Coupling stable isotope studies with food web manipulations to predict the effects of exotic fish: Lessons from Cuatro Ciénegas, Mexico

1. Exotic species threaten native species worldwide, but their impacts are difficult to predict. 2. Stable isotope analysis was combined with field competition experiments to predict how an invasive African cichlid fish, Hemichromis guttatus, might affect native fish in the desert springs of Cuatro Ciénegas, Mexico. 3. Stable isotope analysis suggested diet overlap between the invader and juvenile endemic cichlids, and field experiments verified that the invader reduces growth rates of the juvenile endemics through competition, but has smaller effects on adults. 4. Competition between juvenile endemic cichlids and the invader was asymmetric, with the exotic out-competing the native, suggesting the potential for competitive exclusion if the invasion is not stopped. 5. These results suggest that exotic removal programmes in Cuatro Ciénegas should focus on removing/reducing populations of the exotic cichlid in habitats where juvenile native cichlids are concentrated. 6. This approach could help focus efforts to manage exotic species before populations of native species have crashed, when it is too late to intervene.