Invasive Species

Plants that grow aggressively and outcompete other species are called invasive species. Invasive plants are usually those that were introduced, either intentionally or unintentionally, into a locality where they previously did not grow. Introduced plants, also called exotics or alien species, form an important part of our environment, contributing immensely to agriculture, horticulture, landscaping, and soil stabilization. But among the thousands of plant species introduced to North America, approximately 10 percent display the aggressive growth tendencies of invasive species. Although the terms exotic, alien, and invasive are sometimes used interchangeably, not all exotic plants are invasive. In addition, some native species, those plants that grew in an area prior to European settlement, can be invasive, especially as natural landscapes are altered.

Characteristics of Invasive Species

Invasive species are not a separate biological category, and all types of plants, including vines, trees, shrubs, ferns, and herbs, are represented by invasive species. They do, however, share certain characteristics that help them rapidly grow and invade new areas. Invasive plants typically exhibit at least some of the following:

• production of many seeds
• highly successful seed dispersal
• no special seed germination requirements
• grow in disturbed ground
• high photosynthetic rates
• thrive in high-nutrient conditions
• rapid growth and maturity
• early maturation
• reproduction by both seeds and vegetative means
• long flowering and fruiting periods

Most exotic plants do not pose an obvious threat to native plants when they are first introduced, but we do not fully understand the dynamics of what makes plants invasive. The same plant species can be invasive in one habitat or area and not aggressive in another. Sometimes many years separate the first introduction of a plant and its later spread as an invasive species. For example, Atlantic cord grass (Spartina alterniflora ) was present in small areas on the Pacific coast for more than fifty years before it became invasive.

Often by the time a plant is recognized as being a major problem it has become so well established that eradication is difficult or impossible. Even when plants are recognized as a potential problem, finding the money and manpower needed to eliminate them may not be easy. For example, leafy spurge (Euphorbia esula ), which forms dense stands that cattle refuse to graze, was seen as a potential problem in Ward County, North Dakota, in the 1950s. By the time funding was available to deal with the problem on both public and private lands, leafy spurge was present in all townships in the county and had increased from one small patch to about 12,000 acres.

Spread of Invasive Species

People have been the major factor in the spread of invasive species. Humans have always carried plants with them for food, medicine, fiber, ornament, or just curiosity. As human population has increased, so has the demand for food, housing, transportation, and other necessities of life. More and more land is disturbed to provide people with what they need and want, and disturbed land is where invasive species get their footholds. Increased international travel and global world trade also contribute to the problem. Invasive species have arrived in North America in the cargo holds of airplanes, as seeds in grain shipments, in the soil of ornamental plants, and as ship ballast. Improvements in transportation technology allow both people and plants to travel thousands of miles in just a few hours.

New environments provide an ideal place for invasive plants. These species leave behind the natural controls (usually insects) that kept them under control in their native habitats and can often spread unchecked. Some, such as the common dandelion (Taraxacum officinale ), ox-eye daisy (Chrysanthemum leucanthemum ) or tree-of-heaven (Ailanthus altissima ), have become integrated over time into the flora of urban areas and are the dominant and familiar vegetation.

Most of the invasive species in North America are originally from Europe or Asia, areas with very similar climate. Many of these species were first introduced as ornamental plants. An excellent example is honeysuckle (Lonicera spp.), which was introduced in the late 1890s as horticultural shrubs and vines and for wildlife habitat improvement. Honeysuckle often out-competes native plants due to earlier leaf expansion and later fall leaf retention. Large thickets of honeysuckle interfere with the life cycles of many native shrubs and herbs. These stands alter habitats by decreasing light and depleting soil moisture and nutrients. Some honeysuckle species also release chemicals into the soil that inhibit the growth of other plants. Fruits are consumed and passed by birds, which makes effective control difficult.

Another ornamental that turned invasive is kudzu (Pueraria lobata ), a vine with attractive purple flowers that was first exhibited in the United States at the Philadelphia Centennial Exposition in 1876. It is now listed as a noxious weed in many states, especially in the South, where it smothers large trees as it clambers for light.

