Biological invasions represent both a pressing environmental challenge and an opportunity to investigate fundamental ecological processes, such as the role of top predators in regulating species diversity and food-web structure. In whole-ecosystem manipulations of small Caribbean islands where brown anole lizards (Anolis sagrei) were the native top predator, we experimentally staged invasions by competitors (green anoles, A. smaragdinus) and/or novel top predators (curly-tailed lizards, Leiocephalus carinatus). We show that curly-tails destabilized coexistence of competing prey...