Predation Refuge Mediates Multiple Predator Effects on a Non-Native Fish

Multiple predators may have non-additive effects, facilitating or interfering with one another. These effects, along with predation refuges in the environment, have profound consequences for non-native prey during the process of invasion. We conducted tank experiments to test for multiple predator effects in a three species system with intraguild predation, native largemouth bass Micropterus salmoides and eastern mosquitofish Gambusia holbrooki, and the non-native green swordtail Xiphophorus hellerii. Green swordtail is a common aquarium fish and has been introduced into 12 U.S. states and Puerto Rico. Native predators were tested alone and in combination with a weak (artificial vegetation) or strong (vegetation plus barrier) refuge from largemouth bass predation. Habitat refuge played a critical role in the interaction of native predators and non-native prey, and the interaction between predators. Swordtails avoided vegetated areas with mosquitofish in the absence of largemouth bass. Swordtail survival in the predator combination treatment was two times higher than with largemouth bass alone with a weak refuge but was three times lower with a strong refuge. With a weak refuge, largemouth bass ate mosquitofish and mosquitofish attacked swordtails less (interference). With a strong refuge, swordtails were excluded from refuges by mosquitofish attacks and were consumed by largemouth bass (facilitation). Swordtails were caught “between a rock and a hard place,” with no refuge from predatory fishes. Under these conditions, persistence of swordtails in the environment is unlikely.