Invasive plants
Invasive plants are non-native species that, when introduced to new environments, can cause significant ecological and economic harm. In the United States, these plants are responsible for the decline of many native species and contribute to billions of dollars in control costs annually. Many invasive species originate from Asia or Europe and are often introduced accidentally through human activities, such as transportation of goods. Once established, invasive plants can outcompete native flora due to their rapid growth, prolific seed production, and lack of natural predators, leading to reduced biodiversity and altered ecosystems.
Invasive plants can also deplete soil nutrients and moisture, sometimes releasing toxic chemicals that inhibit the growth of surrounding plants. Effective management strategies include preventing their introduction, implementing plant quarantines, and controlling small infestations. However, once invasive species become widespread, management often requires costly chemical or biological controls, which may inadvertently affect native species as well. Notable examples of invasive plants include Canada thistle, purple loosestrife, and the aquatic water hyacinth. Understanding the implications of invasive plants is crucial for maintaining healthy ecosystems and preserving biodiversity.
Invasive plants
Categories: Environmental issues; poisonous, toxic, and invasive plants; water-related life
Between the damage they cause and the cost of control efforts, invasive plants cost the United States billions of dollars each year. According to the US Forest Service, invasive species have contributed to the decline of 42 percent of all endangered or threatened US species. Thus, invasive plants are capable of causing irreparable changes in ecosystems.
Most invasive species invading the United States originated in Asia or Europe. A key factor in this problem is that seeds or spores of these plants are accidentally transported into new habitats by humans, but the plants’ natural enemies and competitors are left behind. Without natural biological controls, the alien species can thrive and outcompete the native flora, driving the native plants toward extinction and creating a near monoculture of the invader.
Invasive plants are weedy species that grow rapidly, produce large numbers of long-lived seeds, and frequently have perennial roots, or rhizomes, that enhance asexual propagation. Invasive plants have a variety of effects on invaded ecosystems. Many invasive species deplete soil moisture and nutrient levels, either by growing more vigorously than native plants early in the growing season or by being more tolerant of reduced levels of water and nutrients than are natives. Some invasive species produce toxic chemicals (allelopathy) that are released into the soil and inhibit the growth of competitors. By outcompeting native plants, the invader decreases species diversity as it replaces many native species. As a result, animal species dependent on native flora are also affected. Fungi and seed plants are among the most disruptive invasive plants in the United States today.
Control Methods
Invasive species are carried to new habitats, either in or on machinery or organisms, and are usually transported by humans, so prevention is the most cost-effective method of control. Once an invasive species has entered an area, plant quarantine is an effective first line of defense. For example, living plants and animals brought into the United States must pass inspection by the US Department of Agriculture Animal and Plant Health Inspection Service (APHIS) to ensure that they are not carrying potentially invasive species. Particular care is taken to ensure that imports from known areas of infestation are clean of seeds, spores, or propagules.
The next most effective strategy is detection and control of small infestations. When there is a known threat of invasion, the affected area should be surveyed periodically and individual plants removed by hand or, in extreme cases, by “spot-spraying” herbicide. Eradication is possible when the infestation is small.
Once an invasive species becomes established, the only means of management are expensive chemical or biological controls which, at best, will only minimize damage. A variety of chemicals may be used to kill invasive plants. Most chemicals, however, affect a broad spectrum of plants, including native species. Biological controls, including natural enemies from the invasive plant’s native ecosystem, can be more specific but may also be capable of displacing native species and becoming “invaders.”
Fungi
Many of the most serious plant pathogens are invasive species introduced into the Americas since the beginning of European settlement. Two classic examples are Dutch elm disease, caused by the fungi Ophiostoma ulmi and Ophiostoma novo-ulmi and chestnut blight, caused by the fungus Cryphonectria parasitica. At the beginning of the twentieth century, the most common street tree growing in the cities of the eastern United States was the American elm. About 1910, the European bark beetle was introduced into the United States. It was not until the 1930s that Dutch elm disease was observed in Ohio and a few eastern states. The fungal spores are carried by the beetles, which burrow under the elm bark. The native elm has little resistance to this fungus, whose spores rapidly germinate and form extensive mycelia within the phloem of the host tree, killing it within a few years. After its initial contact, the fungus spread throughout the cities and forests of the East and gradually westward, so that by 1990 nearly all of the native American elm trees in the United States had been killed.
