Nonnative & Invasive Species


Non-native species are those that are introduced to an area that is not within the range in which they have historically occurred. Non-native species are also known by other synonymous names such as introduced, exotic, and non-indigenous. In contrast, species that occur within their historical range are referred to as native or indigenous species. Organisms may be transported to new areas by natural means, such as extreme weather events, or by humans. Transportation by humans may be either accidental or intentional. The term nonnative is also applied to species that have been moved outside the range where they normally occur. For example, species native to the west coast of the U.S. that are introduced to the east coast would be considered nonnative in their new location.

There are numerous examples of species introductions around the world, dating back centuries. Many intentional introductions are the result of agricultural activities and there are many examples. The genus Citrus originated in southeast Asia, India, and the Near East, and has been transported over time to parts of Europe, Africa, Australia, and the Americas. Potatoes were discovered growing along the Andes from Columbia to Chile and are now one of the most important foods grown in temperate regions, with large areas of production in China, Poland, the United States and parts of the former Soviet Union. Many other non-native species also exist in their new ecosystems without causing any harm.


Unfortunately, some of the most devastating pests and diseases around the world are also nonnative species. In the United States, Dutch elm disease (DED) and chestnut blight are two particular historical examples that have led to significant changes to the composition of the forests of the northeastern U.S. as well as the appearance of many towns and cities where elm trees used to line the streets. More recent examples that are currently impacting ecosystems in the northeast include zebra mussels (Dreissena polymorpha), Asian longhorn beetle (Anoplophora glabripennis), and the emerald ash borer (Agrilus planipennis).

Nonnative species that cause, or are likely to cause, economic or environmental harm, such as those described above, are termed invasive species.