Invasive Species

America’s Silent Killer: How Invasive Species Threaten America’s Unique Wildlife Heritage

A Nationwide Assault

Jetbead - Invasive species removal | FWS

Foreign plants, animals and pathogens make their way into the United States every day. Some are imported intentionally as crops or ornamental plants, or pet animals. Others arrive accidentally as riders on produce, nursery stock, timber or in ballast water. Freed from their natural predators or other limits on their populations, some alien species spread rapidly. These invasive plants and animals can harm our native wildlife directly through predation or indirectly through degradation and destruction of crucial habitat.

Experts believe that invasive plants already exist in all 50 states on more than 100 million acres of land and water – an area roughly the size of California – and that they continue to spread at a rate of about 14 million acres per year. At least 4,500 species of foreign plants and animals have established free-living populations throughout the country, and at least 15 percent of these are known to be harmful. The worst invasive species caused documented losses of $97 billion from 1906 to 1991.

The environmental havoc alien species wreak is staggering. 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. Alien invaders are considered the top threat to wildlife refuges across the United States.

Ecological Impacts

Invasive species such as purple loosestrife, melaleuca, zebra mussels and nutria can cause a range of impacts, from wiping out native species and degrading ecosystems to more subtle ecological shifts. Invasive plants harm the environment by damaging soil and water resources, ruining fish spawning habitat and crowding out native species.Read more...

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. 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 the impacts of invasive species. More than half of Hawaii’s plants and wildlife are nonindigenous. At least 86 introduced plants played a significant role in past extinctions of native species and continue to threaten ones that are now imperiled. In addition, nonnative species alter the islands’ natural fire cycles.

In Florida, several nonindigenous 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 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 alien invaders change our landscapes and imperil native species, they also impact our economy and cost taxpayers billions of dollars every year. One study of a worst-case scenario for just 15 potential high-impact species estimates $134 billion in economic losses over the next 50 years.Read more...

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 United States, 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,human health and 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. 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. Currently, invasive species control project needs total nearly $150 million. With a nearly $2 billion backlog in refuge funding needs, the rising cost of invasive species control threatens the future of the entire Refuge System, and alien invaders rank as the largest peril facing refuges.

Putting a Face on the Problem

At 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 on the tender roots of marsh plants. The refuge loses 500 to 1,000 acres every year as a result of nutria damage, and the entire estuary loses several times that amount. 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 creatures living in the Chesapeake Bay, including commercially valuable crabs and finfish.

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.

The Mississippi Sandhill Crane Refuge, which was established to protect the endangered Mississippi sandhill crane, is overrun by invasive fire ants that kill the chicks 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 invasive species on their local refuge lands, and in their communities.

More information:

What is NWRA doing:

NWRA helped build support for the Invasives and Volunteers Program by publishing Silent Invasion, a call to action to train and mobilize volunteers to assist refuges in invasive species removal.
Engaging volunteers in the fight against invasive species is an integral part of the National Wildlife Refuge System’s management approach. It helps expand citizen participation in refuge operations while supporting the early detection of newly invading alien species on refuge lands.

This ground-breaking initiative  has so far resulted in $4 million in federal funding to mobilize volunteers at refuges around the threat. To date, over 24,000 acres of refuge lands, in addition to hundreds of water bodies, have been inventoried and mapped through the mapping project by a corps of nearly 200 trained volunteers. Learn more.


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