What is an Invasive Species?
An organism is considered invasive(alien, foreign, nonindigenous, 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 to these new places 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 intentionally to western North American shoreline habitats, where it did not occur previously. Sadly, it has become established and is now considered a serious invasive species, displacing native species and adversely impacting wetland communities and several endangered native bird and plant species.
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 increasing due to a large and ever-expanding human population, increased international travel and trade, and other factors.
Once an Invasive, Always an Invasive
An estimated 3,500 species of exotic plants have escaped cultivation in the U.S., are able to reproduce in the wild, and have become established, or “naturalized”. These plants, however much a part of our current landscapes and ecosystems, are nonetheless exotic, since they were moved here by people. For centuries, horticulturists have imported and disseminated interesting new exotic plants. Unfortunately, many of these have become invasive pests that are having serious impacts to native species and ecosystems. What Makes an Exotic Species Invasive?
When is a Guest a Pest?
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 spreads and establishes over large areas, and persists. Invasiveness may be characterized and enhanced 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.
How Many Plants are Invasive?
According to the Plant Conservation Alliance’s Alien Plant Working Group, about 1,050 plant species have been reported as being invasive in natural areas in the United States (see list of links). This 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.
This information has bee adapted from the National Park Service Weeds Gone Wild website and fact sheet.
Characteristics of Invasive Plants
Like invasive species in general, invasive plants possess characteristics that make them especially suited for colonizing new ecosystems. In addition to the characteristics listed above “How Does a Species Become Invasive?”, these plant-specific characteristics can include invasive plants’ ability to:
- Produce abundant, easily dispersed seeds that can withstand adverse conditions
- Reproduce via multiple pathways: roots, stems, and seeds
- Release chemicals that inhibit the growth of or kill surrounding native plants
- Have very few natural predators
Source(s):
- Pacific Northwest Research Station