By Haley Depner
Take a look at the nearest garden in your vicinity. Do you know which plants have negative effects on your ecosystem? Most people would probably answer that they don’t.
Invasive species can cause a variety of problems for an ecosystem, ranging from the introduction and/or spread of parasites and disease to outcompeting native species for their resources. Any of these scenarios have the potential to be devastating for an ecosystem and everyone, including the humans, that depend on it.
When a species is introduced to an ecosystem in which there is no existing population of that species, a variety of outcomes are possible: the introduced species could die off relatively uneventfully. It could survive to establish a population and become integrated into the ecosystem and become depended upon by local species, as apple trees have in North America. It could also survive to establish a population that can’t be integrated into the ecosystem. In this case the introduced species is taking or altering resources depended upon by other species without anything to counterbalance its reproduction and spread, such as predators.
These resources may include food, water, space, light and adequate quality soil (in the case of flora), and (in the case of fauna) a place to raise their young. If a resource is taken over by a new species and/or altered by that species so that native species that rely upon it can no longer make use of the resource, then the flow of energy in the ecosystem can be significantly altered and can lead to entire populations dying off. The introduced species can also cause die-offs of native species via the introduction of parasites and diseases of which native species have no immunity to or by facilitating the growth and spread of already present parasites and diseases.
Which of these outcomes ends up being the result relies on a variety of factors including the suitability of the habitat for the species introduced as well as when and where it is introduced and the overall health of the specimens when they reach the new ecosystem. Loss of biodiversity is a slippery slope that, once started down, is a lot harder to backpedal. Losing one species can quickly lead to the loss of others that rely on it, which in turn affects still more species throughout the food web.
It is not only flora and fauna that are effected; changes in the ecosystem can affect an environment right down to its landforms and its soil quality; a different composition of communities leads to different compositions of decomposing matter, different ways that the soil is agitated (or not), and how the land is impacted by weather (e.g. how much rain the soil it is directly exposed to, susceptibility to erosion, how flood prone the area is, or how much sun exposure is allowed, etc.).
To answer the question I posed in the opening sentence of the article, I reached out to Nancy Olmstead, an invasive plant biologist from the Maine Natural Areas Program.
“We heard that you work with the Maine Natural Areas Program. What is this program and how are you involved?”
“The Maine Natural Areas Program is a program within the Maine Department of Agriculture, Conservation, and Forestry. The mission of the Natural Areas Program is to protect Biodiversity in Maine. We do that through a variety of programs. We do botanical inventories, rare plant survey, provide information for review of development projects and forestry projects. We also have this invasive plant initiative. My position as invasive plant biologist was created in 2014 to be a full time resource for terrestrial and wetland invasive plants. You may know that our sister department, the Maine Department of Environmental Protection, already has had, for many years, a robust program for aquatic invasive plants, but we haven’t had anyone in state government working exclusively on terrestrial and wetland invasive plants, so that’s my role. “
“At what point is a plant species considered invasive?”
“There are several sort of definitions of what is considered an invasive plant. We generally agree that an invasive species is one that is not native to the habitat, the area where it is exhibiting invasive tendencies, and it causes economic or environmental harm, or harm to human health or with the potential to cause economic or environmental harm, or harm to human health. So, that’s kind of a working definition. Different states and Canadian provinces have different definitions for the sake of regulation, but when I talk about it from a scientific perspective as to what makes a plant invasive, it has to be not native to that area, it has to cause some kind of harm to the natural environment.”
“What invasive species should landscapers and gardeners in Maine be particularly aware of and why?”
“Boy, that could be a long answer. There are many species that have traditionally been used in landscaping and in gardening which are now known to be invasive. I’ll start by listing several shrubs.
- Winged Dewnamis, or Burning Bush [(Euonymus alatus)]. That’s a shrub that’s been widely used in landscaping and it definitely invades forests, as well as forest edges and open habitats.
- Japanese Barberry [(Berberis thunbergii)]. Which is a thorny shrub that was promoted as a sort of “living fence” that is also a very invasive plant in forests and forest edges and also in open areas.
- Autumn Olive [(Elaeagnus umbellata)]. In some circles it is being promoted as sort of a permaculture plant for human consumption. The fruits can be consumed, and that is a plant which is invasive, mostly in sunnier habitats, open habitats, forest edges, and those kind of habitats.
- Rugosa Rose [(Rosa rugosa)], which is unfortunately invasive in coastal habitats. Which is a plant which has widely been used in landscaping, both in the coastal area and also inland. It’s of much less concern inland, because of the primary way that it spreads, by the rose hips getting taken by the tide and spread along the coast in that fashion.
- Norway Maple [(Acer platanoides)] is another plant which has become invasive. It is a tree, as opposed to a shrub, but it’s very invasive. It can invade intact forests and is a very prolific reproducer.
- I hope that most gardeners are aware, and wouldn’t be planting Asiatic bittersweet vines [(Celastrus orbiculatus)]. That’s a woody vine that will climb trees in forests, forest edges. But people do like to make wreaths out of their vines in the fall, because they have very distinctive orange and yellow fruits. So, that’s definitely one that landscapers and gardeners should be aware of.
