Invasive Species

America's Silent Killer
How Invasive Species Threaten America's Unique Wildlife Heritage

A Nationwide Assault

Non-indigenous plant, animal and pathogen species make their way into the United States every day through a variety of avenues, though the majority of them arrived here in the last 80 years alone. Some of these species are harmless to our environment, while others take root and threaten native ecosystems. Humans are responsible for introducing some types of non-indigenous species for the purpose of agriculture, horticulture, aquiculture or pets. Other species arrive unintentionally, through ballast water or as unwitting riders on agricultural produce, nursery stock or timber. Regardless of how they get here, through escape or release many of these species become free-living populations and cause the degradation and destruction of millions of acres of wildlife habitat.

Experts believe that invasive plants already exist in all 50 states on more than 100 million acres of land and water in the U.S. - an area roughly the size of California - and that they continue to spread at a rate of about 14 million acres per year. According to the U.S. Congressional Office of Technology Assessment, there are at least 4,500 species of foreign plants and animals that have established free-living populations throughout the country. Of that total, at least 675 species, or 15 percent, cause severe harm. In economic terms, 79 species, or 12 percent, deemed the worst invaders caused documented losses of $97 billion from 1906 to 1991.

And we don't even know the half of it. In the case of insects, it's impossible to get a firm grasp of how many invasive varieties exist in the U.S., since scientists have identified only about half of our own native insect species. But as far as the invasive plant and animal species we have identified, seeing the havoc they wreak on the environment may be all we need to know. Forty-six percent of all federally-listed threatened and endangered species are considered at risk primarily due to competition with or predation by invasive species.

Ecological Impacts

The effects of invasives range from wholesale ecosystems changes and extinction of indigenous species to more subtle ecological changes and increased biological sameness. Invasive plants harm the environment by damaging soil and water resources, ruining fish spawning habitat and crowding out native species.

About 15 percent of all invasive species cause severe harm, such as the gypsy moth, zebra mussel or leafy spurge, which causes more than $144 million in livestock forage damage each year in Montana, North Dakota, South Dakota and Wyoming. Purple loosestrife has invaded wetland habitat in 48 states and crowds out at least 44 native plants species. The State of Florida has spent $28 million trying to control tropical soda apple, a shrub-like herb that now covers 370,000 acres.

Because of their distinctive geography, climate, history and economy, Hawaii and Florida are particularly vulnerable to ecological degradation associated with invasive species. More than half of Hawaii's plants and wildlife are non-indigenous. At least 86 introduced plants played a significant role in past extinctions of native species and continue to threaten ones that are now threatened or endangered. In addition, nonnative species alter the islands' natural fire cycles.

In Florida, several non-indigenous aquatic weeds and invasive trees seriously threaten the Everglades ecosystem and cost the state millions every year. More than 900 nonnative plant species have become established in Florida and constitute at least 27 percent of the total flora of the state. For example, melaleuca trees are severely and rapidly degrading ecosystems by outcompeting indigenous plants and altering topography and soils. State officials estimate that melaleuca infests about 500,000 acres of native wetlands in southern Florida and is expanding at a rate of 50 acres per day.

And then there are the invasive animals like Norway rats; in cities, there is roughly one rat for every human, and on farms, each rat is estimated to destroy grain and other goods worth $15 annually. In the late 1800s, farmers introduced mongooses in Puerto Rico and Hawaii to kill rats in the sugarcane plantations, but now those animals prey on ground-nesting birds and have contributed to the extinction of at least 12 species of amphibians and reptiles - and they also carry diseases like rabies that affect humans.

Economic Impacts

Not only do invasive species change our landscapes and affect native species, they also impact our economy and cost taxpayers and communities billions of dollars every year. One study of a worst-case scenario for just 15 potential high-impact species estimates $134 billion in future economic losses over the next 50 years.

Some of the most damaging and widespread invasive plants include purple loosestrife, Chinese tallow, melaleuca, salt cedar and kudzu. These and the others on the top 15 problem plants list alone cost the U.S. more than $600 million in damages from 1906-1991. During that same time period, the 43 most harmful insects (more than 370 insects are known to be invasive residents) cost nearly $92 billion. Purple loosestrife, an extremely hardy wetland plant, is perhaps the most prevalent invasive species in the U.S., covering about 400,000 acres of federal land, at an annual cost of control upwards of $45 million. These and other invasive species impact many of our national interests: agriculture, industry and human health - not to mention the protection of natural areas. And experts estimate that for every year we delay addressing the issue, the costs of controlling invasive species may increase two- to three-fold.

