Biodiversity Protects Space From Invaders

Invasion plots. Two high-diversity plots crowd out alien plants (bottom), while a low-diversity plot leaves more space to invade (above).

Diversity is a buzzword for college admissions officers and conservationists alike, but its value to ecosystems is the subject of a long and bitter debate. Now, a study argues that diversity helps grasslands resist invasive species. The authors say the results could guide ecological restoration. Skeptics call the study unrealistic.

Researchers at the University of Minnesota's Cedar Creek Natural History Area are famous--some say infamous--for their experiments on diversity in 3-by-3-meter grassland plots. By carefully manipulating which plants live where, they have found that species diversity leads to higher productivity and stability, although some researchers say results from the tiny plots are unrealistic. Since the mid-1990s, they have also investigated plant invasions, which cost the U.S. economy billions of dollars a year. Previous studies showed that invaders are more common in areas of greater diversity, most likely because favorable soil and climate encourage invaders and natives alike. But Cedar Creek researchers suspected that if such factors were held constant, a diverse cover of native plants would prevent invaders from gaining a toehold.

Theodore Kennedy, an ecologist at the university's Twin Cities campus, and colleagues painstakingly analyzed the fate of plants invading 147 plots at Cedar Creek. The plots were seeded with up to 24 species of native plants, and unwanted plants were weeded out prior to the study. In a paper published in the 6 June issue of Nature, they show that in more diverse plots, fewer invaders sprout, and those that do sprout end up smaller than in less diverse plots. Using a statistical procedure called neighborhood analysis, they show that this happens because diverse groups of native plants tend to crowd out invaders.

But Tom Stohlgren, an ecologist at Colorado State University in Fort Collins, says the experiment is "unlike any natural landscape--it's too controlled." His research suggests more diverse communities are in fact more vulnerable. "I'm surprised that they would think that these general patterns would hold over large landscapes," he says. "In fact they don't." Future experiments must include far more than 24 species, larger plots, and a wider array of ecosystems, he says.

Kennedy acknowledges the study's limitations, but says the conclusions are still valuable. "The relationship between diversity and invasion success in grasslands or plant communities wasn't really known at either small or large scales, and our study identifies the mechanism driving this relationship at small scales."

Related sites
Cedar Creek home page
Tom Stohlgren's home page
Science feature on the diversity debate