When it comes to invasive species, we tend to hear most about the ones that are the most sensational or scariest to human beings, even though their ecological impact is pretty minor. We have all heard a lot of buzz from Blaine, Washington surrounding the giant Asian hornet (commonly referred to as a “murder hornet” in popular media), but its impact remains to be seen. Tigger the house cat is much more dangerous to native ecosystems than a lot of other invasive species, but she’s cute and cuddly so we tend to turn a blind eye. Let’s take a look at some common invasive species in our backyard and across the globe that researchers at UW Environment are studying.

European Green Crab

A hand with glove holding a crab
This European green crab was one of 96 found on Dungeness Spit in 2017.

A more invasive species with a far bigger ecological impact than the giant Asian hornet also calls Blaine home: the European green crab. Native to the Eastern Atlantic countries, green crabs are the most common crab seen in beaches from North Africa to northern Europe. More than 200 years ago, they arrived on the shores of the U.S. East Coast, likely accidentally brought on ships by sailors making the transatlantic journey or accidentally packed into seafood shipments. Once on U.S. soil, green crabs spread rapidly and widely, eventually making their way to the West Coast via San Francisco sometime in the 1980s. 

Their scientific name translates into “raving mad crabs”, and it’s not hard to understand why. Concern over green crabs stem from how they spread — in temperate waterways all over the world, but especially in Canadian and northwest U.S. waterways. They prey on shellfish in the northeast, causing economic and ecological damage to commercial shellfish beds in the area and have dramatic impacts on seagrass nurseries and decimate meadows of eelgrass and marsh ecosystems, which are important for migratory birds. 

Here’s why that matters for humans: green crabs like to eat the same shellfish we do, and when they get to high abundance, they have the potential to mess up important habitats for the fish and shellfish species we care about. By changing habitats, they have the potential to damage ecosystems and impact shellfish resources ecologically, economically and socially. The current fear is that they will reach high abundance and cause major problems to the local shellfish industry (which is huge in Washington state), get into marshes and eelgrass meadows and affect entire ecosystems up to salmon, Dungeness crab and other local exports, which has huge impacts on the economies and cultures of coastal communities.

The Washington Sea Grant green crab team works with partners like state agencies and tribal governments to monitor green crab populations and keep them at low abundance. The Green Crab team also helps initiate rapid response to eliminate as many crabs as possible and sustain eradication whenever possible. 

“Well, we eat Dungeness and king crab, why can’t we just eat green crab as well?” some of you might be thinking. In their native Europe, green crab can be found in seafood markets and in restaurant menus, but it’s far better ecologically here in Washington to try and keep the populations  to protect Dungeness crab and other natural resources. Also, these crabs are relatively small (a little larger than the size of a fist), so there’s not much meat to be found. 

“Green crabs tend to be a problem when they reach high abundance so anything we can do to limit their spread and keep populations down is going to be beneficial to our local ecosystem, economies and our local region”, said Green Crab Team member P. Sean McDonald. “Folks should be informed about what invasive species look like and what native ecosystems are like so they know what’s invasive and alert authorities so we can monitor and address quickly so they don’t become a problem.”

Brown Tree Snake

hand holding a brown tree snakeNative to Australia, Papua New Guinea and surrounding islands, the brown tree snake was accidentally brought to Guam in the late 1940s when the U.S. was moving freight and cargo in the Pacific corridor at the end of World War II. The brown tree snake population in Guam started with less than 10 snakes, and in about 40 years time the population erupted to about two million, with many snakes malnourished and extremely skinny as their prey became more scarce. Nowadays, an estimated 20-30 snakes per 10,000 square meters can be found on the island.

This invasive species was and is especially bad news for animals on the island, who became food for the brown tree snake. The snake has already caused 10 out of the 12 native forest bird species to go extinct (either locally in Guam or entirely as they were only found on Guam) and caused declines in native lizards, fruit bats and plants (as birds can no longer disperse seeds). Brown tree snakes also eat nonnative mice and rats, which helps brown tree snake populations persist. With no native predators in Guam, the brown tree snake remains largely unchecked.

