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Perspective
Biodiversity: Friend or foe of invasive species?
They slip past customs officials and become embedded, unnoticed in the community for years, before one day exploding and wreaking havoc. These terrorists are the six-legged, deep-rooted, or even slithery kind known as invasive species. Lori Williams, executive director of the U.S. National Invasive Species Council (NISC), wants them uncovered and eliminated, and she’s drawing up a battle plan to do just that. Good reason for concern exists: 3 million acres are lost to invasive plants alone every year, she says.
NISC is a coordinating body representing more than a dozen federal departments. “Given the shortage of resources at the federal, local, and state levels, NISC is revising and updating its national management plan to make it better able to prevent and target new invasives (both accidental and intentional), as well as to detect and respond to invasions early,” Williams says. The new plan, which is to be released for public comment early this year, links the latest research to practical tools in the field, she says.
One of those tools should be biodiversity, says Tom Stohlgren, with the U.S. Geological Survey, which is a member of NISC. New research from his group presented at the Ecological Society of America (ESA) meeting last August reveals that the most biologically diverse regions of the U.S. are also the most invaded. Focusing on these areas could lead to early detection and rapid response to new invasives, Stohlgren says.
But not everyone is convinced. Stohlgren’s results run counter to the long-held theory that high native biodiversity acts as a firewall to keep out exotic invaders. In addition, a host of experimental manipulations of biodiversity find that the field plots richest in native species have the fewest invaders.
These seemingly contradictory findings were passionately hashed out at a special symposium at the ESA meeting. Resolving the paradox has important implications for managing invasions, scientists say.
Stohlgren and his colleagues evaluated data on the distribution of native and non-native plant species in U.S. ecosystems that range in scale from individual streams to whole regions. They found a significant and positive correlation between native plant diversity and the establishment of invasive species. “Stream and river areas, which are typically rich in native species, are getting pounded, especially in California,” Stohlgren says.
“There is probably no direct cause-and-effect relationship between native and non-native species richness,” Stohlgren says. Rather, invasive species thrive in areas well-endowed with natural resources such as water, light, and nutrients, he says. Those same resource-rich areas also happen to be the kind of habitats that support large numbers of native species, he explains.
“If you change your scale of reference, you can get a different result,” says Shahid Naeem from Columbia University. He and his colleagues manipulated 1 × 1.5 m plots of Minnesota prairie to contain 1–24 native species and observed them for 2 years. They found that the number, size, and variety of invasive species increased as biodiversity decreased, peaking in the plot that was originally dominated by one native species.
Naeem speculates that the native species in the diverse plots complement each other because they differ in height, rooting depth, and drought and light tolerance. This complementarity means that resources, such as nutrients and water, are being used more efficiently in the diverse plots so that little is left over for invaders to utilize. “The results suggest that if you lose biodiversity, the ecosystem loses its resistance to invasion,” he says.
The findings of Stohlgren and Naeem don’t necessarily exclude each other, because they are asking fundamentally different questions, says Jay Stachowicz, a marine ecologist at the University of California, Davis. Observational studies such as Stohlgren’s show that communities that are good for native species are good for invaders too, he says. On the other hand, experimental work such as Naeem’s shows that in the very diverse areas, loss of native biodiversity will lead to even more invasions, he says. This information has important implications for resource managers, he adds.
“Biodiversity can help prevent invasions, but the trick is understanding the relative importance of biodiversity compared to other factors, such as habitat disturbance, in preventing invasions by exotic species,” says Dov Sax with the University of Georgia. “To what extent can native biodiversity be a management tool in and of itself, and when is biodiversity an indicator of where you should be spending your monitoring efforts to prevent further invasions?” he asks. The ESA symposium showed that no clear answers exist, but it also helped point out the direction of future work that needs to be done to resolve the issue, Sax concludes.
