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Species and Sprawl: Yellow Starthistle

Seeds can’t move by themselves, so they rely on moving things to give them a lift. Some catch rides on rivers or gusts of wind. Others cling to pant or dog legs using barb-tipped hairs. Some are eaten and expelled by birds or bats. But the quickest route to world domination, for plants at least, may be via rapid, efficient, and pervasive hitchhiking on enameled steel, radiator grilles, and muddy tires. The increasing urbanization of natural lands, aided and abetted by the automobile, is taking homespun seed dispersal mechanisms to a new level.


Yellow starthistles' inch-long spikes are a thorny nuisance to humans and can damage the eyes of grazing animals.

Cindy Roche,

Take the Western states’ yellow starthistle (Centaurea solstitialis), for example. This yellow-bloomed, spiny, nonnative weed produces two kinds of sesame-sized seeds, one with bristles, one without. The unbristled seeds catch gravity, dropping within half a meter of the parent plant. The bristled seeds go perhaps three meters better by catching wind currents. Yet since yellow starthistle’s introduction during the gold rush as a contaminant in bags of Chilean alfalfa seed, the plant has managed to carpet 15 million acres of roadside, rangeland, farmland, and grassland in California, mostly by hitching rides on tractors, road maintenance vehicles, and passenger cars. The plant has now invaded 23 states and is northern and central California’s thorniest weed problem, costing the state countless millions in lost grazing land, plummeting ranch revenues, and increased water conservation costs. Overall, the damage and control costs inflicted by invasive species in the United States amount to $138 billion dollars a year.

The spread of development in the United States is tough on many plants and animals. The National Wildlife Federation recently announced that at least 553 of the nation’s most at-risk species are found only in the 35 fastest-growing metropolitan areas. But some species, like yellow starthistle, actually get a population boost from sprawl. These invasive plants—nonnative species that enter an area, outcompete local vegetation for resources, and cause harm environmentally or economically—may even prefer that odd habitat located parallel to the nation’s nearly 6.4 million km of public roads.

Have Seeds, Will Travel

A road’s impact on soil, water, plants, and animals can extend 100 m on either side the pavement, meaning that the country’s public road system has an ecological effect on about one-fifth of all land in the United States. “Roads are the entry point for virtually every human impact on natural systems,” says conservation biologist Jonathan Gelbard, who has conducted a number of studies on how roads affect vegetation.

The habitat alongside a street is by its very nature disturbed, which can cause and accelerate plant invasions. During road construction, the roadsides are usually cleared of their variety of native species in favor of homogenous grass and other nonnative landscaping. Roadsides are also frequently flooded with sun, wet with runoff, compacted, salty, mowed, and replete with car emissions like nitrogen, which acts as a fertilizer. Many opportunistic invasive plants find such unsavory conditions optimal for quick growth. Furthermore, as Gelbard’s research has shown, the more improved the road surface, the more invasives crowd its edges.

After taking root, invasive weeds can swiftly populate the roadside strip. For example, Gelbard analyzed 1,300 sq km of grassland near Napa County, California, and discovered yellow starthistle in 73 percent of areas 10 m from roads, but in only 21 percent of areas further than 1,000 m from roads. Starthistle disperses easily along roadsides by lofting on the high-speed, turbulent wind tunnels generated in road corridors by passing vehicles, or by riding vehicles themselves. After one Montana researcher drove through several feet of knapweed, a starthistle cousin, the car picked up 2,000 seeds and deposited all but 200 of them within 16 km.


The less improved a road, the less susceptible its edges are to weed invasion.


Roadside maintenance also aids one of starthistle’s growth strategies. In early spring, while grasses and other roadside plants are sending up young shoots, yellow starthistle lays low—literally hugging the ground with a fledgling, palm-sized rosette—and quickly sends down an extensive root system that can access deep water sources. “Roadside mowers come when the grass is still green,” says Gelbard. “The mowers will blade off the grass but leave the starthistle to flourish in the absence of competition.”   By late May, starthistle bolts up into a tall prickly stalk with yellow flowers framed by 2 cm long spines. The dense spreads of starthistle have the potential to meander into adjacent wild lands, farms, and pastures, leaving landowners to bear the costs of controlling or eradicating the invader.  “By putting off highway mowing for just two or three weeks they could help reduce the problem by mowing the starthistle stalks themselves, says Gelbard.”

Paving Paradise

To keep pace with development, which consumes 2 million acres of wild land per year, transportation agencies consistently add and expand the nation’s road network. Nearly 70,000 lane-kilometers were steamrolled in 2003 alone. But a great many are not part of the 6.4 million km estimate, including the 600,000 km of National Forest roads and untold hundreds of thousands of kilometers of improved and unimproved “ghost roads” for private, military, and ranch use, oil and gas drilling, and other uses. Gelbard singles out those unofficial tracks for four-wheel drive recreation as a serious yet underestimated vector for plant invasion. “Because off-road vehicles go far from roads,” he says, “they not only affect habitats near roads that are already disturbed, but also remote, roadless habitats that are a lot less invaded.” How far invasives are actually creeping from roadsides into interior lands is still unclear and little studied.

Gelbard is hesitant about road expansion as a whole. “By curtailing and limiting the spread of roads,” he says, “you are essentially sealing off corridors from multiple environmental problems and the economic impacts they bring.” While reining in the pavers is unlikely to happen anytime soon (In December 2004, Texas launched the first phase of construction of the Trans-Texas Corridor, a 6,500 km megahighway project with lanes up to a half a kilometer across), recent years have seen ecologists, geographical information specialists, transportation planners, and conservationists take notice of the new field of road ecology. Its science is cross-disciplinary, exploring roadside vegetation, roadkill mitigation, and water, sediment, and chemical flow. It gives roadside habitats their proper due as unique environments. State and federal agencies are also increasingly incorporating ecological concerns into transportation planning, such as the Transportation Research Board’s Task Force on Ecology and Transportation, which links leading road ecologists with transportation planners. Can our desire for mobility work in concert with the needs of the environment? The answer lies down the road.

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