No evidence for multiple predator effects between a non-native and native wetland fish

Invasive predators may disrupt native predator–prey dynamics by modifying feeding rates and trophic interactions. Yet, the extent to which native and non-native predators jointly influence prey consumption remains insufficiently understood. Functional responses (FRs) provide a powerful framework to quantify these dynamics and assess the combined impacts of multiple predators during biological invasions. This study compares the FRs of the non-native swordtail fish (Xiphophorus hellerii) and the native mouthbrooder (Pseudocrenilabrus philander) from a protected aquatic ecosystem in Mpumalanga, South Africa. Controlled experiments assessed prey consumption rates of both species, individually and in predator pairs, across six prey density levels using freeze-dried Chironomidae larvae as prey. Both fish species exhibited Type II FRs, with the mouthbrooder reaching higher maximum consumption rates (≈ 14.5 prey items) compared to female swordtails (≈ 8.1 prey items) and male swordtails (≈ 3.6 prey items), despite having comparable attack rates. This reflects that the per capita effects of swordtails are relatively weak on native prey compared to those of mouthbrooders. Predator-predator interactions were primarily additive, with minimal non-trophic effects observed. Although swordtails exhibited weaker per capita effects compared to mouthbrooders, their invasion could still heighten overall predation pressure on prey communities if their population abundances increase. These findings underscore the importance of understanding trophic interactions in managing invasive species and maintaining ecological balance in protected areas.