#1 Threat to the Refuge System

About eight million acres of habitat scattered throughout half the units within the Refuge System are infested with invasive plants that interfere with crucial habitat management objectives. In 1998, combined invasive plant and animal control cost the Refuge System about $13 million. By July 2000, the problem had worsened, and the cost had risen to $120 million. With a nearly $2 billion backlog in refuge funding, the rising cost of invasive species control threatens the future of the entire Refuge System. Currently, invasive species control is the fastest growing component of the Refuge Operation Needs (RONS) database, with project needs totaling nearly $150 million. A recent "test run" of the new Refuge System Threats and Conflicts Database listed invasive species problems as the highest ranking threat or conflict affecting refuges, scoring almost double that of the second-ranked threat (illegal activities).

Putting a Face on the Problem

At the Blackwater Refuge on Maryland's eastern shore, nutria, a beaver-like rodent species native to Central America that was originally brought to the United States for its fur, has destroyed thousands of acres of tidal marshes by feeding upon the tender roots of marsh plants. The refuge loses approximately 500-1,000 acres every year as a result of nutria damage and several times that amount over the entire Blackwater/Fishing Bay estuary. As nutria chew up the marsh, they also displace native muskrats and shrink the wetlands needed by more than 250 species of birds, including migratory waterfowl. The marsh loss also affects the Chesapeake Bay. Fewer marsh plants are available to filter out pollution and sediments, and the bay's commercially valuable crabs, finfish, and other creatures receive a thinner diet of plant detritus, and it also threatens one of the most important blue crab nursery areas in the Chesapeake Bay.

At the Don Edwards San Francisco Bay Refuge, one of the biggest invasive species problems is Spartina alterniflora, a common east coast marsh plant also known as giant cordgrass. It competes with and hybridizes with native cordgrass vegetating the Bay's mudflats, and impacts shorebirds and the endangered California clapper rail. The refuge staff is also trying to halt invasion of yellow star thistle and nonnative grasses that impact two endangered plants and an endangered butterfly. On top of all that, nonnative red fox, which only arrived in the Bay in the mid-1980s, pose a major threat to the endangered California clapper rail and threatened western snowy plover.

At the Mississippi Sandhill Crane Refuge, which was established to protect the endangered Mississippi sandhill cranes, is overrun by invasive fire ants that kill the pips and eggs of these ground-nesting birds.

More than 30 invasive exotic plant species have taken root at the A.R.M. Loxahatchee Refuge in Florida. Although most of the invasive plants infest very little acreage, a few of the species, such as melaleuca, Old World climbing fern, Brazilian pepper, water lettuce and water hyacinth are major problems and impact significant acreage. A 1995 study estimated that 36,000 acres of the refuge were moderately to heavily infested and that melaleuca was spreading at a rate of 10 acres per day. Infestation by melaleuca affects threatened and endangered species such as the snail kite and wood stork by destroying nesting habitat and reducing forage for prey species. In addition to the resulting loss of diversity, dense stands of melaleuca impede water flow, leading to sediment accumulation. However, at the current level of funding, the refuge only has enough money to remove one acre of melaleuca each day.

Nearly every refuge suffers the harmful impacts of invasive species, and most refuges don't have the financial or human resources to address the problem. Even if refuge staff could control invasive plants and animals, these species do not yield to geopolitical boundaries. It is ultimately up to local citizens to educate their communities about the insidious nature of this threat and work both with refuge professionals and adjacent landowners to ensure that remedies are consistently implemented. Given proper information, skills and guidance, refuge Friends groups will play an instrumental role in controlling the proliferation of invasive species on their local refuge lands, and in their communities.

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Purple Loosestrife

Where It Came From

Purple loosestrife is native to Great Britain, central and southern Europe, central Russia, Japan, Manchuria, China, southeast Asia and northern India.

The plant was first introduced to northeastern United States and Canada in the 1800s for its medicinal, decorative and horticultural values. It could have also been introduced through various seed sources such as: ship ballast, livestock feed and bedding. Soon after its entry, purple loosestrife was entrenched in the northeastern area and spreading.

Where and How It Spreads

Purple loosestrife is perhaps the most prevalent invasive species in the U.S., covering about 400,000 acres of federal land, including wetlands, marshes, pastures and riparian meadows.

