Ecological Effects of Roads on Wildlife

1. Mortality from road construction

The actual construction of a road, from clearing to paving, will often result in the death of any sessile or slow-moving organisms in the path of the road. Obviously, trees and any other vegetation will be removed, as well as any organisms living in that vegetation.

2. Mortality from collisions with vehicles

Roadkill is the greatest directly human-caused source of wildlife mortality throughout the U.S. More than a million vertebrates are killed on our roadways every day.

3. Modification of animal behavior

The presence of a road may cause wildlife to shift home ranges, and alter their movement pattern, reproductive behavior, escape response and physiological state. When roads act as barriers to movement, they also bar gene flow where individuals are reluctant to cross for breeding.

4. Alteration of the physical environment

A road transforms the physical conditions on and adjacent to it, creating edge effects with consequences that extend beyond the white lines. Roads alter the following physical characteristics of the environment:

  • Soil density - Soil becomes compacted and remains so long after a road is in use.
  • Temperature - Dark pavement absorbs radiant heat and releases it at night, creating a "heat island" around roads. This can attract heat-seeking species such as birds and snakes to roads, increasing their mortality by vehicle collision.
  • Soil water content - Porosity of soil is reduced, allowing for less absorption of water.
  • Light - In order for a road to be built, trees must be cleared. Without the protective shade of the canopy, the area is exposed to more sunlight and inviting to light-loving species that otherwise would not thrive there.
  • Dust - Passing cars will stir up dust from the road. Dust will settle on nearby plants, blocking photosynthesis. Amphibians, such as frogs and salamanders, are also affected by traffic dust.
  • Surface water flow - Roads are impermeable, and act as channels to redirect water flow, including sediments and nutrient flow.
  • Pattern of run-off - Roads are often built with parallel ditching, which diverts rainwater run-off along roadways, rather than the natural flow pattern.
  • Sedimentation - Roads act to route fine sediments into streams.

5. Alteration of the chemical environment

Maintenance and use of roads contribute at least five different general classes of chemicals to the environment:

  • Heavy metals - gasoline additives
  • Salt - de-icing
  • Organic molecules - dioxins, hydrocarbons
  • Ozone - produced by vehicles
  • Nutrients - nitrogen

6. Spread of exotics

Roads provide opportunities for invasive species by:

  • providing habitat by altering conditions;
  • stressing or removing native species; and
  • allowing easier movement by wild or human vectors.

7. Increased use of areas by humans

Roads facilitate increased human access to formerly remote areas. In addition to the additional disturbance and pollution often associated with heavy human access, roads increase the likelihood and efficiency with which natural resources can be extracted.

*(adapted from "Review of Ecological Effects of Roads on Terrestrial and Aquatic Communites," by Stephen C. Trombulak and Christopher A. Frissell. Conservation Biology, Pages 18-30. Volume 14, No. 1. February 2000.)