Wildlife crossing

Roads and Habitat Fragmentation

Habitat fragmentation occurs when human-made barriers such as roads, railroads, canals, power lines, and oil pipelines penetrate and divide wildlife habitat (Primack, 2006). Of these, roads have the most widespread and detrimental impacts (Spellerberg, 1998). Scientists estimate that the system of roads in the United States impacts the ecology of at least one-fifth of the land area of the country (Forman, 1999). For many years ecologists and conservationists have documented the adverse relationship between roads and wildlife. Jaeger, et al., (2005) identify four ways that roads and traffic detrimentally impact wildlife populations: (1) they decrease habitat amount and quality, (2) they increase mortality due to wildlife-vehicle collisions (road kill), (3) they prevent access to resources on the other side of the road, and (4) they subdivide wildlife populations into smaller and more vulnerable subpopulations (fragmentation).

The first three impacts (loss of habitat, road kill, and isolation from resources) exert pressure on various animal populations by reducing available resources and directly killing individuals in a population. For instance, Bennet (1991) found that road kills do not pose a significant threat to healthy populations but can be devastating to small, shrinking, or threatened populations. Road mortality has significantly impacted a number of prominent species in the United States, including white-tailed deer (Odocoileus virginianus), Florida panthers (Puma concolor coryi), and black bears (Ursus americanus) (Clevenger, et al., 2001). In addition, habitat loss can be direct, if habitat is destroyed to make room for a road, or indirect, if habitat quality close to roads is compromised due to emissions from the roads (e.g. noise, light, runoff, pollution, etc.) (Jaeger et al., 2005). Finally, species that are unable to migrate across roads to reach resources such as food, shelter and mates will experience reduced reproductive and survival rates, which can compromise population viability (Noss et al., 1996).

In addition to the first three factors, numerous studies have shown that the construction and use of roads is a direct source of habitat fragmentation (Spellerberg, 1998). As mentioned above, populations surrounded by roads are less likely to receive immigrants from other habitats and as a result, they suffer from a lack of genetic diversity. These small populations are particularly vulnerable to extinction due to demographic, genetic, and environmental stochasticity because they do not contain enough alleles to adapt to new selective pressures such as changes in temperature, habitat, and food availability (Primack, 2006).

The relationship between roads and habitat fragmentation is well documented. One study found that roads contribute more to fragmentation in forest habitats than clear cuts (Reed et al., 1996). Another study concluded that road fragmentation of formerly contiguous forest in eastern North America is the primary cause for the decline of forest bird species and has also significantly harmed small mammals, insects, and reptiles in the United States (Spellerberg, 1998). After years of research, biologists agree that roads and traffic lead to habitat fragmentation, isolation and road kill, all of which combine to significantly compromise the viability of wildlife populations throughout the world.

Wildlife-Vehicle Collisions

In addition to conservation concerns, wildlife-vehicle collisions have a significant cost for human populations because collisions damage property and injure and kill passengers and drivers (Clevenger et al., 2001). Bruinderink and Hazebroek (1996) estimated the number of collisions with ungulates in traffic in Europe at 507,000 per year, resulting in 300 people killed, 30,000 injured, and property damage exceeding $1 billion. In parallel, 1.5 million traffic accidents involving deer in the United States cause an estimated $1.1 billion in vehicle damage each year (Donaldson, 2005).

The conservation issues associated with roads (wildlife mortality and habitat fragmentation) coupled with the substantial human and economic costs resulting from wildlife-vehicle collisions have caused scientists, engineers, and transportation authorities to consider a number of mitigation tools for reducing the conflict between roads and wildlife. Of the currently available options, structures known as wildlife crossings have been the most successful at reducing both habitat fragmentation and wildlife-vehicle collisions caused by roads (Knapp et al., 2004, Clevenger, 2006).

