Posterior probability distributions of estimated Bayesian 95% ellipse area for mesocosm experiment and field dataset, modelled with nicheROVER package (Lysy et al. 2021). The isotopic niche of topmouth gudgeon (TMG, Pseudorasbora parva) was considerably reduced in syntopy (Syn), while the isotopic niche size of native crucian carp (CC, Carassius carassius) remained similar in the experimental setup (A). The topmouth gudgeon (TMG, Pseudorasbora parva) had a generally smaller isotopic niche size when compared to native crucian carp (CC, Carassius carassius) in the presence of gibel carp (GC, C. gibelio). In the field, its niche was substantially larger in syntopy only with native crucian carp (B). The boxplot boundaries represent upper and lower quartiles; the thick lines represent medians and the whiskers represent 1.5 times the interquartile range.