Roads and Biological Invasions

A widely cited generalization about biological invasion is that it is promoted by disturbance. Building roads and subsequently maintaining them (including ditch clearing, road grading, and vegetation clearing) in the interior of a forest are disturbances that create and maintain new edge habitat. These roadside habitats can be invaded by an array of exotic plant species, which may be dispersed by "natural" agents such as wind and water as well as by vehicles and other agents related to human activity. Roads may be the first point of entry for exotic species into a new landscape, and the road can serve as a corridor along which plants move farther into the landscape (Greenberg and others 1997, Lonsdale and Lane 1994). Some exotic plants may then be able to move away from the roadside into adjacent patches of suitable habitat.

Invasion by exotic plants may have significant biological and ecological effects if the species are able to disrupt the structure or function of an ecosystem. Cowbirds (Molothrus ater), for example, can be introduced into forested environments by roads and subsequently affect populations of neotropical migratory birds through nest parasitism. Roads can also act as vectors for the spread of pathogens (see Forest Diseases).

Invasion may be of concern to land managers, if the exotic species disrupt management goals and present costly eradication problems. However, few environmentally benign approaches to exotic plant control or eradication have been tested.

Although few habitats are immune to at least some invasion by exotic plants, predicting which species will become pests usually is difficult. Assessing the scale of a biological invasion problem is complicated by the lag between when an exotic is introduced and when it begins to expand its distribution and population size in a new area. Also, observations in different settings suggest that the exotic species that successfully invade and the scale of invasion problems differ regionally. A less-than-ideal science base exists for identifying which exotic species pose the greatest threat and what preventive or remedial measures are appropriate.