As predators play a central role in prey population regulation, predicting the impact of a novel predator requires determining how the invader affects the compensatory dynamics that underlie native prey persistence. The Indo-Pacific lionfish Pterois volitans is an invasive meso-predator that voraciously consumes native coral-reef fishes of the tropical western Atlantic and Caribbean. The fairy basslet Gramma loreto is a common prey of lionfish, and pre-invasion research has demonstrated that basslet populations undergo regulating density-dependent mortality due to predation. To unequivocally measure lionfish effects on prey mortality and to test whether prey survival remained density-dependent when exposed to predation by the invader, a controlled field experiment was conducted wherein both fairy basslet settlement density and lionfish presence were manipulated by divers on natural coral reefs. On reefs with and without lionfish, fairy basslet populations were repeatedly censused over the 28 d experimental period and mortality rates across a gradient of prey densities were quantified. Per capita loss of fairy basslet was density-dependent on reefs with and without introduced lionfish; however, the magnitude of this loss was significantly higher on reefs with the invader present. High mortality rates at low prey density resulted in local extinction of 2 of 14 fairy basslet populations exposed to the invader, a phenomenon observed only on lionfish reefs. Further, 9 out of 14 lionfish-exposed prey populations showed loss rates of >50% compared with just 3 prey populations with such rates on nativeonly reefs.