Accidental introduction is also a common way for invasive species to become established. Mile-a-minute weed (Polygonum perfoliatum ), an Asian vine named for its fast growth rate, appeared in rhododendron nurseries in Pennsylvania in 1946, presumably the result of seeds mixed with imported plants. Since then it has spread to other areas in Pennsylvania as well as to surrounding states and is rapidly becoming a major problem along roadsides and other disturbed areas.

Impact and Eradication

The economic impact of invasive plants is staggering. They affect agriculture, the environment, and health. Invasive plants cause reductions in crop harvests as well as increased production costs. Farmers worldwide spend billions of dollars annually on chemicals and other methods to control weeds. The toll in human time is enormous, as hand-weeding of crops is the number one work task of 80 percent of people in the world. Some invasive species that contaminate harvested crops or pastures are toxic and pose a threat to both people and animals ingesting that food or milk.

Invasive plants are also a major threat to native plants and animals, including rare and endangered species. In fact, alien species are considered by some experts to be second only to habitat destruction as a threat to biodiversity . In the United States, for every acre of federal land destroyed by fire in 1995, two acres were lost to invasive plants. Two-thirds of all endangered species are impacted by invasive plants. Wetlands, home to many endangered plants, are especially susceptible to invasive species, such as purple loosestrife (Lythrum salicaria ), which has taken over thousands of acres in at least forty-two states.

The problem of invasive species affects all fifty states. Introduced species make up 8 to more than 50 percent of the total plant species of most states. Nowhere is the problem more serious than in Hawaii, where exotic species now outnumber native species. In Florida, at least 1.5 million acres of natural areas are infested with nonnative plants. Of mainland states, New York and Pennsylvania have the highest ratio of introduced-to-native species.

Methods for eradicating invasive plants range from hand-pulling to chemical controls. When weeding plants, it is important to disturb the soil as little as possible because disturbed areas are where invasive species can grow well. Other mechanical means include mulching soil to prevent or reduce seed germination, applying heat to seedlings, mowing, and girdling trees (pulling a strip of bark off all the way around the trunk to prevent the flow of nutrients). As more and more noxious weeds become resistant to chemical treatments, attempts at biocontrol (using natural predators) are increasing. Researchers have identified thirteen different insect species that may potentially control leafy spurge, and a beetle that eats the leaves of purple loosestrife has already been released in some areas.

Perhaps most important is public awareness and participation in the problem. People should avoid using invasive plants in their yards and gardens. This can be a complicated task as some invasive species, such as purple loosestrife, are sold in garden stores and catalogs. Beware of any plants described as "spreading rapidly." Another important defense is being on the lookout for alien plants and removing them before they become a problem. Organized efforts at invasive plant removal are a major weapon in preventing their spread. In Utah, middle and high school students who participate in a Scotch Thistle Day each spring have significantly reduced the amount of this noxious weed in their area.

Although most invasive species have been introduced from other areas of the world, native plants can become aggressive, especially as habitats are altered or destroyed. Boxelder (Acer negundo ) and wild grapes (Vitis spp.) as well as other native species can form fairly exclusive monocultures that thrive in disturbed environments. On the other hand, some otherwise invasive species can be useful in heavily disturbed sites. For example, tree-of-heaven grows where other plants cannot, thus providing just the foothold needed by other species to colonize .

The problem of invasive species is a costly one in terms of time, money, and loss of native habitats and species. Since the 1950s, weed-associated losses and costs worldwide have increased exponentially and are continuing to spiral upward. Of the more than sixty-seven hundred plants worldwide that are considered to be invasive, only about two thousand presently occur in North America. This leaves more than four thousand invasive plants now growing in other countries that could in the future become a problem in the United States.

see also Endangered Species; Human Impacts; Kudzu; Seed Dispersal; Wetlands.

Sue A. Thompson

Bibliography

Collins, Tim, David Dzomback, John E. Rawlins, Ken Tamminga, and Sue A. Thompson. Nine Mile Run Watershed Rivers Conservation and Natural Resources. Harrisburg, PA: Pennsylvania Department of Conservation and Natural Resources, 1998.

Randall, John M., and Janet Marinelli, eds. Invasive Plants: Weeds of the Global Garden. Brooklyn, NY: Brooklyn Botanic Garden, 1996.

Westbrooks, R. Invasive Plants, Changing the Landscape of America: Fact Book. Washington, DC: Federal Interagency Committee for the Management of Noxious and Exotic Weeds, 1998.