The humble tumbleweed — that icon of the American West, blowing across the dusty, dry landscape of every old Western movie — is an immigrant.
And it isn’t a single species, but several. The first known tumbleweed species to arrive in the United States, Salsola tragus, or Russian thistle, is thought to have hitched a ride in a package of flax seed that some Russian immigrants brought with them to South Dakota in 1873. Over the years, other tumbleweed species arrived, including S. australis, which is thought to be a native of Australia or South Africa, though their paths into the country are less well known.
The species all look pretty similar, though despite the name, they don’t all tumble. They are all weeds, and ones that can pose a fire hazard during drought — a flaming ball of dry plant material that can be blown from place to place. It’s such a serious problem that scientists have even suggested importing fungi from Russia to control the plants.
So scientists have incentive to keep track of the tumbleweed invasion. In 2002, researchers reported that there was a new tumbleweed on the scene in California, S. ryanii. The new species was truly new; it combined the 36 chromosomes of S. tragus with the 18 chromosomes of S. australis to form a hybrid species with 54 chromosomes. S. ryanii was an intermediate of its two parents, with traits like fruit size and tumbling behavior falling square in the middle of the two others. And in 2008, scientists predicted that this made it likely that S. ryanii wouldn’t be as much of a problem as its parent species because it wouldn’t be as well adapted to the landscape.
It appears that isn’t the case. Shana Welles, now at the University of Arizona, and Norman Ellstrand of the University of California, Riverside surveyed tumbleweeds at 53 sites across California. In 2002, S. ryanii had been found in just two places in the San Joaquin Valley, but in 2012, the researchers found the plant in nine. In addition, the species also showed up at six other sites, including in coastal areas near San Francisco and Ventura. Clearly, the weed is spreading, Welles and Ellstrand report March 29 in the American Journal of Botany.
“It seems likely that the range of S. ryanii will continue to expand and [the species] is likely to become an important invasive species,” the team writes. It’s now another lookalike invader that can cause problems in the drought-prone West.
It’s even possible that S. ryanii could become an invasive species in other countries, the scientists say, should its seeds find a way to hitchhike across international borders, just like its great-great-great-great-great-grandparents did.