Skip to main content

Invasive Species in King County

Invasive Species in King County

Invasive species are typically non-native plants or animals that are highly competitive over native species, often difficult to control or eliminate, and in extreme cases may be quite destructive of native ecosystems or economically valuable plant and animal resources. Invasive plants that are highly destructive are termed "noxious weeds," and destructive invasive animals are "pests."

Washington Invasive Species Council points out, on their home page, that "Invasive species are . . . one of the leading threats to the world's diversity of plants, animals, and the places they live. For example, invasive species impact nearly half of the plants and animals listed as threatened or endangered under the U.S. Endangered Species Act."

Invasive Plants

In King County alone, there are over 100 noxious weeds that have been identified by the King County Noxious Weed Control Board. Many of these species are so widespread that control and eradication is virtually impossible. One such species is Eurasian water milfoil (Myriophyllum spicatum), an aquatic plant found in lakes, ponds, reservoirs, rivers, and streams. If you have been swimming along the shores of Lake Washington, you may have wrangled with this plant. Water milfoil dramatically alters the ecology of a water body because it rapidly reproduces and forms dense mats that choke out native plants and animals (and swimmers).

swimmer with milfoil
photo by Roxie Dufour

Many noxious weeds are so widespread in King County that eradicating them is considered impossible -- these weeds include yellow-flag iris, reed canarygrass, Scot's broom, Himalayan and evergreen blackberry, English and Irish ivy, and purple loosestrife. Many of these species create monocultures, meaning they completely carpet an area, and in so doing decrease native biodiversity. When an area is covered with one single species, there is no structural diversity and no food diversity, for example. Shockingly, most of those plants are legal and available to purchase at a nursery and plant in your yard. Here are some examples of plants that are invasive yet remain available for purchase in nurseries:

  • English ivy - can take over hillsides and forests, and kill mature trees. Best to eliminate it from your yard if you have it.
  • Fragrant water lily - can completely choke out ponds and wetlands. Bad for biodiversity (and not so good for kayakers either).
  • Butterfly bush - it is pretty and attracts butterflies, yes? Well, two big problems there: (1) butterflies won't lay their eggs on it because it offers no nutritional value for the new caterpillars, and (2) because it provides a nectar source for insects, they often go to it instead of native plants, and therefore do not end up pollinating our native plants.

Your Pet Goldfish

You can also add goldfish to the list above. Goldfish, a "pest"? Yes, goldfish. Often times when people need to get rid of their private fish tanks, instead of disposing of the creatures, by giving them away to friends or returning to your local pet store, or euthanasia and flushing, they dump the tank and its contents into our local ponds and lakes. This is a very bad idea. Gold fish released into our freshwater bodies can do great damage to native salmon. Please don't dump your home aquariums into ponds and lakes.

Freshwater Invasive Animals

bullfrog in wetland
Aquatic invasive animals are another big threat to our native biodiversity. Here are just a few. Each of these animals has the potential to devastate populations of native species or their habitat, or both.


  • New Zealand mudsnail - these snails are tiny animals, about the size of a pencil tip. Large populations may consume up to half of the available food in a stream and starve out stream bugs essential to trout and salmon. They now live in some in streams in the Lake Washington Watershed -- learn more so you don't accidentally transport them.
  • Nutria (external link) - a large aquatic rodent, is a relative newcomer to King County. If populations increase, we could see destruction to wild and agricultural lands. If you have nutria on your property and want to know what to do, look at WDFW's Living with Wildlife Nutria page.
  • Brook trout - now found in historically fishless alpine lakes, these fish will eat up all the insects and amphibians.
  • American bullfrog - if you catch one, feel free to eat it. That is why they were originally introduced -- for their frog legs. Unfortunately, they do a fine job of eating our native amphibians and even the Western pond turtle.
  • Zebra and quagga mussels - they are not currently in King County waters, but they could be if a contaminated boat is put in a lake or pond here. It is critically important to clean your boat and gear before moving from one water body to another.

Washington Department of Fish and Wildlife's Aquatic Invasive Species home page. Click on the fact sheet links on the left side of the page to learn about these many invasive animals.

Marine Invasive Species

The marine environment is host to numerous non-native species. Over 40 non-native species of plants and animals have been found in Puget Sound and many of those in King County. Non-native species may be introduced through ballast water discharges, transport on ships' hulls, importation of aquaculture species, and importation in live seafood shipments. Clean your boat to stop the spread of nuisance species!

The colonial sea squirt, Didemnum lahillei, is an invasive marine invertebrate from Japan recently found in the marine waters of King County. Sea squirts, or tunicates, are a type of invertebrate that can spread rapidly, and non-natives can crowd out or kill populations of local native marine species. They have no known predators and can quickly blanket the hull of boats, pilings, and other hard surfaces, and out-compete or suffocate other sea life including clams, mussels, and oysters. Two other non-native tunicates have been found in Puget Sound: the club tunicate (Styela clava) and the solitary sea squirt (Ciona savignyi). Additional information on these species and contact numbers to call when you find them can be found on the Invasive Species in Puget Sound website.

The large brown alga Saragassum muticum was accidentally introduced into Puget Sound through the importation of oysters from Asia. It is found in lower intertidal and shallow subtidal rocky/cobbly habitats, including a large portion of the shoreline in King County. Although Pacific herring are known to lay eggs on it and it provides a benefit, it out-competes native kelp and can grow in dense clumps that can smother and kill intertidal organisms during low tide events.

Terrestrial Invasive Species

  • European Starling - these birds often evict cavity nesters from their nest holes. So they are impacting our woodpeckers, as well as swallows and wood ducks.
  • House Sparrow - these aggressive birds also evict some of our native birds, including house finches and chickadees, from their nests.
  • Eastern gray squirrel and fox squirrel - they out-compete native squirrels in urban areas.
  • Domestic cats - the family cat, when let outside to roam, contributes to a significant loss of biodiversity among our native birds. Each year, across the U.S. it is estimated that 2.4 billion birds are killed by cats. It is most often our native birds that become prey to these non-native cats. Read more on our birds page.

Reporting Invasive Species

In King County, to report a noxious weed infestation, use this form. To report sightings of other invasive species, please contact the Washington Invasive Species Council by visiting the website. In the event of an Aquatic Invasive Species emergency, please immediately call 1-888-WDFW-AIS to schedule a boat decontamination or report a violation to law enforcement.

News and announcements

Guardian Environment Network, January 5, 2010
Invasive Species Threaten U.S. Biodiversity

Seattle Times, August 31, 2009
Mussel Invasion Closes in on Northwest Waters