Roads and Fragmentation

Authored By: J. Knoepp

Roads fragment habitats by changing landscape structure and by directly and indirectly affecting species. Habitat effects of roads on the landscape include:

Dissecting vegetation patches.
Increasing the edge-affected area and decreasing interior area.
Increasing the uniformity of patch characteristics, such as shape and size (Reed and others 1996).

Whenever forest roads are built, changes in habitat and modified animal behavior will lead to changes in wildlife populations (Lyon 1983). Large mammals such as elk (Cervus canadensis), bighorn sheep (Ovis canadensis), grizzlies (Ursus arctos horribilis), caribou (Rangifer tarandus), and wolves (Canis lupus) avoid roads. Avoidance distances of 300 to 600 feet are common for these species (Lyon 1985). Road usage by people and their vehicles has a significant role in determining road avoidance by animals. In a telemetry study of movement by black bears (Ursus americanus), they almost never crossed interstate highways, and they crossed roads with little traffic more frequently than those with high traffic volumes (Brody and Pelton 1989). Bobcats (Lynx rufus) crossed paved roads in Wisconsin forests less than expected, possibly to minimize interactions with vehicles and people (Lovallo and Anderson 1996).

A few studies have related genetic changes in populations simply to the presence of roads (Forman and others 1997), but the distribution of roads in the environment also must be considered. Road density is a useful index of the effect of roads on wildlife populations (Forman and others 1997). Wolves in Wisconsin are limited to places with pack-area mean road densities of 0.7 mile/square mile or less (Mladenoff and others 1995). Some studies have shown that a few large areas of low road density, even in a landscape of high average road density, make suitable habitat for large vertebrates (Rudis 1995).