Why Invasive Plants Love Climate Change

The dandelion may be the ultimate invasive species, thanks to human beings. Bess Hamitii/Shutterstock

America is under attack by a group of bad actors with deceptively innocent names. Their roles could have been written for a bizarre science-fiction movie.

Carp. Bullfrogs. Beetles. Mosquitoes. Grasses. Diatoms. As vastly different as these swimming, croaking, creeping, biting, waving-in-the-slightest-breeze and microscopic single-cell organisms are, some types have one truly scary thing in common: They are invasive species. Even worse, climate change is fueling their spread around the United States and beyond in ways that can affect all of us.

Because an invasive species can be almost any kind of animal, plant or microbe, there's no hard and fast definition for just what an invasive species is, said A. Townsend Peterson, University Distinguished Professor in the Biodiversity Institute and Department of Ecology and Evolutionary Biology at the University of Kansas. Even so, Peterson said, there are characteristics that define when a species becomes invasive.

What exactly is an invasive species?

"First, an invasive species is a species that's not from that place originally," said Peterson. "Those are called alien species. Frequently, that means they came from another continent." But, he added, just being a non-native or alien species is not enough to make it invasive. A second requirement to be invasive, he said, "is that the species essentially has to become enough of a problem or affect the natural systems of the place so that you notice it." It’s worth adding, he said, that there are probably thousands of species that have come in from another continent but aren't causing any trouble. "The question is whether the species is affecting natural ecosystems or agro-ecosystems," he pointed out. "That could be anything that gets out of control. Then you call it an invasive species."

Climate change is affecting the spread of invasive species through their response to three factors, Peterson said. The first of those is the physiology of the species, which, in the case of a plant, for example, includes its requirements for temperature, humidity, sunlight and the number of days it needs to set flowers or fruit — anything that is a feature of its environment. These are called abiotic factors.

"Clearly, climate change moves these conditions around," Peterson said, citing two ways this happens. "The general wisdom is that with changing climate, especially warming climates, species will tend to move their distribution toward the North or South poles, away from the equator, and into higher elevations, into the mountains." Another way species respond to climate change is by shifting activity patterns to earlier in the season. "For example," he said, "plants have been dividing, flowering and fruiting earlier and earlier during the past four decades."

Ardisia elliptica's flowers and fruits growing in Hawaii
The Ardisia elliptica is native to southeast Asia but can be found in a number of other tropical locales. Its high rate of reproduction often muscles out native species. Forest & Kim Starr/Wikimedia Commons

The second factor is called biotic and occurs when there's an absence or presence of some pathogen, parasite or any other species that modifies where a species can be. This could occur, for example, if there's a long enough winter that the cold kills back some parasites or pathogens, Peterson said. "Whatever it might be, you can definitely imagine a particular species could be either absent or present and that modifies the presence of another species. Biotic factors can get really complicated and unpredictable," he emphasized, "because a species must respond to the species around it."

The third factor is access. "This is especially important to invasive species," Peterson said. To understand this factor, Peterson said you need to ask this question: What places are available or accessible to the species? Peterson answers the question by telling a story about dandelions. The story illustrates how scientists think about the distribution and the ecology of species.

"If we were having this conversation in the year 1300, we might talk about dandelion being an Asian species. Dandelions were distributed across Asia out to the limits of their abiotic requirements subject to the modifications of their biotic requirements. That means they could spread to the oceans but not beyond. But changing technology allowed humans to move around more, and oceans were no longer a barrier to dandelion distribution. And, so, dandelions were essentially transported by humans onto every continent except Antarctica, but were still subject to the abiotic and biotic requirements of the species."

The A-listers of the invasive species world

Cecropia peltata growing in Venezuela
The Cecropia peltata cuts a stunning profile against the sky, but it's also no. 20 on the IUCN's 100 worst invasive alien species. It favors disturbed areas — like gaps in canopies or roadsides — to the detriment of natives. Cristóbal Alvarado Minic/flickr

Given that there is a great diversity of invasive species and climate change is affecting their distribution and impact, is there a poster child for invasive species? Not really, said Peterson. "That would be totally dependent on the individual with whom you are speaking," he cautioned.

