By simulating ecological networks with microbes, researchers revealed properties that may make natural communities susceptible to invasion. Introduction For decades, ecologists have puzzled over a mystery: Why do some natural habitats get overrun by invasive species while others seem to repel outside threats? In a classic 1958 book on the subject, the ecologist Charles Elton argued that an ecosystem with more species should be more resilient. In a diverse ecosystem, he wrote, so many species are...