- Although only a minority of introduced species become established and have noticeable consequences in their new communities, some can displace native species, alter food webs, and cause local extinctions. Studying these invasive species can provide new insights into basic ecological questions as well as inform management strategies. Pacific lionfish (Pterois volitans miles) are the first non-native marine fish to become established throughout the tropical and sub-tropical western Atlantic. Since the early 2000s, lionfish populations have spread rapidly and grown exponentially, reaching densities that are several orders of magnitude greater than those in their native range. Combined with these high population sizes, lionfish have strong negative effects on native coral-reef fish populations via direct predation. The main goals of this dissertation were to determine how the local density of lionfish influences their demographic rates and behavior, and in turn, how lionfish affect native fish populations at different lionfish densities, across multiple habitats, and via non-consumptive effects.In the first experiment, I manipulated densities of juvenile lionfish on ten small artificial patch reefs The Bahamas and for 8 weeks monitored their demographic rates (Chapter 2) and effects on native-fish communities (Chapter 4). Although there was no evidence for density-dependent per capita rates of loss (mortality and emigration) or gain (recruitment and immigration), individual growth rates decreased at higher densities. For each increase in lionfish density by 1 fish m², lionfish grew an estimated 0.02 mm day slower. In addition, native fish abundance and biomass declined non-linearly with increasing lionfish density, such that lionfish had diminishing per-capita effects on native fishes at higher densities. Observations of lionfish conducted on sixteen coral patch reefs encompassing a natural range of lionfish densities revealed that lionfish foraging behavior and movements also vary with local lionfish density (Chapter 3). At higher densities, lionfish exhibited greater activity levels and increased time away from shelter, including more short-term foraging movements between coral patch reefs and surrounding seagrass habitats. Combined, these patterns suggest that invasive lionfish experience intraspecific competition for food.The foraging movements of lionfish may also have implications for which native species are susceptible to lionfish predation. By manipulating sixteen patch reefs in The Bahamas to have either high or low lionfish densities and comparing changes in native fish populations on and around the reefs for 7 weeks, I determined that lionfish first caused an approximately 60% reduction in the abundance of native fishes on the patch reefs and then caused similar declines on small structures in seagrass beds that surround these reefs (Chapter 5). Unlike the effects of native predators on prey fishes, the effects of lionfish did not diminish rapidly with increasing distance from coral patch reefs, likely because lionfish have few natural predators and thus may forage with impunity over extended distances.The negative effects of lionfish predation both on and around coral reefs are likely compounded by the fact that some prey fishes are naïve to the threat posed by lionfish. During a critical life-history transition from pelagic larvae to reef-associated juvenile (‘settlement’, measured as ‘recruitment’), some coral-reef fishes recognize cues from predators and consequently preferentially settle to reefs without predators. To test whether lionfish have similar non-consumptive effects on the recruitment of coral-reef fishes, I manipulated the presence, identity, and diet of prior resident fishes and measured daily recruitment to fifteen small standardized reefs in Bonaire (Chapter 6). Regardless of predator diet, one species of reef fish had 55-59% lower recruitment to reefs with a native predator compared to predator-free control reefs and reefs with lionfish, suggesting that they recognize and avoids cues from native predators but not invasive lionfish.Overall, this research clarifies the extent and mechanisms underlying the ecological effects of invasive lionfish. In terms of management, the lack of density- dependent gain and loss rates suggest that current efforts to reduce local densities via manual removal by divers are likely to remain the most effective management strategy for the foreseeable future. Due to the non-linear effects of lionfish on native fish populations on coral patch reefs and their effects in surrounding areas, management efforts that greatly reduce lionfish densities on coral patch reefs will have the greatest benefit for native fishes across multiple habitats. Finally, because at least one species of reef fish has lower recruitment to reefs with native predators compared to lionfish, monitoring strategies that simply compare the abundance of native fishes on reefs with lionfish to reefs with native predators will likely underestimate the consumptive effects of lionfish.This research also reveals that invasive lionfish are different than native coral-reef mesopredators in many ways: they do not experience density-dependent population gain and loss rates at current high densities, they forage over broad distances encompassing multiple habitats, and they are unrecognizable to at least some native prey fishes. In addition, typical anti-predator strategies of small native fishes, including inhabiting seagrass beds instead of coral patch reefs and avoiding reefs with predators at settlement, are ineffective against lionfish. These characteristics likely at least partially explain why lionfish have such strong negative effects on native coral-reef fishes, and some of these traits are likely shared by other successful invasive predators.