Invasive plant species such as spotted knapweed aren't only taking a toll on native plant species, but have a detrimental effect on the biodiversity of microbes in the soil, says a new Colorado State University study to be published in the journal International Society for Microbial Ecology.

"Spotted knapweed originated in Eurasia where it's held in check by pathogens, herbivores and other plant competitors that evolved along side of it," says Amanda Broz, a graduate student in Colorado State's Center for Rhizosphere Biology, who conducted the research. "When knapweed was introduced to the American West, it escaped these natural enemies, allowing it to spread and take over many of our native grasslands."

Spotted knapweed arrived on both coasts of North America in the late 1800s as a contaminant of alfalfa seed. In addition to displacing native plant species, the weed increases water runoff leading to erosion and reduces forage for livestock and wildlife.

Researchers collected soil samples from areas near Missoula, MT, where spotted knapweed is particularly problematic, infesting more than 4.7 million acres in the state. In areas with very high-densities of spotted knapweed, there was 80% less DNA of fungi than areas with low-densities of spotted knapweed. Even areas with a low-density of spotted knapweed showed changes in the amount and types of soil microbes naturally found in the area.

Soil microbes can have a profound influence on molecular and biochemical processes in individual plants, plant community and ultimately the entire ecosystem. The disruption of the balance between native plants and microbial communities in the soil can have a negative effect on native plants while benefiting invasive species.

"A better understanding of the interactions between native plants, invasive species and the native soil community will help in developing more effective strategies in managing invasive species and restoring the landscape to its natural state," Broz says.