American chestnut was also one of the early dominant trees of the eastern US forest. In addition to providing edible fruit, the chestnut became a commercially important timber tree. Chestnut blight fungus was first reported in 1904 on chestnut trees in the New York Zoological Garden and quickly began to spread. This infestation led directly to passage of the Plant Quarantine Act of 1912, the forerunner of APHIS. By 1950 most native chestnut trees were reduced to minor understory shrubs.
Terrestrial Green Plants
Many of the plants commonly called “weeds” are foreign invaders that are difficult, if not impossible, to control. Some of the most severe include Canada thistle (Circium arvense), leafy spurge (Euphorbia esula), and purple loosestrife (Lythrum salicaria). Canada thistle is the most widespread and difficult species of thistle to control. It was introduced to Canada from Europe in the 1600s and in 1795 was listed as a noxious weed in Vermont. It later spread throughout most of the United States as well as in Canada. Single herbicide applications do not provide long-term control, and there are no effective biological controls that do not also attack native species.
Leafy spurge was first reported in Newbury, Massachusetts, in 1827, where it arrived in ship ballast. By 1900 it had reached the West Coast, and later thrived in more than half the states and in Canada. Multiple species of insects are approved for biological control, and several herbicides can be used to control infestations effectively. Sheep and goats will browse on spurge.
Purple loosestrife was introduced into the United States as an ornamental plant in the early 1800s and became established in New England by 1830. Its early spread into the Great Lakes region was by barge and other canal traffic. Rapid expansion of the pest, particularly in the West, occurred after 1940, primarily due to the plant’s “escape” from ornamental cultivation into irrigation projects. It was later found in all the lower forty-eight states except Florida, with no effective controls.
Aquatic Green Plants
Invasive plants are not limited to the terrestrial habitat or to vascular plants. One dramatic example is the alga Caulerpa taxifolia, the so-called killer alga. This attractive tropical alga was found to be easy to grow in saltwater aquaria and useful as a secondary food source for herbivorous tropical fish. It began to be used this way at the Oceanographic Museum of Monaco in 1982. Two years later, a meter-square patch was found growing in the Mediterranean Sea, visible from a window of the museum. By 1990 the alga had reached France, and by 1995 it could be found from Spain to Croatia. Caulerpa produces a number of toxins that inhibit foraging by native fish, and it is a prolific vegetative reproducer. Fragments of the alga, stuck on an anchor for example, can start a new infestation wherever the anchor is next dropped. This species has been discovered in Southern California, and a related species has become dominant in Sydney Harbor, Australia. Other aquatic invasive plants in the United States include the mosquito fern (Azolla), the Eurasian water milfoil (Myriophyllum), and the water hyacinth (Eichhornia crassipes).
Bibliography
"Invasive Plants." U.S. Forest Service, U.S. Department of Agriculture, www.fs.usda.gov/wildflowers/invasives/index.shtml. Accessed 11 July 2024.
Meinesz, Alexandre. Killer Algae. Chicago: University of Chicago Press, 1999. A noted scientist describes the spread of an exotic seaweed in the Mediterranean Sea.
Randall, John M., and Janet Marinelli, eds. Invasive Plants: Weeds of the Global Garden. Brooklyn, N.Y.: Brooklyn Botanic Garden, 1996. Simply written, useful book for identification of and removal techniques for invasive species. Discusses weed origins, environmental impacts, and U.S. distribution. Includes illustrations of about fifty species.
Sheley, Roger L., and Janet K. Petroff. Biology and Management of Noxious Rangeland Weeds. Corvalis: Oregon State University Press, 1999. General information about invasive plants with individual chapters on specific species.