This is the third and final post in a series looking at wildlife movement, corridors and roads. Read the first post, Ecology Lesson: Population Movements, which was followed by Corridor Ecology and Planning.

Roads traverse all terrain


Roads crisscross the entire country covering much of the land. Highways take us through states or across the whole country, streets cover cities and dirt roads link rural areas to cities. They traverse over rivers, lakes, mountains, valleys, deserts and forests. Some highways are many lanes wide and some bridges are modern engineering marvels. They bring us commercial goods, foods and fuel across great distances. There are over 4 million miles of roads in the United States alone.

Road Ecology

Road ecology is a field that addresses the relationship between roads, wildlife and the environment. The topic has received a lot of attention over the past 30 years with a lot of focus on roads and wildlife. Germany and Switzerland were two of the earlier countries to look at modifying the current infrastructure to address problems, and amphibians in Western Europe were some of the earliest beneficiaries of wildlife tunnels.

Because roads cover such vast terrain, they are subjected to the many environmental conditions those regions produce such as mudslides, ice storms, windy debris and the shifting earth. They also, in turn, greatly influence environmental conditions by creating impervious surfaces for rain, influencing the vegetation structure adjacent to them, they cause erosion where they’ve cut into  hills, and when crossing over a stream, they funnel the water through a culvert. Throw bridges, pipes, overpasses and noise barriers into the mix and it’s a complicated system of man-made structures and the natural environment.

The vegetated edge of roads are often mowed or sprayed with pesticides which creates a hard edge to the adjacent habitat and a severe barrier. This disturbed area creates a zone for plant opportunists and many invasive plants such as blackberry do well there and once established, have a nice road to spread along. The roadsides, which have heavily maintained vegetation also create a different habitat for wildlife. Most everyone can recall seeing raptors sitting alongside roads surveying the short vegetation for rodents. Other species that find this type of habitat beneficial are reptiles, butterflies, amphibians and bees. If the roadside is managed for biodiversity, it could become an effective habitat. Designing successful roadside habitat is possible if different, conflicting goals can be achieved such as visibility for both wildlife and drivers, cover and a way to cross the road safely. There’s an excellent digram in chapter 4 of the very worthwhile book, Road Ecology: Science And Solutions, that illustrates the various patterns of roadside vegetation.

Roads also can influence aquatic ecosystems by crossing waterways or running alongside of them. Stream crossings with culverts can alter the form and hydraulics of the stream and change the processes downstream as well. Small culverts can backup with water during heavy flows and spill onto the road, and they can also impede movement of aquatic species as well as movement of debris downstream. However, culverts can be designed for aquatic crossings that can be suitable for the aquatic species and minimize impact on the water flow.

To learn much more about the aspects of road ecology such as water and sediment, wind  and atmosphere and chemicals see Road Ecology.

Wildlife and Roads

“In front of a motel room in Ottumwa I finger-scrape the dry, stiff carcasses of bumblebees, wasps and butterflies from the grille and headlight mountings, and I scrub with a wet cloth to soften and wipe away the nap of crumbles, the insects, the aerial plankton of spiders and mites. I am uneasy carrying so many of the dead. The carnage is so obvious.”

-Barry Lopez, Roadkills, 1998

Roads have a huge impact on wildlife. They act as a barrier to many types of species during their daily or seasonal movements, they provide an unsafe basking site for reptiles and they can also make streams impassible for aquatic species. When large mammals cross in front of vehicles, the result can be deadly to both humans and wildlife, and also very costly. Many species of wildlife are killed on roads including mammals, reptiles and amphibians, insects and birds.  There has not been much study in the past about wildlife road fatalities, however one estimate suggests upwards of 72,000 deer are killed every year. One study of painted turtles in Montana found that during a four month period, on a stretch of four and a half miles, 205 turtles were killed. A two-year study on a 2.2 mile stretch near Lake Erie in Canada counted 32,000 vertebrates killed.


Birds can be victims of roads

Toads and frogs are extremely susceptible to road fatalities because they often migrate seasonally in large numbers. In the U.K., citizens have formed groups, such as the Toads on Roads project, to help toads get across roads during their peak times. It is estimated that on a busy highway, a frog or toad has an 89% or greater likelihood of being killed while trying to cross. One way of looking at wildlife fatality on roads is found in Road Ecology; ask not “How many birds are killed by speeding automobiles? but rather, how many pairs of birds are prevented form successfully rearing broods?”

Behavior of certain species often indicates how likely they are to cross roads such as those with high mobility like large mammals. Seasonal movements of species during the breeding season also make many wildlife species more susceptible such as is the case with the frogs and toads mentioned above. Species attracted to the road, such as reptiles who bask and scavengers who eat the roadkill also have a higher risk due to their behavior.

Road fatality is only half of the problem though, the other piece is the effect of roads on species behavior and population dynamics. We already know the problem of habitat fragmentation that roads create, from the last post in the series, Corridor Ecology and Planning. Roads act as barriers to many species impacting distribution, which was discussed in the first post, Ecology Lesson: Population Movements. For smaller animals, roads create serious barriers. A study mentioned in Road Ecology found that a four-lane road was the equivalent of a body of water twice that size to small mammals.

The more we study, the more we can change the surrounding landscape to help reduce vehicle and wildlife conflicts. For example we know from studies that deer fatalities decrease in areas with higher density of buildings close to the road. Other studies indicate a lower fatality rate when there is more visibility and greater distance from forest edge to road. The other ways we can mitigate roads is the use of wildlife crossings.

