Species and Sprawl: A Road Runs Through It
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.
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.
