UIC study: Invasive plant roots may weaken soil ecosystems

Invasive plants can dramatically change an ecosystem, outcompeting native species and harming animal life, agriculture and human health. Now, a new study led by University of Illinois Chicago researcher Gabriela Nunez-Mir finds that these negative effects reach underground, too.

The study published in Proceedings of the National Academy of Sciences found that invasive plant species have traits indicative of faster root growth than their native counterparts. And these qualities may have a dramatic effect on the microbes living in the surrounding soil.

Invasive species can impact microbes in the soil through their natural defenses. For example, the roots of one notorious invader, garlic mustard, produce toxic chemicals that kill fungus and deter other plants from growing in its vicinity.

When researchers examined microbial communities in soil samples from across the United States, they found that plots with invasive species were more alike than plots with only native species. Samples from as far away as Alaska and Hawaii to Washington, Illinois and Florida contained similar compositions of bacteria and other microbes, despite their far-flung locations.

"They have very similar microbial systems, even though their climate, geography and vegetation are very different," said Nunez-Mir, assistant professor of biological sciences at UIC. "This suggests that invasive plants are homogenizing the microbial communities, regardless of where they are."

Those changes could weaken the ability of ecosystems to protect themselves from disruptions such as disease and climate change and make it easier for additional invasions to occur, the authors warn.

The study looked at plant and soil data from 377 plots in 42 sites around the United States. Researchers Nunez-Mir and Matthew McCary - a UIC PhD alum and now an assistant professor at Rice University - compiled a new dataset of belowground plant traits, which have been understudied compared with easily observable aboveground traits such as leaf chemistry, seed size and plant height.

The ecologists used this dataset to compare the belowground traits of invasive and native plants and assess the impact of invaders on soil microbes. They first observed that invasive species have longer and less dense roots than native species, a sign of their aggressive growth.

"Basically, what that means is that these roots are optimized for foraging for nutrients," Nunez-Mir said. "They grow fast, making them very good at acquisition of resources, which makes sense - these invaders become invasive because they're proficient at colonizing new areas and surviving."

However, questions remain around whether invaders trigger the change in soil microbes, the homogeneous soil makes it easier for invaders to take root, or both.

"Maybe it is the invaders causing these changes, but maybe those changes create positive feedback in which now more invaders are becoming established because the environment is better for them," Nunez-Mir said.

Regardless, the converging microbial communities found in soil from widely distributed locations is likely bad news for ecological resilience. Future field experiments will test the precise nature of how invasive species affect their microbial surroundings, and how it may increase the risk of further invasions or damage.

"We haven't really seen this homogenization below ground before," Nunez-Mir said. "That is new and important to talk about because it could mean losing certain processes that ecosystems use to protect themselves."