What is an Invasive Species?
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:
knapweed, Scotch broom, Himalayan blackberry (plants)
salmon, European green crab (fish, aquatic invertebrate)
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:
tolerate a variety of habitat conditions
grow and reproduce rapidly
compete aggressively for resources (like food, water, and nesting
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:
native wildlife habitat
forest health and productivity
Are Invasives Ever Good?
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
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
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.
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:
abundant, easily dispersed seeds that can withstand adverse conditions
via multiple pathways: roots, stems, and seeds
chemicals that inhibit the growth of or kill surrounding native
What You Can Do About Invasive Plants
which invasive plants threaten the ecosystems in your area
not collect invasive plants, their seeds, or reproductive bodies
invasive plants on your property
the growth of ornamental plants on your property
driving or recreating in areas where invasive plants grow
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.
Ecology and Management Team
Inventory and Analysis Program
National Invasive Species Information Center
Pacific Northwest Region, Invasive Plant Program
USDA Forest Service,
Invasive Species Program