Invasive species are an expensive problem in the United States – federal agencies spent more than $104 million last year to control them. But a recent study on garlic mustard shows that it might be better to leave some invasive species alone.
Garlic mustard is a forest plant with heart-like leaves and clusters of white flowers. It can grow up to about four feet tall and is often the first green plant you’ll see in the spring. Europeans settlers brought it to the United States in the 1800s as an herb for cooking. It was also used to treat ulcers and gangrene.
Paul Vugteveen, a Michigan chef, uses the plant in his cooking. He says it has a garlicky, oniony flavor and is best served raw.
“You can just throw it in at the end of a sauté just to kind of brighten it up,” he says.
But this culinary herb has outstayed its welcome. Garlic mustard has spread all over the northeast and Great Lakes region — wiping out native species in its path.
The plant has a secret weapon that makes it so invasive — a chemical called sinigrin. When sinigrin leaches into the soil, it kills off a beneficial fungus that other plants rely on to get nutrients.
But now, scientists have spotted a weakness. After years of domination, garlic mustard starts giving up the fight.
Richard Lankau, who teaches plant pathology at the University of Wisconsin-Madison, co-authored a recent study on this in the journal Functional Ecology.
He says, "This weapon if you will, it’s not useful when you’re competing with other members of your own species."
Lankau’s team found that over the span of decades, large populations of garlic mustard stop producing as much sinigrin — which might allow native plants to move back in. Basically, it becomes less invasive.
If a big, bad invasive species like garlic mustard can stop being as big and bad, should we still try to control it?
Ken Thompson says maybe not.
Thompson, a retired ecologist from the University of Sheffield in England, wrote a book called “Where Do Camels Belong? The Story and Science of Invasive Species.” He says invasive species have a way of balancing themselves out.
“Over time what happens is the species evolves, the species with which it is competing evolve and this apparent superiority slowly begins to decline," says Thompson.
He says the invasive species we should be worried about the most is us. People have destroyed native habitats and brought in invasive species without thinking of the consequences.
“Very often the root cause of the invasion are the changes to the environment that we have made," he says. "I mean, how likely would it be—after all the changes we’ve made to the world—that the native species that used to live in all those very changed habitats are now the best species adapted to live there?”
Thompson says often methods used to control invasive species—like spraying herbicides—do more damage than the species itself.
Thompson says in a century, garlic mustard probably won’t be any more of a problem in the U.S. than it is in the U.K. But it’s what might happen in that hundred years that worries the authors of the study.
Lankau says when invasive species take over, there’s less diversity of the plants and animals that make up a healthy ecosystem.
“One sort of take-home message is that we shouldn’t necessarily apply the same strategies all over the country," he says. "There are places where the invasion is sort of hot and heavy where it’s probably worth investing a lot of resources in and there are places where it’s cooled off a little bit and maybe our resources are better used in other ways.”
Ryan Koziatek is a local invasive species manager and the stewardship field director for the Kalamazoo Nature Center. He says last year volunteers pulled 10,000 pounds of garlic mustard at the nature center alone—that’s the weight of two rhinos.
Koziatek isn’t ready to change his strategy for dealing with garlic mustard just yet—but it’s worth talking about.
"So at this point it’s kind of a nice to know and something to consider moving forward."