Wildlife crossings are structural passages beneath or above roadways that are designed to facilitate safe wildlife movement across roadways (Donaldson, 2005). In recent years, conservation biologists and wildlife managers have advocated wildlife crossings coupled with roadside fencing as a way to increase road permeability and habitat connectivity while decreasing wildlife-vehicle collisions. Wildlife crossing is the umbrella term encompassing underpasses, overpasses, ecoducts, green bridges, amphibian/small mammal tunnels, and wildlife viaducts (Bank et al., 2002). All of these structures are designed to provide semi-natural corridors above and below roads so that animals can safely cross without endangering themselves and motorists.

History and location of wildlife crossings

The first wildlife crossings were constructed in France during the 1950’s (Chilson, 2003). European countries including the Netherlands, Switzerland, Germany, and France have been using various crossing structures to reduce the conflict between wildlife and roads for several decades and use a variety of overpasses and underpasses to protect amphibians, badgers, ungulates, invertebrates, and other small mammals (Bank et al., 2002). In addition to Western Europe, wildlife crossings are becoming increasingly common in Canada and the United States. The most recognizable wildlife crossings in the world are found in Banff National Park in Alberta, where vegetated overpasses provide safe passage over the Trans-Canada Highway for bears, moose, deer, wolves, elk, and many other species (Clevenger, 2007). The 24 wildlife crossings in Banff were constructed as part of a road improvement project in 1978 (Clevenger, 2007). In the United States, Chilson (2003) reports that thousands of wildlife crossings have been built in the past 30 years, including culverts, bridges, and overpasses. These have been used to protect mountain goats in Montana, spotted salamanders in Massachusetts, bighorn sheep in Colorado, desert tortoises in California, and endangered panthers in Florida (Chilson, 2003).

Costs and benefits

The benefits derived from constructing wildlife crossings to extend wildlife migration corridors over and under major roads appear to outweigh the costs of construction and maintenance. One study estimates that adding wildlife crossings to a road project is only a 7-8% increase in the total cost of the project (Bank et al., 2002). Theoretically, the monetary costs associated with constructing and maintaining wildlife crossings in ecologically important areas are trumped by the benefits associated with protecting wildlife populations, reducing property damage to vehicles, and saving the lives of drivers and passengers by reducing the number of wildlife-vehicle collisions.


A number of studies have been conducted to determine the effectiveness of wildlife corridors at providing habitat connectivity (by providing viable migration corridors) and reducing wildlife-vehicle collisions. The effectiveness of these structures appears to be highly site-specific (due to differences in location, structure, species, habitat, etc.) but crossings have been beneficial to a number of species in a variety of locations. Some of the wildlife crossing success stories are detailed below.

Banff National Park, Alberta, Canada

Banff National Park offers one of the best opportunities to study the effectiveness of wildlife crossings because the park contains a wide variety of species and is bisected by a large commercial road called the Trans-Canada Highway (TCH). To reduce the effects of the four-lane TCH, 24 wildlife crossings (22 underpasses and 2 overpasses) were built to ensure habitat connectivity and protect motorists (Clevenger, 2007). In 1996, Parks Canada developed a contract with university researchers to assess the effectiveness of the crossings. The past decade has produced a number of publications that analyze the crossings’ impact on various species and overall wildlife mortality (see Clevenger and Waltho, 2000, Clevenger, et al., 2001, and Clevenger, 2007).

Using a variety of technique to monitor the crossings over the last 25 years, scientists report that 10 species of large mammals (including deer, elk, black bear, grizzly bear, mountain lion, wolf, moose, and coyote) have used the 24 crossings in Banff a total of 84,000 times as of January 2007 (Clevenger, 2007). The research also identified a “learning curve” such that animals need time to acclimate to the structures before they feel comfortable using them. For example, grizzly bear crossings increased from 7 in 1996 to more than 100 in 2006, although the actual number of individual bears using the structures remained constant over this time at between 2 and 4 bears (Parks Canada, unpublished results). A similar set of observations was made for wolves, with crossings increasing from 2 to approximately 140 over the same 10 year period. However, in this case the actual number of wolves in the packs using the crossings increased dramatically, from a low of 2 up to a high of over 20 individuals. In continuation with these positive results, Clevenger et al (2001) reported that the use of wildlife crossings and fencing reduced traffic-induced mortality of large ungulates on the TCH by more than 80 percent. Recent analysis for carnivores showed results were not as positive however, with bear mortality increasing by an average of 116 percent in direct parallel to an equal doubling of traffic volumes on the highway, clearly showing no effect of fencing to reduce bear mortality (Hallstrom, Clevenger, Maher and Whittington, in prep). Research on the crossings in Banff has thus shown mixed value of wildlife crossings depending on the species in question. Parks Canada is currently planning to build 17 additional crossing structures across the TCH to increase driver safety near the hamlet of Lake Louise. Lack of effectiveness of standard fencing in reducing bear mortlity demontrates that additional measures such as wire 'T-caps' on the fence may be needed for fencing to mitigate effectively for bears (Hallstrom, Clevenger, Maher and Whittington, in prep).