Wildlife Crossings

States have tried many techniques to prevent wildlife and vehicle conflicts. One study analyzed mitigation techniques for deer, including techniques such as highway lighting, public relations, habitat alteration and wildlife fencing. Among the least successful of techniques was lower speed limits, highway lighting and warning signs. On the other hand, some of the most successful techniques included wildlife fencing (91%) and overpasses/underpasses (63%). A combination of various techniques may be very successful. Unfortunately this hasn’t changed the way states implement techniques….yet.


Existing highway bridge

An understanding of what species cross roads and a thorough understanding of the behavior of those species can effectively help mitigate the impact of roads. A wildlife crossing on it’s own will not be as effective as a crossing combined with other mitigation techniques such as one-way gates, fencing, culverts, barriers and signs. There are many different types of crossings, some designed for toads, some for bears and many can accommodate a variety of species.

Small crossings are used for species such as frogs, toads and salamanders and can often be successful as converted culverts or pipes. The materials are important considerations, as well as the size, shape, length and substrate. Many amphibian species require moist substrates and slow moving waters. Small mammals and reptiles on the other hand, require dry substrates with no running water. Many species have similar preferences in some aspects of crossings, such as having vegetation close to openings.

Larger crossings can go under roads or over them and are usually designed for large mammals such as deer and bears. Underpasses can also be converted from existing structures, or designed with a new road. They can be a single or multi-span bridge, viaduct, or some variety of culvert. Overpasses are built like a bridge going over the road and vary in size. They are still uncommon and only a small handful exist in the US. In studies of use of both underpasses and overpasses in Banff (see below) species preference was recorded. Elk, deer, wolves and Grizzly bears by far preferred overpasses while cougars preferred underpasses and Black bears used both equally. Because it’s still such a new concept, we still have a great amount to learn about design such as shape, height, vegetation and length of the crossings.

There are many factors involved with the design of a wildlife crossing and fortunately, there are many resources available. See the Wildlife Crossings page under the Resources tab on this blog for a list of those resources.

Wildlife Crossing Examples

One of the earliest examples in the United States was in Florida with Alligator Alley. Interstate 75 runs along Everglades National Park and a series of underpasses were built to help move water to the Everglades Park as well as giving the endangered Florida Panther a way to move under the road. Vehicle collisions accounted for nearly half of all Panther deaths and their numbers dropped to about 30 individuals. Since the installation of 23 crossings no Panthers have been hit where there was a crossing available. (This is based off slightly older data which may have changed. Two panthers have been killed by cars in Florida in this month alone.) The most recent estimate is that there are over 100 individuals today and the general consensus is that growth is in large part due to the crossings. The Panther is not the only species to benefit however, other wildlife seen using the crossings include Black Bears, armadillos, alligators, Gray Foxes,  White-tailed Deer and a variety of birds.

The Banff Highway, in Canada is another early example of wildlife crossings set along a major highway. The project consists of over 20 individual crossings, both over and under and in a variety of shapes and sizes and incorporates fences with escape structures. They have also been studied in great detail and have found to be quite successful. Studies in 2001 found that from the beginning of the research, in 1996, there had been 26,279 through-passes at underpasses and 5,515 at the overpasses and that number continues to grow as it’s continually monitored.

Recent Designs

Wildlife crossings have been gaining fast traction and most recently there was a design competition for a road in Colorado. The ARC International Wildlife Crossing Infrastructure Design Competition attracted 36 entries from 9 countries and represented 100 firms. There were five finalists which I covered in the blog post ARC Wildlife Crossing Finalists, and the winner was just announced. Watch the video of the competition and the finalists below.

In Washington, a similar project is currently underway for Interstate 90. The I-90 Wildlife Bridges Coalition is working with WSDOT to plan a series of wildlife crossings over a 15-mile stretch that is currently scheduled for safety improvements. They are currently conducting Wildlife Watch, a data collection program aimed to help document wildlife, dead and alive, along the stretch of I-90 for better planning. View a video of the design visualization below.

On the opposite side of the country, another project has been developed by the Nature Conservancy in Massachusetts and the University of Massachusetts. Part of the project is studying several salamander tunnels which were installed in the state, some as early as 1987. They have also developed the new Critical Linkages program, which takes a look at where linkage improvements would make the biggest impact. They have created a model to pinpoint which stream and road junctions would have the best results for the surrounding habitat. Take a look at the videos below, the second one shows an example of the model.

Further Reading::

This is just the tip of the iceberg of the subjects of road ecology and wildlife crossings. For more information about wildlife crossings, see Wildlife Crossing Resources, which contains links to websites, documents and other resources.

Road Ecology: Science And Solutions (Book)

Kelly Brenner
Follow me
Latest posts by Kelly Brenner (see all)
Liked this? Take a second to support Kelly Brenner on Patreon!

Commenting area

  1. Fascinating post. I’ll have to read the whole series.

  2. As of July 2010, eleven species of large mammals have used 24 wildlife crossings more than 220,000 times since 1996. This includes grizzly and black bears, wolves, coyotes, cougars, moose, elk, deer, bighorn sheep, and more recently wolverine and lynx.

Trackbacks for this post

Leave a Reply

You can use these tags: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>