Invasive Species Laws: Why Are Non-Native Species Dangerous to Ecosystems?

Conservationists, academics, and the green industry united in supporting the law, because they agreed the commercial sales of invasive species were causing widespread environmental damage. Research conducted by the University of Delaware and others has concluded that habitat loss is causing natural ecosystems in the bottom half of the state to collapse. Perhaps no voice was more important in supporting the law than that of the Delaware Nursery & Landscape Association. Besides controlling the worst-of-the-worst plant offenders, the group wanted to create a level playing field by putting all nurseries, including big-box stores, on the same legal footing.

An amendment indirectly addressed another nursery industry concern with far broader implications: whether sterile cultivars of banned species should be exempt. When the legal language was written, sterile cultivars hadn’t been on anyone’s radar. The nursery industry contends that sterile cultivars are either seedless (think of the seedless grapes you buy at the grocery store) or, if they have seeds, the seeds can’t sprout and therefore can’t be spread into the wild by birds, who are the main culprits in dispersing invasive plants.

The industry has established precedents for requesting exemption of sterile cultivars, particularly for Japanese barberry (Berberis thunbergii), which is native to China and Japan. Other states invasive species laws have also addressed the barberry. New York has conditionally exempted four sterile barberry cultivars from its state law banning invasive plants, and a request is pending in Pennsylvania to exempt four sterile cultivars from the state’s alien species ban.

Three of New York’s conditionally exempt barberries — ‘Crimson Cutie,’ ‘Lemon Cutie,’ and ‘Lemon Glow’ — are from a 10-plus-year breeding program directed by Mark Brand, a professor of horticulture and plant breeding at the University of Connecticut. “There’s no valid argument against using cultivars that’ve been documented to be non-invasive,” says Brand. He also downplays an often-cited criticism that barberries harbor ticks by arguing that thickets of native species also provide humid environments that support ticks.

Other academics are also researching and developing cultivars of invasive plants. North Carolina State University, for example, has put two non-invasive barberry cultivars, ‘Sunjoy Mini Maroon’ and ‘Sunjoy Todo,’ into commerce. Instead of saying they’re sterile, though, the university describes them as “low fertility.” “It’s impossible to prove a negative, absence of evidence, so we go with ‘low fertility’ or ‘high infertility,'” says Thomas Ranney, JC Raulston Distinguished Professor in the Department of Horticultural Science. Resolving the cultivar issue is important to the Delaware nursery industry, because there isn’t a native alternative to the color forms of the popular barberry species, making them difficult to replace in the landscape, says Joe Wick, third-generation owner of the wholesale Joseph Wick Nurseries in Dover, Delaware.

Also in development at the University of Connecticut are potentially sterile cultivars of another popular alien landscape plant, winged euonymus, or “burning bush” (Euonymus alatus). While the industry awaits these results, Wick believes taking burning bush out of commerce will have minimal impact because of what he calls great native alternatives. “In its place, we’ve been using Iteas and blueberries, which also turn bright-red in the fall.” However, he admits efforts to get the public to accept them as standard substitutes are “a work in progress.”

While barberry, burning bush, and other showy landscape shrubs have garnered attention from Delaware’s new law, a small creeping plant on the banned list “has been an especially tough pill for the garden centers to swallow,” says Valann Budischak, executive director of the Delaware Nursery & Landscape Association. That’s the yellow form of creeping Jenny (Lysimachia nummularia), popular for brightening up and spilling over the sides of pots. “I think this hit the nursery industry the most,” says Wick. “It will be very difficult to find an alternative.”

Why are non-native species dangerous to ecosystems? One of the ways invasive plants destroy ecosystems is by getting a head start on native plants. “In Delaware in March, when our spring season is beginning, you’ll notice the landscape beginning to green up, and you’ll think spring’s here!” says Jim White, chair of the Delaware Native Species Commission, tasked with hopefully stopping the decline of native species in the state. “But if you look closely, all those green plants are non-native. They’re multiflora rose (Rosa multiflora), autumn olive (Elaeagnus umbellata), Amur honeysuckle (Lonicera maackii), or Japanese honeysuckle (Lonicera japonica) — all alien plants. One of the ways they outcompete native plants, up here in the North, anyway, is they leaf out early and get going quickly. So, by the time the natives get going, these [invasive plants] are already rocking and rolling, leaving the natives behind.”

