Introduced marine predator alters density dependent patterns of mortality in prey

Background/Question/Methods

Species introductions can drastically alter population, community and ecosystem-level properties of invaded systems. As predators play a central role in prey population dynamics, it is essential to determine whether novel predators alter predation-mediated regulatory mechanisms, potentially destabilizing prey populations. The invasive Indo-Pacific lionfish (Pterois volitans) is a new mesopredator 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. We previously compared mortality rates in G. loreto before vs. after the arrival of P. volitans and demonstrated that prey mortality remains density-dependant, but has undergone a density-independent increase since the introduction of the novel predator. Because such before-after comparisons confound the effects of the invasion with uncontrolled factors, we followed up this work with a second field experiment, manipulating both basslet settlement density and lionfish presence/absence in a split-plot, cross-factored design.  We censused prey populations repeatedly, measuring per capita loss to determine whether native predators produce density-dependent patterns of loss alone and whether P. volitans predation alters the magnitude of loss and/or the intensity of density dependence.  

 Results/Conclusions

After four weeks, G. loreto populations demonstrated strongly density-dependent patterns of loss on reefs both with and without introduced P. volitans. Further, after accounting for the effects of prey density, reefs with lionfish present had higher mean per capita loss compared to reefs with native predators alone. Qualitatively similar patterns emerged after only 48 hours, indicating high post-settlement mortality. We observed high variability in per capita loss of unmanipulated (low-density) G. loreto populations; however, several populations exposed to P. volitans predation approached net 100% loss- despite low initial density. For G. loreto populations with experimentally enhanced prey density, per capita loss was nearly three times higher in the presence of lionfish. Treating prey density as a continuous predictor of prey loss rates, we found no evidence for a difference in slope between predator treatments. These results indicate that, while the overall magnitude of prey loss is higher in the presence of lionfish, the intensity of compensatory density dependence remains similar to that observed on native predator only reefs, with implications for the probability of local extirpation.