For the average person, non-native, invasives species are often experienced as a nuisance: a stubborn patch of Japanese knotweed encroaching on a flower bed, or zebra mussels that slice your heel during a summer swim.
For the planet, however, invasive species represent a serious threat to biodiversity, economies, food and water security, and human health. A 2023 report estimated that invasives introduced to new ecosystems result in $423 million in losses to the global economy annually. That same report found that invasive species contributed to 60 percent of recorded global extinctions.
In a Q&A, two Yale invasive species experts explain the complex challenges posed by invasive species and solutions that may help control their spread. Marlyse Duguid is the Thomas J. Siccama senior lecturer in field ecology at Yale School of the Environment and director of research for Yale Forests. Sara Kuebbing is a lecturer and research scientist at Yale School of the Environment and Yale Center for Natural Carbon Capture and research director at the Yale Applied Science Synthesis Program. This interview was condensed and edited for clarity.
What are invasive species?
Marlyse Duguid: Invasive species are a subset of non-native species that didn’t co-evolve in a place but instead were brought there over a significant geographic barrier by humans. This includes a suite of disparate organisms from fungi and bacteria to giant mammals to plants. To be considered invasive, a non-native species also has to cause some sort of perceived negative impact, whether that’s economic, health, or ecological.
Sara Kuebbing: It’s important to note that there is disagreement on what specific non-native species meet the “invasive” definition. It’s totally hodgepodge how states and the federal government define invasive species. For example, the definition Marlyse just gave is what the U.S. federal government uses to define invasive species. Other people, myself included, might consider any non-native species that spreads far and wide beyond the original point of introduction to be an invasive, even if a researcher has not measured the possible economic, health, or ecological impacts of its spreading. You see this variation in state invasive species lists. There are some states that have six invasive plant lists made by six different organizations, with different species on each list. So just defining what we mean is really hard and creates confusion.
How are invasive species transported?
Duguid: Humans have been moving organisms for as long as humans have been moving—especially plants, but also animals. The modern invasive concept arrived with the modern global trade network. Some species were purposely introduced and some were brought accidentally. For instance, the emerald ash borer, a metallic green wood-boring beetle that is decimating native ash trees across the United States, was likely brought into eastern North America accidentally from Asia on packing materials such as wood pallets. A lot of invasive shrubs and trees were purposely introduced as ornamentals, or agencies thought they would be good for erosion control. For example, kudzu was promoted for planting to farmers in the south as a nitrogen fixer that could improve soil nutrients and prevent soil erosion. Other species were brought in for biocontrol, like the cane toads in Australia that were introduced to control crop-damaging cane beetles. People thought the toads would eat the insects and they didn’t, and now cane toads are a huge problem.
How easy is it for invasive species to become established?
Kuebbing: We actually have to work pretty hard to introduce many species, especially plants and animals. Early British colonists who missed their homeland tried to introduce bird species from the UK, most notably the house sparrow and European starling. They would release the birds and they would all die, and then they would do it again and again. So, it can actually take a lot of effort. Japanese knotweed is one of the rare examples of a species that took over easily. It was first brought from Asia and planted in a couple of ornamental gardens like the New York Botanical Garden and Arnold Arboretum in Boston, and then all of a sudden it started spreading down waterways and was able to colonize.
How do invasive species harm ecosystems?
Kuebbing: A lot of times invasive species kill other species. Dutch elm disease was a combination of an introduced beetle and an introduced pathogen that wiped out elm trees across the eastern United States, including here in New Haven. A more modern example would be the white nose fungus that is extirpating bat colonies across the eastern U.S. Another way invasives can cause ecological impacts is through direct competition with resident species in the ecosystem. Japanese knotweed, for instance, is so good at growing quickly—and using up space and resources—that nothing else can grow there. Invasive species also can alter the hydrology or fire regime in an ecosystem, which then will have downstream effects on other resident species who may not be tolerant to changes in water depth or fire intensity.
In what ways do invasives impact human health?
