Exotic Species Disrupt
Local Plant Communities
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.
Thats 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 its 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 dont understand yet. Invaders may
succeed by interrupting this long-term association of species within a community.
Certainly biocontrols [such as bugs that eat it] dont seem to be the total answer
against knapweed. Lots of things eat it and it thrives.
Callaways 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
knapweeds 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
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