Inland Invasion: The Impact of Eurasian Watermilfoil on Water Habitats
by Emily Chang
The Eurasian watermilfoil, also called Myriophyllum spicatum or simply the milfoil, is an invasive plant species introduced to the United States from Europe in the 1940s, and it has spread to various places in North America from British Columbia to South Carolina. Its leaves, which are jade-green in color and consist of approximately twelve to twenty-one paired leaflets, are grouped into three to six whorls, or coils. The stems and small flowers of the milfoil are usually red-brown but have some variation in color. Because the Eurasian watermilfoil looks similar to certain other plant species, particularly the native northern watermilfoil and the native coontail, it requires close observation to identify this submersed plant. While the native northern watermilfoil (Myriophyllum sibiricum, or M. exalbescens) has fewer than twelve pairs of leaflets per leaf group, the Eurasian watermilfoil generally has twelve to twenty-one per leaf. Unlike Eurasian watermilfoil, the native coontail (Ceratophyllum demersum) has a rough texture and toothed leaves.
Although the Eurasian watermilfoil can spread to inland lakes and ponds via boats, boating equipment, and waterbirds, its primary means of dispersal occurs through plant fragmentation. The milfoil generally reproduces and spreads when parts of its stems break off and travel to other waterbodies, where they would take root and create new colonies. This plant fragmentation process often takes place in two ways: when the milfoil’s stems become brittle and easily broken around late summer to fall annually, and when motorboats sever the stems and disperse them to new locations. Horizontal plant segments called stolons contribute to the local colonization of water habitats by milfoils as well. These dispersal methods, coupled with the plant’s ability to withstand a wide range of temperatures, allow the milfoil to spread quickly; it can colonize a lake in less than two years’ time. Once a colony is established in a body of water, the milfoil could cause substantial harm to its environment. For one thing, it can grow into dense mats that cover the surface of a lake; this can crowd out other native plant species, obstruct waterways and water intake passages, and decrease the oxygen levels of the lake. Dense milfoil growth can also hinder human activity such as boating and fishing. A variety of methods have been employed to control the growth and spread of milfoil. Biocontrol methods include the use of pathogens, insects, and fish; aquatic herbicides and other chemical methods are somewhat effective but must be implemented every one to three years. Humans have also employed hand-pulling, rototilling, underwater vacuuming, and mechanical harvesting to keep milfoil populations under control. Although these means of control are effective, they have not managed to completely eliminate Eurasian watermilfoil colonies in bodies of water.
Because the spread of the milfoil can occur through human activities, notably boating, it is reasonable to question the ties between humans and this invasive plant species. As I read on some of these websites, aquatic wildlife and ecological organizations advise boaters and fishers to remove plant material from their boats and other equipment and discard this debris in a container or location reasonably far from lakes and ponds. Also, I saw information concerning aquatic gardening and how to choose, plant, and discard aquatic plants for a garden. Such notices and advice generate some doubts in my mind as to whether they are effective. When organizations put up helpful tips like these, people often do not heed them, which render them ineffective. An analogous situation would be putting recycle bins along trails to prevent littering while people still choose to throw their trash among the trees and other plant vegetation that could be potentially harmed. Since scientists and researchers have not achieved milfoil eradication, then it is reasonable to take a step back and reflect on the situation at hand. How could people effectively conquer and defeat this detrimental species? If scientists do find a way to control milfoil colonies, will this method harm other plant and animal species? Will it involve unreasonable measures for humans to take – in other words, will it ask too much of both the scientific community and recreational boaters alike to implement? Like I have said, the various control methods currently used to keep milfoil colonies under control are only successful to a certain extent, so I believe that scientists and conservationists should either look for alternative methods to eradicate milfoil populations or research new ways to eliminate them. The wildlife organizations also could impose inspections of boating equipment that could result in heavy fines for boaters who fail to clean their equipment well.