Sparingly introduced in waste places
by Cherie Winner | © Washington State University
Although scientists have been aware of biological invasions at least since the mid-1800s, when Charles Darwin noted the rampant spread of European species in South America, only recently has the scientific community recognized the broader threat invaders pose to biodiversity and environmental quality. Richard Mack of Washington State University recalls that when he first started talking about the cheatgrass invasion at annual meetings of the Ecological Society of America (ESA), his presentations would be scheduled for the "Miscellaneous" session on the meeting's last day.
"Thirty years ago, it wasn't on the radar screen as an academic topic worthy of investigation," he says. "It felt lonely."
That has changed. The program for the 2007 meeting of the ESA listed hundreds of papers directly concerned with biological invasions; at least two major sessions every day for a week were devoted to the issue.
Mack says he got into research on invasive species at its inception and has simply "ridden the rocket." For him, it all started with a lucky choice of study species when he began his career at WSU in 1975. As a new faculty member keenly interested in plant population biology, he looked for a readily available annual plant that completed its growth and reproduction within a single year. The rangelands of eastern Washington offered a prolific candidate: cheatgrass. Mack knew it was a non-native that had become a nuisance since its introduction here in the late 1800s. The deeper significance of his choice didn't hit him until a couple of years later. He knows exactly when the light went on.
One Saturday afternoon he wandered down to the University's Ownbey Herbarium and began browsing through old floras (plant lists). He looked up cheatgrass in one of the first floras of the region, published in 1902. "Bromus tectorum," it said. "Sparingly introduced in waste places."
He's still stunned at the realization. "I said, 'That's sure not the case now!' It just suddenly clicked for me that this was something that had gone through this enormous spatial transformation. In some ways I should have tumbled to this a lot earlier. I knew the kind of damage that cheatgrass does…but I never really thought about it in a temporal sense. Once I saw that statement, everything clicked." Mack went on to explore the causes, progress, and effects of the cheatgrass invasion.
Cheatgrass (also called downy brome) likely arrived in eastern Washington as an accidental introduction. It "simmered along" in low numbers for about 20 years, says Mack, and by World War I had become enough of a problem to alarm farmers and weed agents around Pasco and neighboring areas of the Columbia Basin. Cheatgrass went on to spread throughout the Intermountain West, infesting croplands and reducing the abundance of native plants on rangelands. Farmers, livestock, and wildlife all suffer as a result.
"Whatever the nutritional value and amount of forage that was on these lands before, it was certainly greater than what cheatgrass provides," says Mack. "And cheatgrass is worthless after it dies. A cow will die of hunger eating cheatgrass straw all day. The same would go for wildlife."
The damage extends beyond effects on forage. When cheatgrass dies back in June, there's little left to hold the soil; erosion from cheatgrass-dominated rangelands has destroyed salmon beds and clogged dams throughout the Snake and Columbia River drainages. Dead cheatgrass burns more readily than the native grasses and forbs it displaced, leading to frequent wildfires that further damage rangelands and cost millions to battle. In one recent year, says Mack, nine percent of Nevada burned, largely due to the ready fuel of cheatgrass. The fires have hit sagebrush especially hard. Animals that need sagebrush, including the endangered sage grouse and Columbia basin pygmy rabbit, suffer.
"Not everyone is as desperately concerned about the conservation of species as I am," acknowledges Mack. "That's OK. But everybody's concerned about what comes out of their pocket. And biological invasions definitely tap everybody's pocket. If anybody could tally up the bill [due to cheatgrass], I think the public would be stunned as to how much it costs us. And that's just one invasion. It's an extremely damaging one, but we start adding these all up, and in effect we as a society are paying a huge hidden tax."
In the past few years, Mack and his students have been using molecular markers to trace where the various populations of cheatgrass in North America came from. They've collected specimens from hundreds of sites in the grass's native Eurasian range and in its new North American range. By analyzing DNA from each specimen, they are developing a "genotype map" showing where the different strains occur all across the continent, something never done with a plant species before. He hopes the map will provide clues about what makes some strains invasive, and how the invaders might be beaten back.
So far his team has identified seven distinct types of cheatgrass in North America. One appears to be native to Afghanistan; the strain that blankets southeastern Washington came from a small area in central Europe. Three strains are responsible for about 99 percent of the area occupied by cheatgrass in North America. The other four have stayed close to their site of introduction; they're naturalized, but not invasive.
Ann Kennedy, a soil microbiologist with the USDA's Agricultural Research Service and an adjunct professor in WSU's department of crop and soil sciences, says such information could be crucial in efforts to develop a biological control agent that targets cheatgrass. She has identified a soil bacterium that inhibits the growth of cheatgrass on Washington rangelands, but doesn't know whether the bacterium will have the same effect on cheatgrass in other regions. The genetic differences among strains of cheatgrass may mean they're not all susceptible to the same control measures. Prospective control agents will have to be tested on, and perhaps tailored for, the strain of cheatgrass that afflicts a given region.
While his work with cheatgrass continues, Mack is thinking and writing more about biological invasions in general.
"The thing to keep in mind is that we haven't necessarily seen the worst invasions yet," he says. "There are other species out there, elsewhere in the world, that have yet to arrive or been put in a position here to become invasive. If they do, then cheatgrass will look like a fairly benign invader."