Invasive plants threaten native species, cause environmental problems, and can be nearly impossible to eradicate once established.
By Lowell and Kaye Christie, F47246
Besides chasing birds and watching animals, we spend much of our time exploring the plant life in our favorite places, from the national parks to wildlife refuges, and sometimes just in our own backyard. And the more we learn about plants, the more we become aware of one of the most serious threats to our natural environment: plants that end up where they don’t belong.
When the National Park Service was created, part of its mandate was “to conserve the scenery and the natural and historic objects and the wild life therein and to provide for the enjoyment of the same in such manner and by such means as will leave them unimpaired for the enjoyment of future generations.” The United States Fish and Wildlife Service has a similar directive for the National Wildlife Refuge system. But both of these organizations and many other natural areas in the country are under attack by invasive plants.
Exotic non-native plants occur on 2.6 million acres of U.S. national parks. More than 2 million acres of the national wildlife refuge system have been overrun by plants that don’t belong there. Nationwide, approximately 1.7 million acres of wildlife habitat are invaded each year. Some of these problem plants arrived on their own, hitching a ride with other plants or animals, or perhaps used as packing material protecting other imports. But a surprising number were intentionally released without enough research into their potential for creating havoc with the native vegetation.
One of the best examples of good intentions gone astray is kudzu, introduced from Japan as an ornamental plant and a forage crop at the Philadelphia Centennial Exposition in 1876. In the early 1900s the Soil Conservation Service recommended planting kudzu to prevent soil erosion. Now it covers more than 7 million acres in the Southeast, climbing telephone poles, smothering trees, and turning the landscape into an unintentional topiary garden. The estimated cost of lost cropland and various control measures may be as high as $500 million annually.
Unintended importation occurs when a product such as a crop seed also includes an unexpected and unwanted bonus. Spotted knapweed, originally from Eastern Europe, probably made its way to North America in contaminated supplies of alfalfa. It now has been identified as invasive in natural areas of 26 states. It covers more than 5 million acres in Montana alone. The plant can live up to nine years, producing tremendous numbers of seeds that can survive in the soil for five to eight years. It also produces a chemical that prevents native plants from growing nearby.
In northern California, 10 million acres of former grazing land now contain yellow starthistle. It also arrived in contaminated supplies of alfalfa, and it takes over areas by crowding out the native plants. Five of the United States’ western national parks have infestations of this plant.
This problem is not just one of imports. During a study to find the source of weed seeds entering Japan, it was determined that the main source was feed grain imported from the United States. Invasive species are a worldwide dilemma, and every continent has its share of invasive plants.
Not every alien plant is a problem. Many of our major food crops are not natives of this continent, and many of our landscaping plants come from other countries. But if an introduced plant has high seed production, develops an extensive root system, and can survive in many different environments, it may become invasive. Of every 100 plants introduced into this country, only 10 find conditions where they can become established, and of those 10, only one will be considered invasive.
But that one in a hundred can be one too many. It’s estimated that approximately 40 percent of all threatened and endangered species in the United States are in trouble as a direct result of invasive species. The only more serious problem for species is habitat loss, and in some cases both the loss of habitat and trouble from invasive species combine.
In 1975 the Mississippi Sandhill Crane National Wildlife Refuge was the first refuge established under the Endangered Species Act. The cranes had lost much of their nesting and foraging habitat, and this refuge was an attempt to give the birds a chance at survival. But an invasive plant called cogon grass is threatening areas of the refuge and is complicating the efforts to protect these birds. Controlling cogon grass is just one of the many invasive species management efforts taking place in U.S. national parks and refuges.
According to the U.S. Fish and Wildlife Service, ornamentals make up 56 percent of plants on the World’s Worst Invasive Alien Species list. What you put in your backyard may escape your neighborhood and become a widespread problem. Purple loosestrife was imported into the United States as an ornamental in the early 1800s and is now found in every state except Florida. It is a beautiful plant and is still widely sold, except in a few states where it has been outlawed. It forms dense stands and crowds out native vegetation, taking over many wetland areas. A mature plant can produce more than 2 million seeds each year, and the seeds adhere to both wildlife and people. Purple loosestrife, once it gets a good start, is almost impossible to eradicate.
The financial consequences can be severe when troublesome plants show up in the wrong place. A study at Cornell University puts the total cost of invasive plants at more than $137 billion per year. Some of this is a direct cost from trying to combat the invaders, while some are hidden expenses, such as the higher cost of meat when an inedible plant takes over grazing lands used for livestock.
What can we do about invasive plants? Make sure you don’t help them spread. When you add plants to your yard, use either natives or non-invasive ornamentals. If you’re not positive, check with your local nursery. And remember that many seeds are sticky. That’s so animals (including humans) can inadvertently pick them up and carry them to a new location. So, check your socks for seeds that may have attached themselves for a free ride. Wash off fishing equipment; boats; ATVs; and tires from motorhomes, cars, and bikes before you leave a natural area.
Also, check with representatives at a local park or refuge to see how you can help eliminate these intruders. Many facilities operate volunteer programs in an effort to remove invasive plants. It’s a great way to help protect our native environment.