Competitive dominance and broad environmental tolerance favour invasive success of Nile tilapia

Invasive species cause substantial changes to the biodiversity of freshwater systems. The African Nile tilapia (Oreochromis niloticus) is now widely distributed in tropical freshwaters globally. Despite indications that feral populations can influence native species through competitive effects, direct evidence of competition between Nile tilapia and native species is rare. Moreover, it is not clear if environmental variables such as temperature and oxygen concentration modulate competition. Here, interactions between Nile tilapia and the native Mayan cichlid (Mayaheros urophthalmus) were studied in experimental mesocosms in south-eastern Mexico. We found that Nile tilapia was the more active and aggressive of the two species, and their movement was only weakly influenced by temperature and oxygen concentration. By contrast, movement of the Mayan cichlid was strongly predicted by the movement and aggression of Nile tilapia, and the Mayan cichlid showed a steep decline in behaviours with increased water temperature and reduced oxygen. Our results suggest that broad environmental tolerance of the intrinsically aggressive Nile tilapia provides it with an advantage over native species. Collectively these traits may help to exacerbate its invasive success as those environmental conditions become more commonplace in a changing world.