A short essay on Biodiversity, roadways and road crossings by Justin Londer

Title: An Informal Essay on Roadways, Biodiversity and Conservation through wildlife crossings
By Justin Londer

Introduction, Background, Design Information

Roadways in North America total over five million miles and are the largest ever human constructed artifact on earth (Forman 2003). In the U.S. there are 6.2 million km of roads that 200 million vehicles use everyday (Forman 1998,2003). The density of roads in the U.S is on average 1.2 km/km^2. An estimated (and probably underestimated) 1 million animals are killed on these roads daily making road-kill the greatest directly caused anthropogenic source of wildlife mortality in the country (Forman 1998). An estimated 15-20% of the United States is ecologically impacted by roadways (Forman 1998). Localized biodiversity can be severely affected by roadways that fractionize already marginal habitat.

Biodiversity is affected by roadways in many ways that are really too numerous to list and describe in detail. Some of these include: habitat fragmentation, restriction of gene flows between seperated individuals of species, reproductive isolation, road or vehicle related mortality, migration barriers, noise, chemical and light pollution, behavior alteration, invasive species corridors, promotion of additional urbanization and development and and many others. A potential solution is that of administering "road crossings" of various sorts for impacted wildlife. This idea is part of a larger topic in ecology just beginning to emerge deemed "Road Ecology" (Forman 2003), and in this context we must consider animal crossings as an integral part of the design and implementation of new roadways as well as a mitigation solution for already constructed roadways if we want to preserve biodiversity.

Luckily, in the United States there is a high potential for animal crossings because most roadways are in the public domain. Crossings would involve many ideas and opinions from many fields of research including the social, political, economical and ecological realms — thus, road crossings could provide an avenue for multidisciplinary action. While ecology was emerging as a prevalent field in the 60's and 70's, roadways were being constructed at a frantic pace and were planned for efficieny and transport without ecological consideration for the most part. Today, as we see all around us, roads are prevalent and can be monolithic in makeup at a given area, sometimes spanning nearly 20 or more lanes. In some places, these emulate the Berlin wall for species. Variously constructed road crossings can help mitigate the ecological effects of roads and can also improve our own safety - less collisions with large animals means less fatalities and lowered costs of insurance for drivers as a whole.

When considering where to potentially build a crossing structure there are many factors that come into play. Landscape ecology, crossing design and the functional group(s) of the species effected are three of biggest considerations and need to be understood well to have a crossing that works effectively. Once an area has been identified using landscape ecology that can provide habitat connectivity (often where high road kill is observed) for a particular species or group of species the next consideration is how to build the structure itself. Species of interest can also be based on which species might be endangered or threatened.

The crossing should not direct animals to a mortality sink, or unsuitable habitats where animals will likely die once there for various reasons. Naturally mimicking overhead or underhead structures for megafauna may be needed to span highways in places like the Rocky Mountains, while small underground culverts might be used to connect wetlands or lotic habitats where amphibians or fish could benefit. Placement and design are limitless to a degree, but are key to success and require a great a deal of beforehand research so that limited funds can be used appropriately.

Other considerations include understanding: surrounding habitats, level of human activity nearby, unnatural lighting, the animals field of view, vegetation and cover within the crossing, temperatures within the crossing, traffic noise near the crossing, substrates in the crossing, structural considerations to provide privacy and longevity, other internal habitat features, an animals travel distance to or from the structure from or to normal habitats, creation of artificial or use of natural funneling to get animals to cross at a particular spot, wildlife accessibility, and future maintainance and upkeep of crossings. This list could include many other factors and all factors should be considered because preferably once built a structure wouldn't have to be moved or modified greatly (especially important if the structure is very large and cost millions to build).


In some cases in the United States and North America, roadkill has been dramatically reduced by implementation of large and small crossing structures and biodiversity has been conserved. In Banff National Park in Canada, the number of elk killed by vehicles on an annual basis has been reduced by 97% (Hartmann 2003). The endangered panther in Florida is a species considered to be on the brink of extinction, and only around 100 individuals are thought to survive. The same highway that runs to the northern tip of mitten of Michigan (I-75) runs south through Florida and effects these panthers. By constructing a series of underpasses that link important habitats and by constructing fences along I-75 near the crossings to funnel the animals into the crossing, no panthers have thought to have been killed by road related mortality since 2007. This mitigation has proven to be one of the best avenues explored for protecting this rare species (Scott 2007). These underpasses also appear to benefit bobcats, deer and raccoons and have lessened vehicle collision rates. Crossings have also been important for protecting biodiversity in Southern California where used by the threatened desert tortoise. Tortoise mortality on the highway was reduced by 93% during the first four years after simple culvert underpasses were constructed (Chilson, 2003). Haas (2000) and Lyren (2001) have also suggested that other species such as: bobcats, coyotes, gray fox, mule deer, and long-tailed weasels are benefitting in addition to the tortoise. Besides these large cuddly species mentioned above, there have also been numerous successes with smaller species like fish and amphibians through various mitigation methods that can easily be identified in a literature review.


Ultimately, there is no single or simple approach to road crossings and mitigation of wildlife mortality or loss of biodiversity caused by roadways. Diverse solutions should be explored as diverse problems abound. A crossing structure that is well planned has a better chance of working effectively and thorough research should be done before the structure is built (especially true in high cost situations). Structures that include as many effective design elements as possible will work better. And, structures that work well could be used for generations and may be learned travel corridors for some species that are long lived. Next time you see a flattened animal on a roadway you're driving on, you might think to yourself "what kind of structure could allow this type of animal to safely disperse or go hunting for food or mates if it were placed here. And, if you do, you as well will be part of the emerging science of "Road Ecology" and potentially part of the solution to the loss of biodiversity that has resulted from our creation of the largest human artifact ever on earth that are the roadways we travel on.

Sources and more information

Chilson, P. 2003. Cutting Edge: Right of way. Audubon magazine (June, 2003).

Forman, R. T. T., 2000. Estimate of the area effected ecologiclly by the road system in the United States. Conservation Biology. 14:31-53

Forman, R. T. T., 2003. Book. Road Ecology. Science and Solutions. Island Press

Forman, R. T. T., and L. E. Alexander. 1998. Roads and thier major ecological effects. Annual Review of Ecology and Systematics. 29: 207-231.

Haas, C. D. 2000. Distribution, relative abundance, and roadway underpass responses of carnivores throughout the Puente-Chino Hills. Master Thesis, California State Polytechnic University.

Hartmann, M. 2003. Evaluation of wildlif crossing structures: Thier use and effectivness. Wildlands CPR website.

Lyren, L. M. 2001. Movement patterns of coyotes and bobcats relative to road underpasses in Chino Hills of southern California. Master Thesis, California State Polytechnic University

Scott, B. 2007. Florida panther deaths increase from collisions with vehicles. Florida Fish and Wildlife Conservation Commission


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