Duguid: Organisms like mosquitos and ticks can be vectors for disease. In Connecticut, we’re familiar with black-legged ticks, also known as deer ticks, and the suite of different diseases they carry. But several new ticks are invading the region as well and we don’t know what potential diseases they might carry. There are also indirect ways that invasive species affect human health. Cheatgrass is an invasive grass that is changing wildfire regimes in the American West, which then impacts air quality. Indirectly, you could have invasive plant species that change habitat structure and disrupt the food web, which might benefit animals like deer and mice that carry ticks. Water quality is another—a lot of our water is filtered by native vegetation and invasive plant species could outcompete those species. Finally, there is a lot of biocultural disruption from invasive species. An example we use in our class is the emerald ash borer which kills black ash, a culturally important tree species for many indigenous tribes in the Midwest and northeast regions. These ash trees are used to make baskets and, for some tribes, even play an important role in their creation stories.
What impact is climate change having on the spread of invasive species?
Kuebbing: Climate change will affect all species, including invasives. It’s promoting some and doing a disservice to others. There was an interesting study out of University of Massachusetts Amherst that looked at the potential distribution, based on climate change, of hundreds of non-native species around the United States. They saw a shift where the northeast was predicted to gain many more invasive species than it currently has as warming climates could support species that are currently only in the south. Their model also predicted that southeastern states were actually going to lose invasive species because it would get too hot for current invaders to persist. But these predictions will only come to bear if humans stop introducing new non-native species—which we are not doing.
How is climate change affecting native species?
Kuebbing: Climate change is putting stress on all species because they are now dealing with higher temperatures, shifting rainfall patterns, and extreme weather and storms—just like humans. You can see the stress of climate change—and invasion—by looking at the trees. In general trees are resilient to a year that is really hot or dry; as long as they have some good years, they can withstand the bad years. But native species are also dealing with the impacts of invasive species, and it’s the combination of climate stressors and invasion that can magnify the impacts. Imagine a forest in summer where extreme temperatures and low moisture are causing leaves to wilt and die, which is more and more frequent in the forests of southern New England. Those trees are stressed from heat and lack of water. Now imagine that same forest with invasive insects like the emerald ash borer killing ash trees, spongy moth caterpillars defoliating oak trees and other hardwoods, and beech leaf disease stunting beech tree leaf growth. The trees in that forest are stressed and more likely to die if the conditions persist.
How difficult is it to contain the spread of invasives once they’re established?
Kuebbing: It depends on how established a species is. A species like Japanese knotweed that is in 46 out of 50 states is a sizeable challenge because of how widespread it is. We would have to expend a lot of money and effort to reduce Japanese knotweed in the United States. Species that are detected early can be more easily and cheaply controlled. Zika virus—which is spread by the invasive Asian tiger mosquito—is a great example of how early detection can lead to eradication of an invasive species. When there was a Zika outbreak in Miami, the fear was that the virus would spread widely within the mosquito’s invasive range, which included a sizeable portion of the southern United States. But people decided that it was worth the effort and money to contain the spread of Zika and they eradicated all the mosquitos in the outbreak area and there was never a Zika infection found outside of Florida. Once a species is everywhere across the landscape, it’s very challenging to control and can require remedies people may not like, such as the use of chemicals or culling animal populations.
Duguid: If the goal is complete eradication of widespread non-native invasives, there are very few instances where that is able to happen. But we can get to a point where an invasive is at an acceptable level for some set of human values.
What methods for controlling invasives have proven successful?
Kuebbing: Any method that detects an invasive species early and rapidly acts to keep the species from increasing is a successful method. A good example where preventive measures have become widespread is in marine shipping. Ships are most stable when weighted down so they will fill their hulls with ballast water if cargo is too light. The ballast water, collected at a port, often contains many marine plants, animals, bacteria, and algae. Upon reaching their destination, ships release this water and all the organisms in it. To combat this, the ‘mid-ocean ballast water exchange’ method, which replaces coastal water with oceanic water, can reduce new invasive species introductions by 85%. Despite the added time and reduced efficiency, the method is codified in international shipping standards and used by many shipping companies. There also are good success stories for managing very large invasive species populations on islands. Islands tend to have a higher proportion of invasive species than mainlands and the impacts of invasive animals can sometimes be devastating for island species that suffer from higher predation or loss of habitat. A recent analysis found that over the past 100 years, humans have an 88% success rate of eradicating invasive invertebrates from 1,550 attempts on 998 islands.
