Geographic Location: Southwest United States, Gulf Coast
Title: Alien Invaders: When Weeds Do Good Things
A host of beautiful plants from Africa, Asia, Europe, and South America have made their way into the United States as ornamental plants only to become agricultural pests or weeds. Particularly along the Gulf Coast states, a host of tropical plants exploited the region's warm and moist climate once they wandered from the confines of yards and gardens. Chinese fallow, Chinese privet, elephant ears, Japanese honeysuckle, and johnsongrass are a few highly successful alien plants overgrowing the area. Even tumbleweed, a symbol of the American West, was introduced from Europe with commercial flax seed.
The notorious Kudzu from Japan is probably the most destructive invasive plant that entered the Southern and Gulf states. It is an aggressive vine that can grow several feet within two days. The plant covers up anything in its path. It overgrows whole forests, choking off the natural vegetation and forming a dense, green carpet. Kudzu has been known to overgrow houses, cars, and fences.
This plant has become the "poster child" of the invasive plant problem. The animal equivalent to Kudzu is the zebra mussel, which has the nasty habit of overgrowing native shellfish and clogging water pipes mainly in the northern states.
Alien invaders are generally, at minimum, a nuisance when introduced into a new habitat. Many inhibit the growth of crops, decline grazing areas for cattle and sheep, and displace native species. So, it is not surprising that the United States government spends millions of dollars annually studying and combating the alien species. Controlling many of these organisms poses few concerns for environmental groups. They see these organisms as being the cause of potentially irreversible environmental damage. However, strategies for controlling an invasive plant called water hyacinth are being debated because of potential benefits of the plant.
Background
Water hyacinth, Eichhornia crassipes, is a pretty pond plant that creates a carpet of green and regularly produces an attractive spray of pink to purple flowers. It is an upright floating plant that reaches up to 1 meter in height. In cooler areas, it averages less than 30 centimeters in height. The plant is native to South America where its growth is controlled by competing species and natural enemies. The plant thrives in warm climates where it can quickly carpet standing and slow-moving bodies of water. It propagates sexually with abundant flowers and asexually by producing numerous offshoots. The plant is easily spread by traveling long distances on the feet of aquatic birds and on the surfaces of boats.
Because of its durability and beauty, water hyacinth was introduced into Louisiana in 1884. Like many alien plants, it was brought to the United States as an ornamental plant for use in public and home gardens. It made its appearance at the 1884-1885 World's Industrial and Cotton Centennial Exposition. Within 20 years, water hyacinth made its way into the waterways of all of the Gulf Coast states. It grew at a rate of 200,000 hectares per year and continues to spread at alarming rates.
Water hyacinth has gained fame as a plant pest in many countries. It was brought to Europe in 1879 where it escaped into many bodies of water. Luckily, the plant is killed by frost and does not survive prolonged winters. So, it did not become a persistent pest in northern Europe. The plant was then introduced into Africa around 1880, where it became a serious pest as it formed thick mats that disrupted boating, commercial fishing, and water pumping stations. The Nile River is currently infested with large populations of water hyacinth that are rapidly spreading throughout the African continent.
Water hyacinth also made its way into Australia and southern Asia by 1900. As expected, it spread rapidly throughout southeast Asia and found its way into farm ponds, rice fields, and irrigation waterways. The plant is currently destroying the area's sustainable agricultural and aquaculture practices.
Most nations where water hyacinth has set roots consider the plant a menace that threatens the natural biota and the regional economy. Many scientists label it the most damaging weed in the world because it displaces native species and changes the overall ecology of the waterway. Yet, water hyacinth does provide many benefits. In many rivers and streams, it prevents the extreme bank erosion caused by coastal construction and boat traffic. It acts as a barrier buffering the wave action known to wash away bank soil and lessens the flow of water, causing soil particles to settle on the bank. This is especially true in densely populated regions where erosion is a serious problem. Water hyacinth has a fibrous root system known to improve fish and invertebrate habitat by providing extra shade and cover from predators. New studies show that water hyacinth is a valuable bioremediation organism. It is capable of removing large amounts of organic pollutants and reducing deleterious levels of nutrients. Some scientists are considering using water hyacinths in artificial wetlands designed to treat agricultural and human sewage.
Nations that consider water hyacinth a pest are involved in aggressive control programs. Traditional control involves pulling up the plants by hand or by using simple machines. Obviously, this is not economically feasible in many countries and does not remove all of the plants. Many nations use herbicides such as 2,4-D, diquat ,and copper compounds to control the pest. Herbicide treatments can be more environmentally destructive than the water hyacinth. Unfortunately, the herbicides kill native plants as well as the water hyacinth. Water hyacinth has many beneficial relatives, such as the native pickerelweed, that easily succumb to the herbicide applications. Eventually, the long-term use of herbicides has limited effect on water hyacinth control. The plant's rapid growth makes it easy for the plant to produce herbicide-resistant populations. More sophisticated treatments include biological control using the water hyacinth's natural enemies. The most popular biological control agents are an insect called the mirid bug (Eccritotarsus catarinensis), a weevil (Neochetina spp.), and a moth (Sameodes albiguttalis), which is known to selectively attack water hyacinth without destroying related species. However, biological control also has its limitations. In many cases, the biological control organisms do not survive well in the new environment and usually do not eradicate the pest species. Sometimes the biological control agent becomes a pest if it attacks or displaces the native biota.
The Issues
History shows that invasive organisms usually harm the native biodiversity. In many cases, the alien invaders will take over the niche of native species, causing the extinction of previously established organisms. Controlling or trying to eradicate the invasive organism in most cases causes worse environmental damage than the invasive organism. Some environmentalists argue that much of the damage of invasive plants is economic. They propose using only environmentally friendly ways to control invasive plants and only in cases when the invader is creating significant environmental damage. Others feel that a new biodiversity is being created. They use the constantly changing evolutionary history of the world to support a view that all ecosystems decline and change naturally.
In the case of water hyacinth, some people have evidence that the plant can be beneficial if managed properly. Researchers doing initial investigations to control kudzu are considering managing and overharvesting the plant for commercial purposes. They feel that plants such as water hyacinth can be put to good use if managed properly in the environment. However, the current emphasis identified by the International Water Hyacinth Consortium entails eradication of the plant. They concluded that all current efforts to control water hyacinth have failed worldwide. Scientists at the conference recommended biocontrol efforts using microbes instead of insects. They also foresee more ecological research to better understand the success of the plant. Nothing was mentioned about managing the plant for commercial use.
References
Literature:
Gopal, B., Junk, W.J. and Davis, J.A. 2000. Biodiversity in Wetlands: Assessment, Function and Conservation. Volume 1. Backhuys Publishers; Leiden, The Netherlands.
Keddy, P.A. 2000. Wetland Ecology: Principles and Conservation. Cambridge University Press; New York, NY.
Mooney, H.A. and Hobbs, R.J. (Editors). 2000. Invasive Species in a Changing World. Island Press; Covelo, CA.
Web Sites:
Problem Aquatic Plants
http://www.wapms.org/plants/index.html
United States Geological Survey International Program
http://edcsnw3.cr.usgs.gov/ip/hyacinth/winam2.html
United State Environmental Protection Agency
http://www.epa.gov
Key Principles