Effects of Roads on Biodiversity and Conservation

Roads can have major adverse effects on biodiversity (Forman and Collinge 1996). A recent review by Forman and Hersperger (1996) distinguishes these aspects of the road-biodiversity interaction:

Road density: As road density increases, thresholds may be passed that cause some species to go locally extinct. The probability of extinction depends, in part, on body size, with larger animals requiring larger residual populations to prevent their extinction.
Road-effect zone: The effects of roads can extend over some distance from their centers, such that their "effective widths" can be many times their actual widths.

Two critical uncertainties must be resolved to understand how roads affect fragmentation and population viability. First, in the mechanistic analysis of the effects of roads and roadlike entities, such as power lines, on landscape fragmentation and species viability, the question of the "effective width" of roads is open. Kiester and Slatkin (1974) predict that, for species using conspecific cuing for movement strategies and habitat selection (likely most vertebrates), a spatially localized source of mortality in an area of otherwise suitable habitat can act as a sink, drawing individuals in as residents die, and making it likely that the new individuals will die as well. Consider a road traversing the habitat of a territorial or conspecific-cuing species. Individuals whose home range overlaps a road have some probability of being hit each time they cross the road. Eventually they may be killed, and their neighbors, in the process of constantly testing the boundaries of their home ranges, may move into the vacated area next to the road and themselves run the risk of road mortality. The question is: How far from a road does this probability of mortality spread?

Second, at the landscape scale, the relation between patterns of dispersal of individual species and measurements of fragmentation must be clarified. Schumaker (1996) suggests that most of the commonly used measures of fragmentation do not predict habitat connectivity for individual endangered species; rather, a model of fragmentation must be derived from species-specific dispersal characteristics. This kind of analysis has been done for only a few species.