Invasive plants change ecosystems from the bottom up
In a common garden at the University of Rhode Island, Laura Meyerson has been growing specimens of Phragmites – also known as the common reed – that she has collected from around the world. And while they are all the same species, each plant lineage exhibits unique traits.
Now Meyerson, a professor of natural resources sciences, and Northeastern University Professor Jennifer Bowen have revealed that even when two different lineages grow side-by-side in the same ecosystem, the bacterial communities in the soil differ dramatically. It's a discovery that will aid in understanding how plant invasions succeed and the conditions necessary for their success.
"It's almost like the different lineages are farming their own microbial communities," said Meyerson. "What's amazing is that an invasive Phragmites population in Rhode Island and California will have microbial communities more similar than a native and invasive population living right next to each other in Rhode Island."
The Phragmites lineage native to North America has inhabited local wetlands for thousands of years, but a lineage introduced from Europe has begun to take over many North American marshes.
"I'm interested in bacteria within salt marshes, but I've never thought about these particular plant-microbe interactions and how microbes in the soil work to both facilitate plant success and inhibit growth," said Bowen. "But it turns out that the evolutionary signatures of the different plant lineages are so strong that it results in similar microbial communities in related plants that are found across the country. And that's incredible."
In a research paper published this week in the journal Nature Communications, Meyerson and Bowen outline their field surveys and controlled experiments on native, invasive and Gulf of Mexico lineages of Phragmites. Both methods found that the bacterial communities in the soil are primarily structured by plant lineage rather than by environmental factors, as was previously thought.
"These findings go against the general dogma that says that the environment determines the microbial community you're going to get," Meyerson said. "Two populations growing close to each other should have microbial communities more similar than those living farther apart. But our results say that's not true. In this case for these plants, it's the plant lineage – below even the species level – that determines the microbial community."
These results are important for understanding more about the success and fitness of invasive species.
"Microbes are really important in terms of determining what happens in a plant community," explained Meyerson. "By selecting for particular microbial communities, they're engineering their ecosystem from the bottom up. What happens at the microbial level affects the fitness and chemistry of the plants, and that affects plant interactions."
The researchers noticed that the microbes associated with the native Phragmites had more kinds of bacteria that are used to defend the plant from enemy attackers than the microbes associated with the invasive variety, which left most of its enemies behind in its native environment.
"The invasive plants didn't need to cultivate these defense mechanisms among their microbial communities," Bowen said. "What our research shows is that these plants are successful as invaders, in part, because they are freed from the need to cultivate a microbial defense shield."
Meyerson said her results provide a new perspective for those managing land and trying to control invasive plants.
"It's another reason to be cautious about invasive species," she said. "We have to look beyond what's going on above ground. We also have to look below at the microbial communities and how they affect ecosystems from the bottom up."
Caption: URI Professor Laura Meyerson investigates Phragmites at a marsh on Block Island as part of her research. (Photo courtesy of Laura Meyerson)