Invasive species get a bad rap—but we humans are usually to blame for their spread. Take Japanese Stiltgrass, an invasive that arrived from Asia nearly 100 years ago as a packing material for porcelain. When it creeps into forests, it forms dense carpets that can choke out native tree seedlings. And in the last 15 years, the grass has infested rural roads throughout Pennsylvania's Rothrock State Forest—much faster than foresters expected.

Researchers thought the cause could be another human activity—road maintenance. They spray-painted 320,000 dead safflower seeds, and placed them along state forest roads. After routine road grading, they combed through the gravel to recover them. And they found that some seeds had been carried hundreds of feet down the road. Much farther than the few feet seeds can travel on their own—perhaps explaining the grass' rapid spread.

They presented those results at a meeting of the Ecological Society of America. [Emily Rauschert and David Mortensen, Human-mediated spread of invasive plants across a landscape]

Still, roads need to be safe for drivers. So the researchers propose smoothing shorter segments at a time, or doing it less frequently. Because where humans go, invasives often follow—whether by sea or on land.