Alien species often wreak havoc in new neighborhoods, severely reducing the variety or abundance of local plants or animals and, in the worst cases, establishing monoculture ghettos.

That’s certainly been the scenario with non-native weeds such as knapweed and leafy spurge. The most popular explanation for these take-overs has been that exotic species are no longer controlled in their new environments by the natural enemies that kept them in check back home. But in the case of knapweed, at least, UM researcher Ragan Callaway has evidence that the invader may be succeeding because it brings new ways of chemically interacting with its adopted community that disrupt the balance of power that has evolved among long-associated native species.

“The big news is that it’s not just the absence of consumers that permits knapweed to take over,” Callaway says. “Plant communities may be real entities with some level of integration and quality that we don’t understand yet. Invaders may succeed by interrupting this long-term association of species within a community. Certainly biocontrols [such as bugs that eat it] don’t seem to be the total answer against knapweed. Lots of things eat it and it thrives.”

Callaway’s concept of the integrated nature of plant communities has emerged from years of work on a variety of systems — from oak forests to salt marshes to noxious weeds. His interest in positive as well as competitive interactions among plants has made him especially sensitive to the fact that more could be going on in the case of invasive weeds than just escaping natural enemies.

In a paper in Science last October, Callaway, an associate professor of biological sciences, and former graduate student Erik Aschehoug, now with The Nature Conservancy in San Francisco, describe the results of experiments that compared the competitive effects of diffuse knapweed (Centaurea diffusa) on six closely related species of bunchgrass. They chose three grass species from the northern Rocky Mountains in Montana and three from the Caucasus Mountains of the Republic of Georgia, where knapweed is native. Each of the seven plants was grown alone in greenhouse pots and in all grass-knapweed combinations.

Their results show that knapweed had much stronger negative effects on the biomass and ability to take up phosphorus of grass species from North America versus species from communities where knapweed is native. Additionally, Eurasian grasses significantly reduced the biomass of knapweed and its ability to acquire phosphorus, whereas North American grasses had no such effects on the interloper. When activated carbon was added to mop up chemicals being exuded by plant roots, the American grasses improved their growth while the Eurasian grasses suffered, suggesting that their competitive advantages against knapweed in their native home also depends in part on chemical interactions.

The researchers say these experiments indicate that Centaurea diffusa produces chemicals to which long-term and familiar Eurasian neighbors have adapted but to which knapweed’s new North American neighbors have not.

With the help of undergraduate student Kami Rogers, Callaway is now setting up similar experiments with leafy spurge and four other species of knapweed. In particular, they hope to learn more about the chemical interactions involved and the role of soil microorganisms.

“We need to figure out if these results are applicable to other weeds,” Callaway says. “Even so, these results may be of immediate importance in how we plan our efforts to control knapweed.”

— Caroline Lupfer Kurtz