Native prey mortality increases but remains density-dependent following lionfish invasion

Predators can regulate prey population dynamics, so the introduction of novel predators may alter predation-mediated regulatory mechanisms, potentially destabilizing prey populations. Compensatory density dependence is an essential condition for population regulation. Thus, understanding whether and how introduced predators alter the relationship between prey density and mortality can aid in predicting the ecological consequences of invasion. Here, we investigate the effects of invasive Indo-Pacific red lionfish Pterois volitans on density-dependent mortality patterns previously documented for a common native Atlantic prey species, the fairy basslet Gramma loreto. By repeating a pre-invasion density manipulation experiment, here in the context of predation by both native piscivores and lionfish, we provide a before-and-after comparison quantifying changes in prey mortality since the introduction of lionfish. Per capita loss of fairy basslet remained density-dependent in the presence of lionfish, but the overall magnitude of loss was higher compared to pre-invasion rates. In the presence of lionfish, 7 of 16 local basslet populations experienced greater than 50% loss over the 8 wk study duration, but there was no evidence of a difference in the slope of the density-mortality curve between pre- and post-introduction experiments. Thus, our experiment revealed a density-independent increase in per capita mortality rates since the start of the invasion. We conclude that local fairy basslet populations are facing an elevated risk of extirpation as a result of increased predation, and suggest that different predator foraging behavior and/or prey naivete may explain the altered prey mortality patterns observed after the lionfish invasion.