- Biological invasions have been identified as one of the prominent drivers of global environmental change. In particular, invasive predators typically have substantial negative effects on populations of native prey, even driving species to extinction in extreme cases. However, beyond direct predatory effects, little is understood regarding the specific mechanisms by which invasive predators influence native communities and ecosystems. Therefore, the objective of this dissertation was to investigate whether and how an invasive predator, the Pacific red lionfish (Pterois volitans), alters native community interactions on Atlantic coral reefs. The lionfish invasion is unprecedented for a marine fish in the extent of rapid geographical spread, successful establishment across numerous habitats, and strong predatory effects on native species. By conducting behavioral observations and manipulative experiments in both the laboratory and field settings, I tested for a variety of direct and indirect mechanisms by which invasive lionfish potentially influence native fish communities and coral-reef ecosystems. I first conducted a model-bottle experiment in The Bahamas and Cayman Islands (Chapter 2) to test for aggression of a native territorial damselfish, Stegastes planifrons, toward invasive lionfish. Such territoriality could provide a possible source of biotic resistance that may provide behavioral refugia for native coral-reef fish recruits from lionfish predation. However, the behavior of this damselfish in response to invasive lionfish in a clear plastic bottle did not differ from the minimal response exhibited toward the empty bottle control. Therefore, the territories of this damselfish are unlikely to provide such biotic resistance to the invasion. To investigate whether invasive lionfish alter competition between native prey fishes, I then performed a manipulative field experiment in The Bahamas whereby I simultaneously tested for the effects of both competition and lionfish predation on two congeneric coral-reef fishes, the fairy and blackcap basslets (Gramma loreto and G. melacara, respectively). In the absence of invasive lionfish, competition within local populations of basslets under reef ledges had symmetrical effects on the juveniles of both species (Chapter 3). Interference between species drove juvenile basslets further back under ledges where feeding and growth rates of individuals were reduced. Within reefs with the invasive predator present (Chapter 4), lionfish reduced the density of juvenile fairy basslet, thereby reducing the effects of competition on juvenile blackcap basslet, and tipping the balance of competition between juveniles of these species from symmetrical to asymmetrical effects. Differential predation of invasive lionfish may be explained by a preference for fairy basslet, as demonstrated by a laboratory experiment (Chapter 5). Lastly, I examined possible mechanisms underlying a potential invasive lionfish-herbivorous fishes-macroalgae trophic cascade on large reefs in The Bahamas (Chapter 6). During a two-year field experiment, lionfish caused a decline in the density of small herbivorous fishes on reefs, and behavioral observations revealed that the presence of lionfish reduced grazing by both small and large fishes, which resulted in 66-80% less algae removed from reef substrata. Therefore, invasive lionfish have both consumptive and non-consumptive effects on the important ecosystem function of native herbivorous fishes: reducing the abundance of benthic algae that could otherwise displace corals. In sum, this dissertation indicates that throughout native coral reefs, invasive lionfish (1) are not attacked by native territorial damselfish that could otherwise provide local refugia for native recruit fishes; (2) alter the outcome of interspecific competition between native basslets via differential predation that tips the balance of competition from symmetrical to asymmetrical; and (3) have both consumptive and non-consumptive effects on native herbivorous fishes, which reduces grazing and indirectly benefits benthic macroalgae to the possible detriment of corals. This research broadens our mechanistic understanding of predation in the context of invasive species, which further informs predictions relevant for management and conservation initiatives.