Although the purple loosestrife is most common and abundant in the northeastern United States and Canada, the plant has spread across North America. According to the U.S. Fish and Wildlife Service, it is in every state except Florida.

The construction of roads and their ditches promoted the spread of Purple Loosestrife. These ditches supply long stretches of favorable habitat for the plant to thrive and are pathways for the plant to spread from exposed to unexposed wetland habitats.

Roadside mowing has also assisted the spread of seeds and stem parts mechanically. The plant is commonly planted in perennial gardens and used in wildflower seed mixes because of its mid-summer magenta blossoms. And, large tracts of unused farmland tend to supply ample moisture allowing purple loosestrife to proliferate to an extent that it has created near monocultures inhabiting vast areas of old pastures and meadows.

Purple loosestrife continues to be sold to gardeners, except in Minnesota, Wisconsin, and Illinois, where buying and selling it violates state regulations.

Ecological Impacts

Purple loosestrife invades various wetlands, such as: freshwater wet meadows, tidal and non-tidal marshes, river and stream banks, pond edges, reservoirs and ditches.

It crowds out at least 44 kinds of native grasses, sedges and other flowering plants that offer higher-quality nutrition for wildlife. The plant confines native wetland plant species including some federally endangered orchids and swamp rose mallow, and it reduces habitat for waterfowl. Some declining species directly affected by its invasion are the bog turtle, black tern and canvasback duck.

Economic Impacts

The continuously expanding purple loosestrife acreage requires an annual cost of control upwards of $45 million for habitat restoration and control methods. In addition to the ecological damage it causes, purple loosestrife also affects farmlands by clogging irrigation and drainage ditches and causing the degradation and loss of forage in lowland pastures.

Control and Removal

Mechanical, biological and chemical removal options exist. The size and location of the invasion determine the control methods. Typically, digging manages small invasions of a few plants, especially when they are only a few years old. Larger infestations require herbicidal and/or biological control agents.

Eliminating all the roots and underground stems of the plant by digging is mechanical removal, which is most effective with small, young invasions. After the initial digging, the area should be monitored over several seasons to ensure the plant's eradication. Drying and burning or composting in an enclosed area will dispose of the plants efficiently. Caution should be taken during every step of the process because small pieces of stem can root and reestablish the invasion. This also means, be careful of clothing worn and equipment used during the removal process.

Herbicides chemically control purple loosestrife in areas too large to manage by digging. They can be applied to individual plants so as not to harm desirable plants nearby. Specialized equipment and treatment by professionals might be needed for removal along streams or in marshy areas. Some effective herbicides are Glyphosate and Garlon (triclopyr).

Biological control is considered the most effective control method for large invasions and long-term treatment. Galerucella pusilla and Galerucella calmariensis are two bio-control insect species that have been the most successful in the treatment of purple loosestrife. The adult and larvae of these leaf-feeding beetles eat the purple loosestrife leaves and flowers and have greatly reduced invasions over several seasons.

Physical Description

Purple loosestrife is an erect, perennial herb that grows from 0.5 to 3 meters tall depending on habitat conditions. It has a square, wooded stem and opposite or whorled leaves that are mainly lance-shaped and stalkless. At the base of the plant the leaves are heart-shaped or rounded. The length of the leaves varies from 3 to 10 cm. Leaves at the base and inside of flower spikes tend to be smaller and attached alternately.

The upper section of the purple loosestrife is generally covered with short hairs. Several specie varieties have been distinguished on the basis of different hair distribution and thickness on the stem and leaves and different leaf shape.

In the summer the plants produce lush magenta-colored flowers. They are practically stalkless, 5 to 7 petal flowers. Mature plants can have 30 to 50 stems coming from a single rootstock.

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Spartina Alterniflora

Where It Came From

Spartina alterniflora, also known as giant cordgrass or smooth cordgrass, is native to the Gulf of Mexico and East Coast wetlands of the United States. The plant was intentionally introduced to countries such as Great Britain, France, The Netherlands, New Zealand and China because of its ability to control erosion.

The plant was unintentionally introduced to the Pacific Coast of the United States along Washington, Oregon and California, between the late 19th century and early 20th century. Seeds may have come in oyster barrels shipped from the East Coast or from packing material, which used the plant to protect cargo in ship holds. The original introduction continues to be disputed though.