Brown tree snakes are very hard for the human eye to detect so the few eradication attempts have not been successful. Trapping snakes, hand capturing snakes in the middle of the night and using toxic-laced mice have knocked the population back, but eradication efforts require substantial time and effort. An important part of managing this invasive species is knowing how removal efforts are working and how many snakes remain in target areas. Yet, due to the difficulty in finding the snakes, it takes a lot of effort for people to catch, mark and recapture snakes to get a good idea of the population size. Detection dogs have been used to sniff out snakes, but even then it is difficult. In the field, there is a 0.07 probability of even seeing a snake; in other words, if there are 100 snakes in an area, then you would only have a chance of finding 7 snakes on average given a night of searching with multiple people.

Aside from the ecological impacts, brown tree snakes also pose a mild danger to humans. Their bites are mildly venomous and, for a healthy adult, little symptoms exist outside of stinging and puffiness. However, infants and children have been hospitalized due to a snake bite. Aside from the physical component of a snake bite, the removal of so many native species is sobering for the native peoples who have a connection to these animals.

The brown tree snakes also pose a risk to Guam’s economy. It is very important to check imported and exported cargo for snakes, but that is a very expensive process for a territory that is a major transportation hub in the Pacific. 

“Brown tree snakes underscore the importance of biosecurity and checking cargo to make sure nonnative species are not transferred to other areas,” says School of Aquatic and Fishery Sciences postdoctoral researcher Staci Amburgey. “I’m hopeful that with management and eradication of the brown tree snakes, we can restore birds that are missing but still alive in neighboring islands and reintroduce them back into Guam.”


In the Caribbean, traveling down past the colorful fish and people scuba diving 150 feet below sea level, lies the “twilight zone”. This area gets very little light and since it’s only accessible by submarines, largely untouched and unstudied. School of Aquatic and Fishery Sciences’ Luke Tornabene and his collaborators are mostly interested in the coral reef fish in the twilight zone, and in studying the fish noticed one in particular was going as deep as the submarines were – the lionfish.

Native to the Indo Pacific from the central Pacific Ocean to the Red Sea, the lionfish was introduced to the Caribbean in the late 1980s through the aquarium trade, and spread along the western Atlantic seaboard once they were in Caribbean waters. Like the brown tree snake, the lionfish exploded in population from a lack of predators while feeding on a steady stream of local reef fish. 

Local dive masters and tourists have tried to keep the population in check by spearing lionfish in shallow waters, but researchers are guessing that this is only a small portion of the entire population.

Through video observation, Tornabene and his team know the lionfish feed on both fish in shallower waters during times of low light — dusk and dawn — and deep down in areas that consistently have no light. They also caught about a hundred lionfish at different depths from shallow to deep to look within their stomachs to see what they’re eating, and to see if lionfish are feeding on different fish at different depths. Through lionfish capturing, Tornabene will also be able to look at other biological clues for how these creatures live; for example, if the lionfish down deeper are spawning more, perhaps their sexual organs will be bigger than their counterparts in shallower waters.

“When thinking of a fishery, you usually try to harvest as many fish as possible without harming the population,” says Tornabene. “The goal of a lionfish fishery is to crash the fishery and harvest as many as possible to eradicate them…this is the only fishery that is trying to wipe out the catch completely.”

So, if you ever find yourself sitting at a restaurant in the Caribbean and order the lionfish, you will know the journey of that fish from the depths of the sea, speared by hand or by submarine and finally brought up to be prepared for your dish, and you will be helping to keep their population in check.

Azalea Lace Bug

Our next invasive species takes us back home, and for some of us, to our own backyards. Ever look at your Azalea or Rhododendron plants and notice that the leaves are discolored? Chances are, it’s because of the Azalea lace bug. While this bug doesn’t usually kill the plant, unless the plant is stressed from drought, it reduces the aesthetic value of plants that were specifically planted and cultivated for their beauty. 

Sometime around 1915, the Azalea lace bug was accidentally introduced from Japan to New Jersey by hitchhiking a way on infested  Azaleas that were brought in for planting. It soon became one of the most serious pests on Azaleas, and battles were waged against this insect all along the east coast, from New Jersey to the U.S. National Arboretum to Augusta National Golf Club. They made their way to Washington around 2008, presumably on an Azalea plant that was infested. The climate of western Washington is highly favorable to their success, and our landscapes have no shortage of suitable host plants. Indeed, our climate allows for the planting and thriving of over 700 different species of Rhododendron, including Azaleas, and including the Washington state flower, the Pacific Rhododendron.

