Human-mediated dispersal via rural road maintenance can move invasive propagules

Although preventing introduction is the best way to avoid invasive species problems, the dynamics of spread into new areas are not well understood. Invasive plant presence has frequently been associated with roads, but quantification of the mechanisms behind this phenomenon is lacking. Although some invasive plants may experience more rapid natural dispersal in a roadside habitat, scaling up this increase often cannot account for regional spread. Typically human activities drive long-distance dispersal, which is critical for predicting large-scale spread rates. We followed the movement of seeds by routine rural road maintenance as a potential factor explaining rapid regional spread of invasives. We found that 23.5% of seeds were not moved by road grading, 33.1% moved short distances (between 0 and 10 m), 41.8% moved intermediate distances (10–50 m), and very few moved long distances (more than 50 m, 1.6%). The furthest movement observed was 273 m. Nearly 1/3 of seeds were moved moderate distances; this intermediate distance dispersal is likely a key driver of the presence of invasives along the entirety of many roadsides and poses a large threat to the forest interior. Propagule spread via grading is a fairly stochastic process; nonetheless, a small but potentially important portion of propagules are moved long distances, accelerating spread rates. We discuss these results from the perspective of the rapid invasion of Microstegium vimineum in central Pennsylvania, USA. To slow the spread of seeds via road maintenance, managers should consider shortening grading passes, inspecting vehicles for invasive propagules, and otherwise ensuring that seeds are not transported long distances. Greater attention should be paid to human aided dispersal of invasive plant propagules particularly in light of an increasing forest roads network associated with logging and natural gas development in some parts of the United States.