USDA Forest Service

Pacific Northwest Research Station

Pacific Northwest Research Station
1220 SW 3rd Ave.
Portland, OR 97204

(503) 808-2100

US Forest Service

Invasive Species

Invasive species collage.
What Is an Invasive Species?

A species is invasive when it is both nonnative to the ecosystem in which it is found and capable of causing environmental, economic, or human harm. Invasive species often compete so successfully in new ecosystems that they displace native species and disrupt important ecosystem processes. Plants, fish, insects, mammals, birds, and diseases all can be invasive.

The Pacific Northwest, like other regions of the country, is inhabited by a variety of invasive species. These species include:

Arrow. Diffuse knapweed, Scotch broom, Himalayan blackberry (plants)
Arrow. Atlantic salmon, European green crab (fish, aquatic invertebrate)
Arrow. Gypsy moth (insect)
Arrow. American bullfrog (amphibian)
Arrow. Nutria (mammal)
Arrow. European starling (bird)
Arrow. White pine blister rust (disease)

How Does a Species Become Invasive?

Often, invasive species owe their success in colonizing new ecosystems to one or more of the following characteristics:

Arrow. They tolerate a variety of habitat conditions
Arrow. They grow and reproduce rapidly
Arrow. They compete aggressively for resources (like food, water, and nesting sites)
Arrow. They lack natural enemies or pests in the new ecosystem


What Effect Can Invasives Have?

Invasive species can negatively impact ecosystems in a variety of ways. They can:

Arrow. Displace native species
Arrow. Reduce native wildlife habitat
Arrow. Reduce forest health and productivity
Arrow. Alter ecosystem processes
Arrow. Degrade recreation areas


Are Invasives Ever Good?

Scotch broom, shown here dominating this hillside, is a common invasive plant in the Pacific Northwest (Photo by Eric Coombs, Oregon Department of Agriculture, addition to their negative effects, some invasive species may have positive traits. These traits are often the reason a species that eventually becomes invasive was introduced to an ecosystem in the first place. In addition to growing rapidly and crowding out native plants in the region, the Himalayan blackberry, for example, produces edible berries that are relished by wildlife and people alike.

Similarly, though it now threatens to crowd out native plants and increase fire danger, scotch broom, with its bright yellow flowers, was originally planted for beautification and landscaping purposes.

Any positive effect an invasive species might have in an ecosystem can easily be outweighed by the damage it causes. According to the National Invasive Species Information Center, some estimates put the economic cost associated with invasive species damage and control efforts at more than $100 billion a year in the United States.


Will Nonnative Species Always Become Invasive?

Simply being nonnative in an ecosystem does not mean that a species will become invasive. It must possess certain characteristics, such as those listed above “How Does a Species Become Invasive?”, that ideally suit it for colonization in a particular area. It also is possible for a species to be invasive in one ecosystem, but non-invasive in another. This can be due to a variety of factors, such as the presence of a predator species or less-than-ideal habitat conditions.


Invasive Plants

Like invasive species in general, invasive plants possess characteristics that make them especially suited for colonizing new ecosystems. In addition to the characteristics listed above “How Does a Species Become Invasive?”, these plant-specific characteristics can include invasive plants’ ability to:

Arrow. Produce abundant, easily dispersed seeds that can withstand adverse conditions
Arrow. Reproduce via multiple pathways: roots, stems, and seeds
Arrow. Release chemicals that inhibit the growth of or kill surrounding native plants


What You Can Do About Invasive Plants

Controlling the growth of ornamental plants on your property, like English ivy, can help prevent infestations (Photo by Chris Evans, The University of Georgia,

Arrow. Learn which invasive plants threaten the ecosystems in your area
Arrow. Do not collect invasive plants, their seeds, or reproductive bodies
Arrow. Control invasive plants on your property
Arrow. Manage the growth of ornamental plants on your property
Arrow. Avoid driving or recreating in areas where invasive plants grow
Arrow. Report invasive plant infestations to your local land management agency


Invasive Species Research at PNW Research Station

Scientists with the Pacific Northwest Research Station are conducting studies to better understand invasive species in the region. Their findings will assist natural resource managers as they work to minimize the impact of invasives and limit their spread.

To learn more about invasive species research at the Station, browse the Web sites below.

Arrow. Disturbance and Restoration Ecology Team

Arrow. Pacific Northwest Region, Invasive Plant Program




National Invasive Species Information Center


Pacific Northwest Region, Invasive Plant Program


USDA Forest Service, Invasive Species Program




US Forest Service - Pacific Northwest Research Station
Last Modified: Wednesday,29April2015 at13:47:18CDT

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