Where and How It Spreads

There are an estimated 1,000 patches of Spartina alterniflora spreading from Puget Sound in Washington down to the San Francisco Bay area. These include places such as: Alameda Island, Hayward Marsh, San Francisco Bay National Wildlife Refuge and San Bruno Slough in California; Siuslaw River estuary in Oregon; Willapa Bay, Grays Harbor, the Copalis River Estuary, Padilla Bay and Whidbey and Camano islands in the Strait of Juan de Fuca in Washington.

Washington's largest population of Spartina alterniflora is in Willapa Bay, where it covers an estimated 15,000 to 25,000 acres. The Spartina alterniflora population is spreading at a rate of about 20 percent annually.

Ecological Impacts

Although Spartina alterniflora is valuable in its native range, when it is introduced into the mudflats and salt marshes of West Coast ecosystems it becomes invasive and damages the native habitat. These West Coast ecosystems do not have the same insects found in the plant's native range, which feed on the plant and control its spread.

Plant communities such as pickleweed, seaside arrow-grass, fleshy jaumea, eel grass and the alga, Fucus distichus are being overtaken.

Loss of mudflat habitat causes ecosystem changes that negatively impact marine species such as the juvenile chum salmon, Dungeness crab and English sole that rely on these habitats as ample food sources.

Commercial oyster production in Willapa Bay has been threatened because mudflat culture beds are being invaded. Also, at the Willapa National Wildlife Refuge, it has displaced approximately 16 to 20 percent of essential habitat for wintering and breeding migratory aquatic birds.

In San Francisco Bay, the Spartina is competing with and hybridizing with native cordgrass and vegetating the Bay's mudflats. It impacts shorebirds and the endangered California clapper rail. Infestations have not only displaced native flora, but have also caused sedimentation changes, invertebrate and algae population decreases on the Bay floor and the destruction of foraging sites used by marsh birds, shorebirds and other animals.

Economic Impacts

In its native habitat there are some economically beneficial uses for Spartina alterniflora because of its wildlife sustenance and bank restoration values. But, in its non-native habitat on the Pacific Coast there are several negative economic effects.

Increased sediment build-up caused by Spartina alterniflora may cause changes in water circulation patterns, reducing tidal flow and increasing flooding, which has serious economic implications.

The loss of mudflat habitat jeopardizes commercial oyster production, which is a $16 million industry in Willapa Bay. In Willapa Bay more than $500,000 per year is spent to control the plant's spread. Additionally, changes caused by the plant could impact eco-tourism because it reduces access to beaches, fishing waters, bird watching locations and wildlife photography sites.

Control and Removal

Mechanical and chemical control methods have been used and biological control methods are being developed for Spartina alterniflora. Typically, mechanical control is used for smaller infestations and chemical control is applied to larger populations, but there is no standard control method for infestations.

Mechanical control consists of hand pulling digging or covering methods. Using shovels and hand clippers, removing the entire root-mat is typically successful with 1- to 3-year-old plants because of their less-developed root system. Sometimes using a large shovel can successfully remove circular bunches up to 15 feet in diameter. All plant remains must be removed from the site to prevent re-sprouting.

Covering the plants with an opaque mat deprives them of sunlight and hastens decomposition, which can be successful for circular bunches up to 10 feet in diameter. Black plastic or geotextile mats are typically used to create this light tight atmosphere and must be left on for two consecutive years.

Mowing Spartina alterniflora can also control growth, restrict seed set, and ultimately kill it. The plant must be mowed regularly throughout the growing season from spring to fall.

Glyphosate is the only herbicide currently labeled for Spartina alterniflora treatment in Washington, although success varies. Varied results might occur because the leaves have high salt and sediment levels that prevent the chemical absorption. Therefore, some people have experimented with wiping the plants before spraying, which is harder work but is effective.

In Washington and California scientists have found that a leaf-hopper insect, Prokelisia marginata, feeds on the sap of Spartina alterniflora and is capable of controlling it. Once released, these insects take 5 to 10 years to increase their population enough to cause significant damage to Spartina alterniflora, while causing minimal damage to native plants. The leaf-hopper insect is seen as an inexpensive, long-term and non-toxic way to control the plant. Experimental releases have been and will continue to be conducted.

Physical Description

Spartina alterniflora is a rhizomatous, perennial grass that grows 0.3 to 3 meters tall and has a leaf blade width of 0.6 to 1.5 centimeters. The plant tends to grow in clumps on salt marshes and tidal mud flats. Its underground, soil-binding stems continually spread and sprout new plants. Its stems are hollow and hairless, but the leaves have hairy ligules. Spartina alterniflora's nondescript flowers bud in crowded, two-to-three-inch-long spikes.

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