Are there examples of solutions that weren’t as effective, or made a problem worse?
Duguid: Asiatic lady beetles are an interesting example. Federal and state agencies as well as private entomologists released them multiple times in multiple locations as a biocontrol. At first, they couldn’t get them established. Decades later, they are generally considered a pest because they overwinter in homes and tend to smell bad. It just took a long time for them to get the populations up. Often we don’t see a species as a problem until it’s too late.
The term ‘invasive species’ has itself become controversial in some quarters. Why is that?
Kuebbing: I believe that many of these controversies boil down to unclear terminology and loss of nuance when talking about invasive species. There are people who feel that the division of species as non-native and native is akin to xenophobia of humans of different cultures. A lot of the terms we use in invasion, like “indigenous” or “alien” or “native” are terms that have alternative meanings in other contexts. Because there are multiple linguistic and cultural issues tied up in these controversies, biologists need to figure out how we can talk about this topic is a way that is welcoming, clear, and sensitive. What I like to emphasize is that not all non-native species are “bad.” We have a lot of non-native species—like avocados, corn, and cows—that are great! But some non-native species do a lot of harm—and those non-natives species are what we consider to be invasive. Biologists have studied this and found that non-native species were 40 times more likely to be considered problematic than native species. And there are biological reasons for this: for instance, when we introduce non-native species we typically do not introduce the predators and pathogens that regulate that species’ population growth.
What can the average person do about invasive species?
Duguid: At the local level, people can start in their yards. There are several plant species that spread into natural areas and are having negative environmental impacts that are still widely planted as ornamentals. Get to know what is in your landscape and your garden and try to remove any invasive species or at least avoid planting more of them.
Kuebbing: There’s a lot of good information about what people can do to reduce the spread of invasive species. University extension services or state invasive plant councils typically have great resources on how to identify invasive species in your state and how to reduce the risk that you might move an invasive species to a new area. Gardeners can think about the plants they select for their gardens, and make sure they are not accidentally purchasing a species that is listed as invasive or likely to be invasive in their state. Pet owners should never release a pet into the wild but instead contact their local humane society or animal welfare organization to find an new home. Campers can buy firewood at their campsite which reduces the likelihood of carrying invasive insects living in firewood to new areas. Hikers and trail bikers can clean the soles of their shoes or the treads of their tires after they have been out in the woods to reduce the spread of invasive plant seeds lodged in the dirt to new areas. Boaters can clean, drain, and dry their boats every time they leave a waterbody to reduce the likelihood they will carry organisms to a new one.
What is Yale doing about invasive species?
According to Bill Carone, supervisor at Yale Landscape and Grounds, the university prioritizes early detection and rapid response on campus, employing mechanical and limited chemical methods for invasive plant removal. University grounds crew use cutting, pulling, and mowing to control species such as Japanese knotweed, oriental bittersweet, multiflora rose, mugwort, and black swallow-wort, always ensuring proper disposal of the plants. Yale also prioritizes native species across campus; favoring native seeds over invasive ones fosters a resilient ecosystem where native flora can thrive and eventually dominate.
Beyond campus, the Yale-affiliated nonprofit Urban Resources Initiative (URI) engages in mechanical removal of invasive plants in parks and open spaces within the City of New Haven, leveraging the dedication of hundreds of volunteers. Additionally, URI hosts educational programs to raise awareness among Yale interns and volunteers about the threats posed by invasive species. URI also has involvement in an oak tree regeneration study at the Yale Preserve, where URI is meticulously controlling invasive plants by hand to support the growth of oak trees.
Further afield, Yale Forests—a combined 10,592 acres of forestland in Connecticut, New Hampshire, and Vermont—is actively managed for invasive species control as part of its Forest Stewardship Council certification.