Nonnative organisms that cause a major change to native ecosystems—once called foreign species, biological invasions, alien invasives, exotics, or biohazards—are now generally referred to as invasive species or invasives. Invasive species of insects, fungi, plants, fish, and other organisms present a rising threat to natural forest ecosystems worldwide. Invasive animals are transferred to ecosystems in which there are no natural predators to keep them from spreading. Many species escape into the new environment where they become established, upsetting the ecological balance of the native forest ecosystem. Some invasives are competitors or predators of native species, whereas others cause disease.