General interest groups, however, have compiled global as well as national lists of non-native species. On a global scale, the Invasive Species Specialist Group (ISSG) of the IUCN Species Survival Commission maintains a Global Invasive Species Database. This group even offers their version of 100 of the World's Worst Invasive Alien Species.

On a domestic level, the U.S. Geological Survey (USGS) serves as a sort of clearinghouse for information about where species, including non-native invasive species, have been observed or collected in the United States and its territories. USGS has compiled data about these observations and collections from numerous resources and made them available on a web-based federal mapping resource called Biodiversity Information Serving Our Nation (BISON). The site contains 375 million records, according to Gerald "Stinger" Guala, BISON director and the branch chief for Eco-Science Synthesis at the USGS in Reston, Virginia.

While much of the USGS's work is national in scope and has focused on some high-profile invasive species, including zebra mussels, Asian carp and cheatgrass, BISON has definite homeowner applications. "It's a tool where [homeowners] can zoom in to their county and get a species list to see what animals and plants occur near where they live," said Annie Simpson, data coordinator for the USGS Eco-Science Synthesis group.

Bromus tectorum, or cheatgrass, growing in Badlands National Park
Cheatgrass has many names. Regardless of what you call it, it's an invasive species in the United States. Badlands National Park/flickr

While the site is used more often by people searching for native species, one of the areas of interest in BISON is invasive species, Guala said. "One of the powerful things in BISON is that if you have an area that you are looking at — a natural area, your farm or, for that matter, your backyard — you can draw a polygon around it and actually get a species list for that area, which can give you an idea of invasive species you may want to be on the lookout for," Guala said. You can also expand that polygon to the surrounding area, which Guala pointed out will give you a heads up on imminent threats in a localized as well as broader search parameter.

USGS's work with invasive species is about to take a huge step forward. Simpson is compiling the first-ever database of all the non-native species in the United States. She and Guala hope to integrate that database with BISON by the end of 2018. "Many people for many reasons have created invasive species lists or watch lists or lists of species that are non-native for a specific region or for a specific group of species," Simpson said.

"My work has been to compile authoritative lists that make assertions about the native or non-native status of these species and accumulate them for different regions of the nation. The lower 48 is one, Hawaii is another and Alaska is a third. Compiling the lists turned out to be more complicated than we anticipated. The big issue we ran into was that species can have many names and, in some cases, those names have varied over time. We found out that one mussel, for example, has 200 names. Invasive species are no different from native species in this regard. Cheatgrass, for instance, has lots of common names, one of which is downy brome."

The Lifemapper project aims to map them

A map of the world crafted in green plants on a tree stump
In an increasingly connected and climate change-influenced world, we need to track invasive species. Lightspring/Shutterstock

Probably the next obvious step in figuring out where invasive species are heading is with a project like one the University of Kansas has developed that predicts the spread of invasive species using ecological niche computer modeling. Called Lifemapper, the project is an online tool that serves as an advance-warning database, said James Beach, assistant director for Informatics with the Biodiversity Institute at the University of Kansas. "We show people the likely change that is going to occur in species in different climate scenarios. Some species are migrating. Some are going extinct. Because of climate, everything is in motion. Regarding invasive species, regardless of the climate, we can predict which species could invade a new territory given a dispersal event."

Beach said to think of Lifemapper as a tool for asking "What if?" questions: "What if somebody brought a species into the country in their suitcase and it got out? Where could the species survive?"

Lifemapper can answer these kinds of questions, Beach said, because it compares and matches models of the organism's known distribution with models of climate in the United States into which the species could expand and survive. By identifying suitable habitats, scientists can see where that species could become an invasive species, Beach said.

It's this research side that Beach said is the most important part of Lifemapper. "That's what we get funding for, supporting researchers to do these distribution models," he said. "We have collaborations with biologists around the country who are using Lifemapper to examine biological hypotheses about the distribution of various organisms."

Beach also thinks Lifemapper could have an application outside of the scientific community. "I've always had a fantasy of hooking up with the real estate industry with Lifemapper's multi-species models," he said.

"We can predict which species would occur in a particular neighborhood, which ones could survive in your backyard. That would be cool on the real estate listings! You go to buy a house now and can see how many square feet it has ... how many bedrooms. Imagine if real estate agents could also list how many kinds of species could potentially occur on the property."

Or, in the case of invasive species, how many bad actors a prospective homeowner might possibly encounter.