A noxious weed, harmful weed or injurious weed is a weed that has been designated by an agricultural or other governing authority as a plant that is harmful to agricultural or horticultural crops, natural habitats or ecosystems, or humans or livestock. Most noxious weeds have been introduced into an ecosystem by ignorance, mismanagement, or accident. Some noxious weeds are native, though many localities define them as necessarily being non-native. Typically, they are plants that grow aggressively, multiply quickly without natural controls, and cause adverse effects upon contact or ingestion. Noxious weeds are a major problem in many parts of the world, greatly affecting areas of agriculture, forest management, nature reserves, parks, and other open spaces.