- [Invasive bittersweet species can easily be confused with the locally native species, American bittersweet (Celastrus scandens). According to the Maine Natural Areas Program, “[the two can be distinguished by examining the locations of the clusters of flowers or fruits on the stems. American bittersweet flowers and fruits are always found in clusters at the ends of stems, while Asiatic bittersweet flowers are found in the joints where the leaves grow out of the stems.”]
- A couple of additional showy herbaceous plants that I think are still sometimes planted. There is a yellow iris [(Iris pseudacorus)] that has a very showy, approximately 2” flower, and unfortunately that one can escape from the garden and can invade wetlands, it can invade some salt marshes as well as freshwater marshes. So that’s a very dangerous plant, because our wetlands are so important for many ecosystem processes.
- Another plant I know is in some parts of Maine and is spreading is ornamental jewelweed, which is sometimes called Himalayan Balsam [(Impatiens glandulifera)]. It’s a tall, flowering plant, has a large pink flower. It’s in the jewelweed group. When you touch the mature seed pods, seeds are expelled violently. It spreads along waterways. I was just working on a site on Friday to remove some of this ornamental jewelweed along the Union river. And unfortunately, it can be really bad, forming dense monoculture along our waterways.
- The jewelweed with the orange-yellow flower is our native jewelweed, [(Impatiens capensis)]. Ornamental jewelweed is a different species, [(Impatiens glandulifera)], has a much larger flower, is more showy, it’s a taller plant overall.
Those are just a subset of species, I could go on, but those are the ones I’m most concerned about.”
“Has there been any economic repercussions as a result of these species invading Maine?”
“Well, sure, there definitely are economic consequences for individual land owners as well as for the state of Maine as a landowner. For example, the state has spent significant staff time and money treating invasive plants on public lands, including state parks, as well as our forested timberland. And for individual landowners, getting rid of invasive plant infestations can be a long term project, so definitely in terms of taking people’s time, which for many of us time is one of our most important resources, so there is a financial cost to that, as well as the direct cost of any tools or herbicide you might need to get rid of the plants.
More broadly speaking, there are studies that show the impacts of invasive species in general, not just invasive plants but invasive species, are in the billions of dollars world wide. The federal government, including in Maine, like at Acadia National Park and at our US Fish and Wildlife refuges, they spend lots of money every year to get rid of invasive plant because they are trying to manage those habitats for wildlife or for natural communities, and invasive plants negatively impact wildlife habitats, they negatively impact native biodiversity, so there definitely are financial consequences that we as taxpayers are helping to fund, but it’s a cost to our economy.
To be clear, foresters are seeing a definite impact. There are foresters in Southern Maine who now spend lot of time treating invasive plants before they can recommend to a landowner to go ahead and do a timber harvest.”
“Do you have any advice you think might be useful to gardeners and landscapers in Maine?”
“I really strongly suggest that they consider native plants when they are planning their gardens and landscapes. There are a variety of resources available suggesting native plants for the lawn and garden landscapes. If you look up the cooperative extension’s website, there are some great resources there, including a brochure called “Gardening to Conserve Maine’s Native Landscapes: plants to use and plants to avoid” [view it at https://extension.umaine.edu/publications/2500e/] That is available online from the University of Maine cooperative extension. Many of our local garden centers are knowledgeable about native plants and can provide advice.
So I would just encourage folks to consider using native plants because they are going to provide habitat for native pollinating insects and other beneficial insects, like parasitoids that attack pests. Native plants are also probably more resilient to some changes in the system, although, you know, invasive plants can be pretty resilient too. We are hoping to maintain some of the native biodiversity of our landscape, so planting native plants is a good step in that direction.”
“What does the future look like for ecosystems in Maine inhabited by these species?”
“That’s a good question. Because of the low overall impact of humans on the land, I’m hopeful there are places we can protect from invasive plants. There’s certainly many places in Southern Maine and along the coast where many invasive plants are already present, but I’m hopeful that we can protect some of the more inland and Northern areas of the state from serious damage by invasive plants. But it’s going to take a concerted effort on the part of folks who live and work in those regions to keep an eye out and monitor the spread, and go ahead and nip any new infestations in the bud before they spread and become well established. So that’s kind of a broad answer.
More generally, I’m concerned about our native system’s ability to cope with things like changing climate, but what I understand is that systems that are already perturbed and disturbed by invasive plants are likely to be less resilient to change in the climate. So reducing stress, reducing the burdens on our natural systems due to invasive plants is a good step towards protecting those environments and helping them be more resilient to climate change.”
For more information about ecologically conscious garden planning and which plants are good to plant for our ecosystems and which to avoid, visit
The Maine Natural Areas Program works in conjunction with the University of Maine Cooperative Extension compiled a collection of fact sheets on invasive species in Maine. For more information on what plant species are invasive in the state of Maine and how to control them, visit either organization’s website at