This phrase describes an organism that is living and causing problems in an ecosystem where it is not native. This can be a plant, an animal, a fungus or even a bacterium or virus. Some invasive species arrive at a new habitat on their own. They may travel by wind or water. More often, though, people imported them either on purpose or by accident. Someone might acquire a foreign plant because they want to grow it for food. Or they might accidentally let an exotic animal out of an aquarium. Invasive species have even hitched rides on ships, cars, horses and shoes.
Most of the time, when a species lands in a new place, it doesn’t survive for long without help. Invasive species, though, survive and thrive. There may be no predators that eat them. Or they may gobble up plants that have no defenses against the invaders. If the species is a microbe, host organisms in the new land may have no way to fight them off. Without these kinds of controls, invasive species can multiply, causing harm to the ecosystem, people and even an entire country’s economy.