When an occupying army takes hold of a village, that place is changed. How long the soldiers stay, how much damage they do, how thoroughly they integrate with a community—these factors can dictate how much changes and how fast. Even if the soldiers decamp and disappear entirely, the effects of the occupation linger. They may, in fact, be irrevocable.
Invasive plants—nonnative species that take over existing ecosystems—are akin to an occupying army. When dealing with plants, however, people rarely consider the long-term trauma invaders exact on ecosystems and their plant communities. “We just think—invasive plant, therefore manage,” says Daniel Tekiela, an assistant professor of invasive plant ecology at the University of Wyoming. “If the plant’s not there any more, we have been successful.”
But Tekiela is more interested in the aftermath, what happens after the invaders are gone. After cutting down, pulling up, and waging chemical warfare on an invasive plant, what becomes of the community that remains? In a paper published in Invasive Plant Science and Management, he and coauthor Jacob Barney, an associate professor at Virginia Tech, investigate what they call “invasion shadows,” the accumulated impacts of the establishment and removal of invasive plants. They are interested in how quickly invading plants change ecosystems, but also how long the changes they bring might linger, even after removal.
In the study, Tekiela and Barney tracked four types of plots over the course of three years. They began their study in a forest at Virginia’s Pandapas Pond, where Japanese stiltweed, one of the most prolific invasive plant species on the East Coast, had invaded. There, they created plots where they left the stiltweed untouched and others where they eradicated the invader by hand-pulling established plants and clearing seedlings every year after that. Nearby, in an uninvaded stretch of forest, they monitored stiltweed-free plots and seeded others with stiltweed to mimic an invasion.
One important question they asked is how quickly the seeded, newly invaded plots would become like the places where stiltweed had been well established. Tekiela and Barney expected to see a steady progression of impacts over the course of their study. What they observed surprised them. When they looked at the ecosystem-wide impact, the seeded plots came to resemble the long-invaded plots rather rapidly.
“It took very little to have a sudden shift in basically everything,” says Tekiela. “It shifted quickly and stayed there.”
The environmental stakes to changes like this can be high. Tekiela is from the Midwest, and before he was an invasive plant ecologist, he visited an East Coast park and took a picture of the pretty area he was hiking through—covered with an understory of bright, lime green vegetation. Years later he found the picture again, and realized that he had been photographing Japanese stiltgrass. If you’ve hiked a forest on the East Coast forest, you’ve probably seen oceans of this plant, and maybe found it enchanting.
But stiltgrass is a powerful shaper of its environment. It creates an understory, which changes which tree seedlings thrive and survive. Over time, these changes percolate upward, and stiltgrass shifts the composition of the upper canopy of the forest. The invasive might look pretty, but it dominates the forest, from ground to crowns.
That dramatic impact is good reason to try to control stiltgrass. But Tekiela and Barney’s study suggests that stiltgrass’ impact survives, even when the plant doesn’t. By the end of the three years of the study, the plant community in the plots where stiltgrass had been removed had diverged even further from the community present in the plots that had never been invaded at all (and from the ones were stiltgrass was allowed to remain). In other words, when the invaders left, the place changed even more than before.
The implication of the findings is that there’s not much time after a plant invasion begins to stop it from having an impact. And once the invasion has occurred, simply kicking out the invaders can inflict still more change on the plants that once called that stretch of land home.
More broadly, it also says that attempts to manage plant ecosystems often have no empirical basis. All the shades of a community and its invaders—who was where, who came and when, how long did they say, what cruelties did they inflict, who benefited, who lost out—can impact the life of a place permanently. In human terms, we understand this instinctively, that the invading force can alter the village. But with plants, it’s not quite so intuitive.
“I talk to people all the time whose job it is to manage plants, and they have this visceral hate of all nonnative plants,” says Tekiela. “We have such a short-term mindset when we think about controlling invasive plants that we don’t even think about at least paying attention to the state of that ecosystem afterwards.
“The goal is to have a better ecosystem, not to kill the invaders,” says Tekiela. “People often mistake those two messages.”