As landscapes become increasingly fragmented by busy roads, housing developments,  commercial areas , and other human activities, wildlife habitat is divided into ever-smaller pieces of land for animals to accomplish everything they require; find food and water, establish territory, reproduce, and meet the rest of the specific living requirements. Roads and highways are excellent for connecting people to their destinations, but they can disconnect many wildlife species from essential habitats.

Roads affect populations in numerous ways, from habitat loss and fragmentation, to barriers to animal movement, and wildlife mortality. The impact of roads on wildlife populations is a significant and growing problem worldwide. The effects of roads on wildlife populations have been the focus of many studies in the last decade and increasing concern for transportation and natural resource management agencies. Wildlife crossing structures on US 93 North attempt to reduce impacts of wildlife-vehicle collisions and to increase habitat connectivity.

To see numerous research papers on the effects of roads and traffic on wildlife populations and landscape function, head to the journal Ecology and Society’s website.

Roads + Wildlife = Wildlife-Vehicle Collisions

Wildlife-vehicle collisions affect wildlife, human safety, and property. The total number of large mammal-vehicle collisions has been estimated at one to two million in the United States and at 45,000 in Canada annually. Wildlife–vehicle collisions represent approximately 5 percent (or 1 in 20) of all reported motor vehicle collisions. From 1990 to 2004, the number of wildlife-vehicle collisions increased 50 percent; this increase appears to be associated with an increase in “vehicle miles traveled” and increases in deer population size in most parts of the United States. In the United States, these collisions were estimated to cause 211 human fatalities, 29,000 human injuries and over $1 billion in property damage annually.

In most cases the animals die immediately or shortly after the collision. In some cases it is not just the individual animals that suffer; road mortality may affect some species on the population level, and some species may even be faced with a serious reduction in population survival probability as a result of road mortality, habitat fragmentation, and other negative effects associated with roads and traffic. Traffic has been shown to be the leading mortality source for some wide-ranging mammals, e.g., Florida panther, and regional bear and bighorn sheep populations. Roads were also shown to be the primary cause of wildlife population declines and habitat fragmentation among many amphibian populations. In addition, some species also represent a monetary value that is lost once an individual animal dies.

The preconstruction research along US 93 North found that deer were by far the most frequently recorded species group. However, rare, threatened, or endangered species may be removed before agency personnel was able to record them, and small and medium sized species such as coyote and smaller are rarely reported. It is notable that the western painted turtle is frequently hit by vehicles in the Ninepipe area.

Roads Fragment Wildlife Habitat

Landscape connectivity is the degree to which the landscape facilitates animal movement and other ecological flows. High levels of landscape connectivity occur when the area between core habitats in the landscape comprise relatively benign types of habitats without barriers, thus allowing wildlife to move freely through them in meeting their biological needs. Landscape connectivity is important for two reasons:

  • Many animals regularly move through the landscape to different habitats to meet their daily, seasonal and basic biological needs.
  • Connectivity allows areas to be recolonized, for dispersal, for maintaining regional metapopulations and minimizing risks of inbreeding within populations.

Reduced landscape connectivity and limited movements due to roads may result in higher wildlife mortality, lower reproduction rates, ultimately smaller populations and overall lower population viability. These harmful effects have underscored the need to maintain and restore essential movements of wildlife across roads to maintain within population movements and genetic interchange. This is particularly important on roads with high traffic volumes that can be complete barriers to movement.

The fragmentation effect of roads begins as animals become reluctant to move across roads to access mates or preferred habitats for food and cover. The degree of aversion to roads may vary by age group and gender. The reasons why roads are avoided can generally be attributed to features associated with the road, e.g., traffic volume, road width or major habitat alterations caused by the road.  High-volume and high-speed roads tend to be the greatest barriers and most effective in disrupting animal movements and population interchange. However, some studies have shown that secondary highways and unpaved roads can also impede animal movements.

To read more, please view Clevenger and Huijser 2009, “Handbook for Design and Evaluation of Wildlife Crossing Structures in North America.”

For more information regarding habitat corridors and landscape connectivity, see Center for Large Landscape Conservation’s Corridors and Landscape Connectivity: Clarifying the Terminology pdf 134 Kb

To learn about tools and methods to help reduce these impacts, head to the ‘Tools to Reduce Impacts‘ page of this website

To learn about crossing structures in other regions, check out the ‘Crossing Structures Around the World‘ page of this website

To see other organizations, agencies, and groups working on roads, wildlife, and connectivity issues, head to the ‘Who Else Works on Roads and Wildlife?‘ page of this website