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Invasive species and biodiversity
Invasive species are typically non-native plants or animals that are highly competitive over native species. They are 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."
Noxious weeds
Photo by Roxie Dufour
In King County alone, there are over 100 noxious weeds that have been identified by the King County Noxious Weed Control Board. 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).
Many of these species are so widespread that control and eradication is virtually impossible. Examples include:
- Yellow-flag iris
- Reed canarygrass
- Scot's broom
- Himalayan and evergreen blackberry
- English ivy
- 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. Additionally, species like English ivy can cover mature trees and kill them.
Removing non-native invasive plants and replacing them with native species can be a years-long process. Avoid planting non-native plants, volunteer with local groups to remove invasives, and report sightings of regulated weeds to the King County Noxious Weed Control Program.
Your pets
Goldfish
Goldfish are a "pest"? Yes, often when people need to get rid of their private fish tanks, they dump the tank and its contents into our local ponds and lakes instead of giving the fish away, returning them to the local pet store, or euthanasia and flushing. This is a very bad idea. Goldfish released into our freshwater bodies can do great damage to native salmon.
Goldfish aren’t the only culprits. Any pet has the potential to harm the ecosystem and native species. Another example is the African clawed frog. This invasive species has been found in several water bodies throughout Washington. African clawed frogs compete with native species for food and can also introduce pathogens.
Other aquatic species can hitchhike on equipment we purchase for our pets. One example from the recent past involves zebra mussels and Marimo moss balls (Aegagropila linnaei) used in aquaria. In 2024 in Renton, Zebra mussels were detected on Marimo moss balls that were sourced from overseas and were being sold in the United States. Thanks to early detection and a rapid response, the situation was resolved. This example highlights the importance of awareness and education regarding invasive species and their different pathways of introduction.
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.
Freshwater invasive animals
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
This 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, or if people dump their infested aquariums into our lakes. It is critically important to clean your boat and gear before moving from one water body to another, and to not dispose of your aquarium contents into our water bodies.
More resources
Washington Department of Fish and Wildlife's Aquatic Invasive Species page has helpful information, such as description and range, on many aquatic invasive species, including this handy fact sheet.
Marine invasive species
Over 40 non-native species of plants and animals have been found in Puget Sound, many of them in King County. There are several common methods for introduction of marine invasive species:
- Ballast water discharges
- Transport on ships' hulls
- Importation of aquaculture species
- Importation in live seafood shipments.
Sea squirts
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).
Sargassum muticum
The large brown alga Sargassum 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.
More resources
- Download the Marine Invasive Species Identification Guide from the Puget Sound Partnership.
- Get more information on invasive species relevant to Puget Sound at the Encyclopedia of Puget Sound.
Report invasive species
Noxious weeds
Find out how to identify a weed, and how to report regulated and non-regulated noxious weeds, on kingcounty.gov's noxious weed reporting page.
Outside of King County, contact your county's noxious weed control board.
Other invasive species
Report sightings of other invasive species to the Washington Invasive Species Council, using their web form or mobile app.
Aquatic invasive species emergency
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.