Exotic species are organisms that are introduced to a region or ecosystem, often unintentionally, through human migration or trade. Some exotic species are useful to man, such as horses, goats, pigs, and edible plants including wheat and oats. These are examples of species that were brought to the Americas intentionally by European colonists. Other exotic species that were introduced accidentally such as the Mediterranean fruit fly (Ceratitis capitata), Japanese beetle (Popillia japonica), Africanized bees (Apis mellifera scutellata)(sometimes called killer bees), and Norway rat (Rattus norvegicus) have become pests. Many exotic species, including most tropical fish, birds, and houseplants brought to colder climates, can survive only under continuous care. A few prove extremely adaptable and thrive in their new environment, sometime becoming invasive and out competing native species.

The federal government's Office of Technology Assessment has estimated that more than 2,000 plant species introduced from around the...