Volunteers and Invasive Plants: Learning and Lending a Hand link

U.S. Fish & Wildlife Service
America's National Wildlife Refuge System

Volunteers and Invasive Plants: Learning and Lending a Hand

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The National Wildlife Refuge System

What’s The Problem With Invasive Plants?

The US Fish and Wildlife Service manages invasive plants because these plants have the potential to degrade habitats and in turn, affect the wildlife that the Service is mandated to protect for the continuing benefit of the public.

Not all nonnative plants are invasive or are a problem. In fact, some nonnative plants may be useful, such as grasses as forage for domestic animal grazing or as food crops. Depending on the land management goals of a refuge, managers may work towards creating desired plant communities which may include both native and nonnative plants.

Many of the invasive plants that are found on refuges are nonnative species that were either purposely or inadvertently brought into the United States by people. The reasons that some of these nonnative plants become such proficient invaders are complex even to scientists. In general, nonnative plants do well in the habitats that are similar to the habitats of their origin, and have characteristics that enable them to out-compete the native plants. For example, a nonnative plant may flower more frequently and produce more seeds. This characteristic may enable the nonnative plant to gain a stronger foothold over the native plants.

Read about the policy that authorizes the management of invasive plants on refuges:

Biological Integrity, Diversity, and Environmental Health Policy

Impacts of Invasive Plants

Decrease Biodiversity

Photo of Kudzu leaf and flower.
Kudzu leaf and flower
Photo of Kudzu cloaking vegetation.
Kudzu cloaking vegetation

View origin

Invasive plants can dominate native plant communities by forming monocultures that use the resources (e.g. nutrients, light, water) native plants need to grow. Kudzu (Pueraria montana) is a vine that grows throughout most of the southeastern United States and in some northern states.

Kudzu grows rapidly and can smother native plants, kill trees by girdling, and uproot trees by the weight of its vines (1). When kudzu grows very few other plants are able to survive, substantially decreasing native biodiversity.


Photo of dried seed heads of cheatgrass.
Dried seed heads of cheatgrass
Photo of expanse of cheatgrass with dried seed heads.
Expanse of cheetgrass with dried seed heads

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European cheatgrass (Bromus tectorum) has spread throughout the sagebrush rangelands of the Great Basin in Idaho and Utah, predisposing the invaded habitats to fire. Some research shows that before cheatgrass invasion fires occurred once every 60 to 110 years, but now fires are more frequent, burning every 3 to 5 years (2).


Smooth Cordgrass
Photo of flower stalks of California cordgrass (left) and hybrid (right).
Flower stalks of California cordgrass (left) and hybrid (right)
Photo of hybrid cordgrass clogging a creek.
Hybrid cordgrass clogging a creek

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Smooth cordgrass (Spartina alterniflora) was transported from the East Coast to West Coast, where it hybridizes with native California cordgrass (Spartina foliosa) (3).

Spreading populations of hybrid plants could result in loss of genetic material of native California cordgrass and lead to local extinction (4). Additionally, the invading hybrid and nonnative species create cordgrass “meadows” in naturally open mudflats. This modification of estuary ecosystems has far-reaching impacts on native species and on people (5).


Canada Thistle
Photo of numerous small flower heads of Canada thistle.
Numerous small flower heads of Canada thistle
Photo of infestation of Canada thistle with purple and white flower heads.
Infestation of Canada thistle with purple and white flower heads

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At Lacreek National Wildlife Refuge in South Dakota, Canada thistle (Cirsium arvense) has expanded vastly in the last 10 years, crowding out native grass and forbs on the refuge. Neighboring cattle that normally eat the grasses won’t eat Canada thistle (6).


Purple Loosestrife
Photo of purple loosestrife flowers on long vertical spikes.
Purple loosestrife flowers on long vertical spikes
Photo of purple loosestrife infestation in forest meadow.
Purple loosestrife infestation in forest meadow

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About 42% of federally threatened and endangered species are at risk primarily because of invasive species (7).

On the Upper Mississippi River National Wildlife and Fish Refuge purple loosestrife (Lythrum salicaria) crowds out native plants. Purple loosestrife may be the most ubiquitous invasive species in the United States; it has invaded about 400,000 acres of federal land, including wetlands, marshes, pastures and riparian meadows (8).

Purple loosestrife confines native wetland plants, including swamp rose mallow and some federally endangered orchids, to small areas. It also reduces habitat for waterfowl, which impacts declining species such as the bog turtle, black tern and canvasback duck (8).


Eurasian Watermilfoil
Photo of Eurasian watermilfoil stems with leaves.
Eurasian watermilfoil stems with leaves
Photo of expansive mat of Eurasian watermilfoil on lake.
Expansive mat of Eurasian watermilfoil on lake

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Recreational activities such as swimming, boating and fishing can be impacted by invasive aquatic plants such as Eurasian watermilfoil (Myriophyllum spicatum). As Eurasian watermilfoil grows up from the bottom of lakes and rivers, it creates expansive mats on the water’s surface with dense tangles of stems below. As a result, recreationists are unable to use these areas and the health of the aquatic ecosystem declines (9).

Invasive plant infestations can also reduce the economic benefits of recreation-based activities to communities (10).


Photo of USFWS service staff treating the invasive melaleuca tree (Melaleuca quinquinerva) with herbicide.
Treating the invasive melaleuca (Melaleuca quinquinerva) tree with herbicide
Photo of dense melaleuca forest.
Dense melaleuca forest

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Environmental damage caused by invasive species such as melaleuca, combined with the cost of controlling them, adds up to almost $120 billion per year (2).


  1. Swearingen J, Reshetiloff K, Slattery B, Zwicker S. 2002. Plant Invaders of Mid-Atlantic Natural Areas. <http://www.invasive.org/eastern/midatlantic>. Accessed 2006 Aug 21.
  2. Pimentel D, Lach L, Zuniga R, Morrison D. 2002. Environmental and economic costs associated with non-indigenous species in the United States. Bioscience 50:53-65.
  3. Western Aquatic Plant Management Society. <http://www.wapms.org/>. Accessed 2006 Aug 21.
  4. Ayres DR, Garcia-Rossi D, Davis HG, Strong DR. 1999 July. Extent and degree of hybridization between exotic (Spartina alterniflora) and native (S. foliosa) cordgrass (Poaceae) in California, USA determined by random amplified polymorphic DNA (RAPDs) [abstract]. In Molecular Ecology 8:1179-1186. <http://www.ingentaconnect.com/content/bsc/mecol>. Accessed 2006 Aug 22.
  5. San Francisco Estuary Invasive Spartina Project. <http://www.spartina.org/invasion.htm>. Accessed 2006 Aug 22.
  6. National Wildlife Refuge Association. 2002. Silent Invasion: A Call to Action from the National Wildlife Refuge Association. <http://www.refugenet.org/new-pdf-files/Silent%20Invasion%20pdf.pdf> Accessed 2006 Aug 21.
  7. Pimentel D, Lach L, Zuniga R, Morrison D. 2005. Update on the environmental and economic costs associated alien-invasive species in the United States. Ecological Economics 52:273-288.
  8. National Wildlife Refuge Association, Refugenet. <http://www.refugenet.org/New-issues/invasives.html#TOC02>. Accessed 2006 Aug 16.
  9. Minnesota Sea Grant. <http://www.seagrant.umn.edu/exotics/eurasian.html>. Accessed 2006 Sept 1.
  10. University of Nevada Cooperative Extension. <http://www.unce.unr.edu/>. Accessed 2006 Sept 1.