The ‘Bradford’ pear (Pyrus calleryana) could be a poster child for non-native invasive species laws, says Wick. “Flowering pear is growing from one end of this state to the other, along highway open spaces and in any places not mowed or taken care [of]. It has 2-inch thorns every 8 inches, spreads by rhizomes, and produces a repulsive amount of seed.” Other red-flag offenders, points out White, are porcelain vine (Ampelopsis glandulosa), which he calls “the kudzu of the North” for its ability to smother everything in its path, and Japanese pachysandra (Pachysandra terminalis), which spreads by rhizomes and “can take over the herbaceous layer of a woodland.”

White thinks one way to ease the pain of the environmental damage caused by invasive species is to see the new law as an opportunity for nurseries to replace non-native plants with natives, and for homeowners to use natives in their landscapes to provide food and nesting places for wildlife.

Invasive Plant List

Delaware banned the following non-natives with a new state invasive species law that took effect on July 1, 2022.

Amur honeysuckle (Lonicera maackii)

Autumn olive (Elaeagnus umbellata)

‘Bradford’ pear (Pyrus calleryana)

Chinese wisteria (Wisteria sinensis)

Common periwinkle (Vinca minor)

Creeping Jenny (Lysimachia nummularia)

Creeping water primrose (Ludwigia peploides subsp. glabrescens)

English ivy (Hedera helix)

European privet (Ligustrum vulgare)

European reed (Phragmites australis subsp. australis)

European sweet flag (Acorus calamus)

Fig buttercup (Ficaria verna)

Garlic mustard (Alliaria petiolata)

Hydrilla (Hydrilla verticillata)

Japanese barberry (Berberis thunbergii)

Japanese honeysuckle (Lonicera japonica)

Japanese knotweed (Fallopia japonica)

Japanese pachysandra (Pachysandra terminalis)

Japanese stiltgrass (Microstegium vimineum)

Marsh dewflower (Murdannia keisak)

Mile-a-minute vine (Persicaria perfoliata)

Morrow’s honeysuckle (Lonicera morrowii)

Multiflora rose (Rosa multiflora)

Norway maple (Acer platanoides)

Orange daylily (Hemerocallis fulva)

Oriental bittersweet (Celastrus orbiculatus)

Parrotfeather (Myriophyllum aquaticum)

Porcelain vine (Ampelopsis glandulosa)

Purple loosestrife (Lythrum salicaria)

Spotted knapweed (Centaurea stoebe subsp. micranthos)

Tatarian honeysuckle (Lonicera tatarica)

Tree of heaven (Ailanthus altissima)

Water hyacinth (Eichhornia crassipes)

Wineberry (Rubus phoenicolasius)

Winged euonymus (Euonymus alatus)

Sweet autumn virginsbower (Clematis terniflora)

Yellow flag (Iris pseudacorus)

Source: Delaware 151st General Assembly, Senate Bill No. 22

Why Are Non-Native Species Dangerous to Ecosystems?

While Doug Tallamy praises people learning how to stop invasive plants and keeping invasive species out of home landscapes, America’s most respected voice in using native plants wants to change the conversation. Instead of talking about whether a plant is invasive or sterile, he thinks the discussion should be about whether it’s a contributor or a non-contributor to local ecosystems. “Whether a plant is invasive isn’t really the issue,” says Tallamy, a professor in the Department of Entomology and Wildlife Ecology at the University of Delaware. “It’s that non-native invasive species aren’t contributing [to pollinators or ecosystems]. The problem is they’re protected with chemicals our native insects can’t eat. We’ve got 3,300 species of invasive plants in this country. About a third of the vegetation in natural areas is non-native because of these escaped plants.” As a result, he says, “They really mess with the food web.”

Choose Plants That Contribute to the Food Web

Plants capture energy from the sun and turn it into food that supports all animal life. Most vertebrates don’t eat plants directly, but they do eat invertebrates (typically insects) that eat plants. When the insects that can’t eat these non-native plants are gone, the food webs that support us collapse. Tallamy says that’s what is happening in Delaware, across the U.S., and globally. “There are 3 million fewer breeding birds in North America today than there were 50 years ago, and we’ve already lost 45% of our insects.”

Ecosystem collapse is happening, he says, because we’ve been loading our residential landscapes with non-contributors. “Trying to find ways to use sterile cultivars of invasive species is actually a negative step,” he says, “because we’re still promoting the use of plants that are non-contributors.”

Homeowners can make a difference because of the vast amount of land in residential landscapes by using native plants in home landscaping and using native plants to attract wildlife. “Something like 484 million acres, which is 280 times bigger than Yellowstone National Park, [are] in residential landscapes. Our decisions will determine the food web in these millions of acres.”

Tom Oder is an independent journalist living in Atlanta, Georgia. He writes about gardening, the environment, and agribusiness.