An "invasive species" is defined as a species that is non-native (or alien) to the ecosystem under consideration and whose introduction causes or is likely to cause economic or environmental harm or harm to human health (Executive Order 13112). Sometimes you will see invasive species referred to as exotic, alien, or non-indigenous species. The problem with these names is that they only refer to the non-native part of the definition above. Many exotic or alien species do not cause harm to our economy, our environment, or our health. In fact, the vast majority of "introduced" species do not survive and only about 15% of those that do go on to become "invasive" or harmful. An invasive species grows/reproduces and spreads rapidly, establishes over large areas, and persists. Species that become invasive succeed due to favorable environmental conditions and lack of natural predators, competitors and diseases that normally regulate their populations.
An organism is considered exotic (alien, foreign, non-indigenous, non-native) when it has been introduced by humans to a location(s) outside its native or natural range. This designation applies to a species introduced from another continent, another ecosystem, and even another habitat within an ecosystem. For example, black locust (Robinia pseudoacacia), a tree that is native to the southern Appalachian region and portions of Indiana, Illinois and Missouri, was planted throughout the U.S. for living fences, erosion control, and other uses for many years. Black locust is considered exotic outside its natural native range because it got there by human introduction rather than by natural dispersal. Another example is saltmarsh cordgrass (Spartina alterniflora), a wetland plant that is native to eastern North American estuaries. Saltmarsh cordgrass was introduced to western North American shoreline habitats, where it did not occur previously. It has established and become a serious invasive species, displacing native species and adversely impacting wetland communities.
European settlers brought hundreds of plants to North America from their home lands for use as food and medicine, and for ornamental, sentimental, and other purposes. Introductions of exotic plants continue today and are greatly increasing due to a large and ever-expanding human population, increased international travel and trade, and other factors.
Many non-native species exist in apparent harmony in environments where they were introduced. For example, a relatively small number of exotic plants (e.g., corn, wheat, rice, oats) form the basis of our agricultural industry and pose little to no known threat to our natural ecosystems. The most important aspect of an alien plant is how it responds to a new environment. An invasive species is one that displays rapid growth and spread, establishes over large areas, and persists. Invasiveness is characterized by robust vegetative growth, high reproductive rate, abundant seed production, high seed germination rate, and longevity. Some native plants exhibit invasive tendencies in certain situations.
According to the Plant Conservation Alliance’s Alien Plant Working Group, about 1,100 plant species have been reported as being invasive in natural areas in the United States. This number represents an astonishing one-third or so of the exotic plant species established and self-reproducing in the wild. Some invasive species were planted intentionally for erosion control, livestock grazing, wildlife habitat enhancement, and ornamental purposes. Others have escaped from arboretums, botanical gardens, and our own backyards. Free from the complex array of natural controls present in their native lands, including herbivores, parasites, and diseases, exotic plants may experience rapid and unrestricted growth in novel environments.
Invasive species impact native plants, animals, and natural ecosystems by, reducing biodiversity, altering hydrologic conditions & soil characteristics, interfering with natural succession, competing for pollinators, displacing rare plant species, serving as reservoirs of plant pathogens, replacing complex communities with single species monocultures, & diluting the genetic composition of native species through hybridization.
The following web sites have additional information about invasive and exotic plant species:
Author: Jil M. Swearingen, National Park Service