopic niche spaces. Few prey items were found in the stomach contents of either species, with Ameiurus melas as the main prey. Based on isotope niche analyses, N. tessellata diet consisted of only alien species, and N. natrix diet mostly relied on alien species (> 80%). Stomach contents revealed high overlap between the two species, although isotopes showed a random resource use (i.e. low isotopic niche space overlap). Overall, we caught a low number of individuals, indicating small population sizes. Moreover, snakes at the study site were comparatively smaller in size than most other populations found in the literature and almost all the recorded individuals were in very poor condition or injured by ingested alien prey. We suggest that the presence of rich allodiversity has negatively affected the snakes' fitness by decreasing their foraging performance, increasing their risk of being preyed upon, or through other mechanisms.