Department of Conservation and Recreation
Conserve. Protect. Enjoy.
Department of Conservation and Recreation
Invasive plants are introduced species that cause health, economic or ecological damage in their new range. More than 30,000 species of plants have been introduced to the United States since the time of Columbus. Most were introduced intentionally, and many provide great benefits to society as agricultural crops and landscape ornamentals. Some were introduced accidentally, for example, in ship ballast, in packing material and as seed contaminants. Of these introduced species, fewer than 3,000 have naturalized and become established in the U.S. landscape outside cultivation. About 1,000 naturalized plant species have become invasive pests that interfere with agriculture, forestry, transportation and utility infrastructure, lawn and garden maintenance, and natural ecosystem processes. Of the 3,200 plant species in Virginia, more than 600, or 18 percent, have been introduced since the founding of Jamestown. The Virginia Department of Conservation and Recreation currently lists 90 species as invasive.
Invasive species are a major threat, second only to habitat destruction, to native plants and wildlife. They can reduce habitat and population size of native species, alter habitat structure and change ecosystem properties. Fifty-seven percent of plant species listed as threatened or endangered by the U.S. Fish and Wildlife Service are directly threatened by invasive species.
In the United States, invasive species cause an estimated $120 billion in annual economic losses, including costs to manage their effects. Annual costs and damages arising from invasive plants alone are estimated at $34 billion.
In North America, plant species are generally described as native if they occurred here prior to European settlement. This distinction is made because of the large-scale changes that have occurred since the arrival of the European settlers. The Europeans imported a variety of plants to this country, many are still the major component of traditional lawns and gardens. They also include many beneficial plants important in farming, such as vegetables and grains. Today, approximately 25% of flowering plants in North America are non-natives or alien species, most of Eurasian origin.
All plants are native to some region, and offer a variety of ecological, economic and aesthetic benefits. In fact, many alien species are beneficial, providing food and other valuable resources to society. It is only when a species is "out of place" that we become concerned. In these instances, invasive alien plants can pose a serious threat to biodiversity. Due to a lack of natural controls such as insect pests and competitors, some alien plants can easily become established in new areas. Once established, alien plant species can out compete and displace the native plant species, disrupting ecological processes and significantly degrading entire plant communities. Many invasive plants spread quickly and grow so densely that other species cannot get established in areas infested by these alien plant species. Common native plants can be crowded out, or their populations threatened due to hybridization with escaped ornamentals. Endangered species may be driven from their last habitats by invasive alien plant species. Aquatic invasive species can clog waterways, disrupt groundwater flows, degrade water quality, and lead to dramatic changes in native plant and animal communities.
Although a majority of invasive alien species come from other countries, they may also be introduced from a different region of the same country. For example, saltmarsh cordgrass is a dominant and important member of coastal saltmarsh communities along the Atlantic and Gulf Coasts. But when used for restoration projects in west coast marshes, it became invasive, out competing and replacing western species.
In contrast to invasive alien species, other non-native plants are unable to thrive without extra effort by land managers. For instance, they may originate in regions with abundant rainfall and soils rich in nutrients. If then introduced into a drier region with less fertile soils, they may require additional watering and fertilizer. The natural defenses plants evolve in their original habitats may not protect them in a new environment, requiring the application of pesticides to aid their growth.
