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Four Threats - Key Messages

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Invasive Species

Invasive species are spreading at alarming rates, adversely affecting people and the ecosystem. An “invasive” is defined as “a species that is nonnative (or alien) to the ecosystem” (htttp://www.invasivespecies.com) where they are found and whose “introduction are likely to cause economic or environmental harm or harms to human health” (Executive Order 13112, App. 1). Nonnatives are organisms that have been moved from their natural habitat to a new environment. However, many nonnative species do not pose danger to man, plants, and animals.

  • Invasive species can be introduced on purpose (such as kudzu vine in the 1930s to control erosion) or by accident (such as Asian longhorned beetle in the 1990s on packing crates from China).
  • With the globalization of commerce and foreign travel to and from the US, the number of new invasive species from abroad is growing.
  • In the US, there are 2,000 nonnative plants, about 400 of which are invasives. Invasive plants now cover about 133 million acres in all ownerships nationwide and marching at the rate of about 1.7 million acres per year.
  • Leafy spurge, knapweeds, starthistle, saltcedar, nonnative thistles, purple loosestrife, and cheatgrass are major invasives. Invaded grasslands lose their livestock carrying capacity and wildlife habitats are destroyed.
  • About 3.5 million acres of national forest land are infested. Leafy spurge, knapweeds and starthistles, saltcedar, nonnative thistles, purple loosestrife, and cheatgrass are the biggest problems. Livestock carrying capacity and wildlife habitat value both approach zero on sites infested by invasive weeds.
    • Seventy million acres of forest in all ownerships are at serious risk of mortality from 26 different insects and diseases, including nonnatives such as gypsy moth, hemlock woolly adelgid, dogwood anthracnose, beech bark disease, Asian longhorned beetle, white pine blister rust, sudden oak death, and Port Orford cedar root disease. Pine bark beetles are currently wreaking havoc on pine forests in the West and in the South.
  • Invasives pose unsustainable social, economic, and ecological impacts:
    • Socially, we are losing part of our national heritage. For example, invasives have led to disappearance of the American elm (Dutch elm disease), once a feature of the urban landscape, and two major forest trees—American chestnut (chestnut blight) and western white pine (white pine blister rust).
    • Economically, invasive weeds cost the U.S. about $13 billion per year. For all invasives combined, it comes to about $138 billion per year in total economic damages and associated control costs.
    • Ecologically, invasives threaten the survival of native species. Scientists estimate that invasives contribute to the decline of up to half of all endangered species. Invasives are the single greatest cause of loss of biodiversity in the US, second only to loss of habitat.
  • Invasive species spread across borders and boundaries. Prevention and control require extraordinary coordination on a landscape level.

Last Update: 8 July 2004

US Forest Service
Last modified March 28, 2013

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