Containing the Green Invasion

As gardening enthusiasts flock to nurseries and garden centers this spring, they’re likely to find a wide assortment of plants that would look beautiful in their gardens—but frankly don’t belong there.

That’s because around 70 percent of plants sold at traditional nurseries are introduced—meaning they’re not native to the region where they’re being sold. Furthermore, some subset of these non-native plants are invasive: likely to spread, become abundant, and damage local ecosystems, often with economic consequences. These harmful plants continue to be sold due to lack of knowledge around them as well as outdated, inconsistent, or absent regulation by states, according to Bethany Bradley, professor of biogeography and spatial ecology in UMass Amherst's Department of Environmental Conservation.

Until recently, invasive species were documented in a hodge podge of locally managed lists, with little information shared across regions. Yet forces like globalization and climate change mean local districts cannot afford to be ignorant of invasive species in other areas, which may soon be coming their way. In her Spatial Ecology Lab, Bradley, her students, and postdoctoral fellows, are gathering systematic information on invasive species, where they grow and are sold, and where they might be heading.

"We’re really interested in what we can learn about invasion risks in the future by studying the locations where invasive plants are currently present, considering factors like climate change,” Bradley said. Her lab studies the abundance of species across space and their interactions with the environment, including soil, climate, and other species.

Critically, Bradley and her team are working to get relevant scientific information directly into the hands of natural resource managers, as well as plant growers and sellers, to reduce the spread of invasive plant species and mitigate their damage to ecosystems.

Below, Bradley, left, and postdoctoral fellow Emily Fusco PhD '19 tabling at the 2019 North America Invasive Species Management Association meeting.

Invasives' Threat to Ecosystems

The damage invasive species causes to ecosystems is visible to the naked eye. In the Northeast, for example, bittersweet vines can be seen strangling trees, while knotweed grows through pavement and causes excessive soil erosion with its thick, shallow root system.

Moreover, Bradley said, “We know that as invasive species spread, the abundance and diversity of native species decline. Introduced plants are generally bad food sources for native herbivores like insects, and the effects move up the food chain to birds and other animals. Supporting the bottom of the food chain is really important ecologically.”  

Climate change is compounding the problem of invasive species in several ways, according to Bradley. A warming climate causes range-shifting of plants, meaning plants currently invading the South are moving north. Longer growing seasons give hearty invasive plants more time to overtake native plants. Increased CO2 in the environment also acts as a kind of fertilizer, accelerating plant growth. And increasing intensity of weather disturbances—such as storms, droughts, and heatwaves—causes damage to native ecosystems, opening the door for fast-growing invasives to gain a foothold.

Spreading Knowledge to Curtail the Spread of Invasive Species

In 2022, Bradley and her research team published in Ecological Applications the first systematic accounting of invasive species around the globe. They reviewed about 10,000 scientific papers and created a database of around 3,000 plant species that have been identified as invasive somewhere in the world.

“If you know a species has invasive tendencies in one location, that’s a clue that would elevate risk in other places,” said Bradley. “By making this information available, we hope to enable informed decisions about introducing new plants to a region.”

The researchers’ statistical modeling suggests that the 3,000 invasive species they identified is an undercount; the total should be closer to 4,700.

Bradley’s team also has studied state regulations on invasive species. “While most states have a list of declared invasive plants, we found most of them were quite outdated or heavily influenced by the nursery plant industry, which doesn’t want to stop selling profitable plants – even if they’re known to be problematic,” she said. Some states, such as New Jersey and Rhode Island, have no regulations at all around selling invasive plants.

The lab is also building a database of historical plant records based on nursery catalogs dating back to the early 20th century. “We hope to use this to identify heirloom varieties of native plants that have been successful in the Northeast in the past, and might be good candidates for planting today,” she said.

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Members of the lab are conducting other lines of research to expand knowledge about invasive species. For example, Will Pfadenhauer, a PhD student in organismic and evolutionary biology, is interrogating the “Tens Rule” in invasion biology, which holds that 10 percent of introduced species become established and, of those, 10 percent become invasive. That is, there is a one in 100 chance of an introduced species becoming invasive – a seemingly low risk. But this rule, which is based on research using a small data set in Great Britain in the 1980s and 1990s, has long been suspect. In fact, Pfadenhauer’s research finds that the chance of an introduced species becoming invasive may actually be 20 percent—twice as high as previously thought.

“This has really significant implications for our understanding of invasion risk. It could mean a difference of potentially billions of dollars previously unaccounted for, and could affect the work of land managers,” he said. 

Putting Science into Action

To help put all this science into action, Bradley is leading the Northeast Regional Invasive Species & Climate Change (RISCC) Management Network, part of the Northeast Climate Adaptation Science Center (NE CASC) hosted by UMass. Founded in 2016, the RISCC network aims to reduce the compounding effects of invasive species and climate change by synthesizing relevant science and building strong relationships between scientists and natural resource managers working at state and federal agencies and NGOs.  

“There’s a well-known barrier for science getting into management. We’re working to do more of the science that natural resource managers need and deliver it in a format that is useful,” said Bradley.

A leadership team of about 25 faculty, staff, and students at UMass and other affiliated institutions meets weekly.  By working across regions of the country, they help managers proactively prevent the spread of invasive species that may be a threat in the future as the planet warms.

Learn more about the RISCC Network in this video on NE CASC:

Recently, Bradley’s lab has launched a new initiative to engage directly with growers and sellers of plants to help fill gaps in state regulations. To start, they are focusing on native plant nurseries. “Our hope is to help them share their knowledge about native plants with traditional plant nurseries, which sell primarily introduced species, in order to support native plant sales,” she said. “Native plants rarely cause problems for ecosystems, and we’ve seen growing interest among gardeners in planting native plants and creating pollinator gardens, so it’s a win-win for everyone. ”

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The lab also has created a “do not sell” list for nurseries, as well as a pamphlet for gardeners of climate-smart native plants for the Northeast. In a survey the lab conducted, most businesses said they’d be willing to change their practices if they had more information about invasive plants they might be selling.

Bradley is also conducting complementary research on restoring ecosystems. Her lab is building models to predict how plant communities might shift with climate change, which can help restoration practitioners choose sets of species most likely to coexist now and in the future.  

“You can spend a lot of time treating an invasive plant infestation, but unless you restore the ecosystem, you’ll be right back where you started,” she said. “With climate change, there are a lot of questions about what restoration should look like and how to source plants and seeds. We hope to answer these questions by combining our science with our partnerships with natural resource managers and native plant nurseries.”

This story was originally published in April 2023.