Invasive species are organisms that are not native to an environment that can cause ecological and economic harm. Some of the worst invasive species include the European rabbit, zebra mussel, and ship rat. Invasive plants and animals are often introduced by humans and then spread rapidly because they lack natural predators in their new environments. Examples of harmful invasive species discussed are the Burmese python, nutria, and English ivy. The document emphasizes that invasive species impact ecosystems and native species, and represent a significant economic problem.