In our area, research by School of Environmental and Forest SciencesPatrick Tobin and Ryan Garrison, a Plant Health Specialist for the University of Washington Botanic Gardens, has shown that 2-3 generations per year are likely, with the first nymphs hatching from overwintering eggs in mid-May. Both the adults and nymphs feed on Rhododendron much like mosquitoes feed on human blood. They pierce the underside of the leaves, and suck out the plant sap, depriving the plant of nutrients. Several infestations can result in leaf drop, reduced photosynthesis, and an overall reduction of plant vitality and their aesthetic value. But not all Rhododendron plants are affected the same way. Research by Tobin and Garrison has shown considerable variation in Rhododendron susceptibility to Azalea lace bug. For example, evergreen azaleas within the Rhododendron section Tsutsusi are especially susceptible to Azalea lace bug attack, while deciduous azaleas within the Rhododendron section Pentanthera are quite resistant to Azalea lace bug attack. Unfortunately, control of Azalea lace bug by natural enemies is not effective at present in protecting some of the more susceptible Rhododendron plants, meaning that the main control options at this point are systemic insecticides, or human tolerance to the damage they cause. 

Rusty Crayfish

Rusty crayfishRusty crayfish are considered a new invasive species to the western U.S., having popped up in the John Day River in Oregon from the Midwest around 20 years ago. Julian Olden, professor in the School of Aquatic and Fishery Sciences, has carefully tracked this species from its initial point of introduction to now many hundreds of river miles and marching downstream towards the Columbia River. When introduced to river systems without native crayfish, it is very easy to gain a foothold to rapidly spread and increase in density – up to 50 crayfish per square meter.

According to Olden, “rusty crayfish are a significant threat to freshwater ecosystems, acting as polytropic consumers which means they feed both above and below them in the food web.” Concerns continue to grow regarding the potential impacts of rusty crayfish on Pacific salmon and their prey.

Rusty crayfish are typically introduced to new places by fisherman who use them as bait, but on the West coast it appears that a relatively unappreciated pathway is to blame. Olden notes that “crayfish are widely used in the classroom in both middle and high schools, and teachers and students have been found to release them into the wild at the end of the year.” From there, rusty crayfish spread rapidly, making population control near impossible.

Eurasian Milfoil

Milfoil warning sign
Milfoil warning sign seen at Lake Union.

Over half a century ago, a plant called the Eurasian milfoil made its way to the U.S. through the aquarium trade and for garden ponds. It is from – you guessed it – Europe and Asia and parts of North Africa, and spread quickly throughout North and South America. This is an invasive species that grows extremely fast, spreads quickly through a variety of ways and has big ecological and economic impacts.

Milfoil clings to boats, fishing gear or boat propellers, traveling from one body to another and wreaking havoc on every single body of water they encounter. It is able to survive out of water for one or two days, often puddling up in the bottom of boats. It only really needs moisture to survive and spreads through fragmentation.

Once in a body of water, Eurasian milfoil grows thick, underwater forests and crowds out native plants by shading them out. In fact, Eurasian milfoil can grow so thick that juvenile fish can’t survive, and when decomposing it lowers oxygen levels in lakes. There are also human impacts. Milfoil can get so thick to prevent any humans from doing any sort of lake activity like fishing, canoeing or boating. Research by Olden found that resale values of houses on lakes with milfoil are significantly less than houses on lakes without. “We aren’t talking about pocket change here, houses sell for tens of thousands dollars less,” says Olden. Weak swimmers have also gotten tangled up in milfoil, and drowned as a result.

Milfoil can be found in hundreds of lakes around Washington, as well as in about three hundred miles of the Columbia River with slower moving water. In terms of milfoil present in lakes, the responsibility of removal is primarily on the homeowner with some assistance from lake associations and grants from the WA Department of Ecology. There are a couple of ways to control milfoil. Physical removal, where milfoil is cut and raked out of lakes, and biological means, where weevils are introduced to eat  the milfoil, are the two most popular options. Chemically treating bodies of water with herbicides is also an option, although rarely used. 

Although none of these names are talked about as sensationally on national media or as threatening sounding as a “murder hornet”, these species have huge impacts on native ecosystems and have the potential to completely wipe out entire native plant and animal populations. Either brought to new places purposefully or accidentally, invasive species have the unique ability to flourish and quickly expand and take control of the environment around them.