In many parts of the world, alien species are harming the environment and causing other problems. An alien species is a species plant or animal that has moved from its original home to a new area. Sometimes people have purposely introduced the new species; sometimes it arrives accidentally, as a side effect of international trade In its new location, the alien species has no natural enemies, so it can grow and multiply without limit. Over time--sometimes decades sometimes a few years-the new species takes living space and food away from the native plants and animals. The result can mean drastic change in the natural landscape and problems for people