Mesocosm experiment (A) with treatments of crucian carp (CC, Carassius carassius), syntopy (Syn) and topmouth gudgeon (TMG, Pseudorasbora parva) and field evidence from six sites (B). Isotopic niches shown as 40% standard ellipse area for three species at each site (darker shading) and 95% ellipse area (lighter shading). Solid lines = ellipse areas, symbols = individual values. CC = Crucian carp (Carassius carassius), GC = Gibel carp (C. gibelio); TMG = Topmouth gudgeon (Pseudorasbora parva).