Distribution of exotic plants along roads in a peninsular nature reserve

To examine the influence of roads on the spread of invasive exotic plants, we studied six species on a peninsular nature reserve. Three species were noxious invasives that we believed to be actively spreading; one of these (goatgrass, Aegilops triuncialis) was a major problem weed that recently invaded the reserve. The other three were ubiquitous and long-established exotics. Based on the idea that roads act as corridors for the spread of weeds, we predicted that the abundances of the three noxious invasives would decline along the roads from the base to the tip of the peninsula, while the abundances of three long-established species would not. However, the only species to show a base-to-tip decline in abundance along the peninsula was a long-established species (oat grass, Avena fatua), and a field experiment showed that habitat suitability could explain this pattern. We did find that the abundance of the invasive A. triuncialis declined with increasing distances from the edges of roads, suggesting that roads act as sources of weed propagules into adjacent grasslands. Our results add support to the idea that roads act as disturbances that promote invasive species, but not to the concept that roads act as corridors for the flow of invasive propagules into new landscapes.