Collier and Lee counties, Florida, USA

Twenty-four wildlife crossings (highway underpasses) and 12 bridges modified for wildlife have been constructed along a 40-mile stretch of Interstate 75 in Collier and Lee counties in Florida (Scott, 2007). These crossings are specifically designed to target and protect the endangered Florida panther, a subspecies of mountain lion found in the southeast United States. Scientists estimate that there are only 80-100 Florida panthers alive in the wild, making them one of the most endangered large mammals in North America (Foster and Humphrey, 1995). The Florida panther is particularly vulnerable to wildlife-vehicle collisions, which claimed 11 panthers in 2006 and 14 panthers in 2007 (Scott, 2007).

The Florida Fish and Wildlife Commission (FWC) has used a number of mitigation tools in an effort to protect Florida panthers and the combination of wildlife crossings and fences have proven the most effective (Scott, 2007). As of 2007, no panthers have been killed in areas equipped with continuous fencing and wildlife crossings and the FWC is planning to construct many more crossing structures in the future. The underpasses on I-75 also appeared to benefit bobcats, deer, and raccoons and significantly reduced wildlife-vehicle collisions along the interstate (Foster and Humphrey, 1995).

Underpasses, southern California, USA

Wildlife crossings have also been important for protecting biodiversity in several areas of southern California. In San Bernardino County, biologists have erected fences along Route 58 to compliment underpasses (culverts) that are being used by the threatened desert tortoise. Tortoise deaths on the highway declined by 93% during the first four years after the introduction of the fences, proving that even makeshift wildlife crossings (storm-drainage culverts in this case) have the ability to increase highway permeability and protect sensitive species (Chilson, 2003). Additionally, studies by Haas (2000) and Lyren (2001) report that underpasses in Orange, Riverside, and Los Angeles counties have drawn significant use from a variety of species including bobcats, coyotes, gray fox, mule deer, and long-tailed weasels. These results could be extremely important for wildlife conservation efforts in California’s Puente-Chino Hills, which have been increasingly fragmented by road construction (Haas, 2000).

Ecoducts, Netherlands

The Netherlands contains an impressive display of over 600 wildlife crossings (including underpasses and ecoducts) that have been used to protect populations of wild boar, red deer, roe deer, and the endangered European badger (US Humane Society, 2007). As of 2007, De Hoge Veluwe (the largest nature reserve in the Netherlands) contains three 50-meter ecoducts that are used to shuttle wildlife across busy roadways that transect the park. For example, two ecoducts crossing A50 (a highway cutting through the middle of the reserve) have reconnected two forests and were used by nearly five thousand deer and wild boar during a one year period (Bank et al., 2002). The Netherlands also boosts the world’s longest ecoduct/wildlife overpass called the Natuurbrug Zanderij Crailo (sand quarry nature bridge at Crailo) (Danby, 2004). The massive structure is over 800 meters long and spans a railway line, business park, river, roadway, and sports complex (Danby, 2004). Monitoring is currently underway to examine the effectiveness of this innovative project combining wildlife protection with urban development.

Slaty Creek Wildlife Underpass, Calder Freeway, Black Forest, Australia

The final case study of the effectiveness of wildlife crossings comes from an underpass built to minimize the ecological impact of the Calder Freeway as it travels through the Black Forest in Victoria, Australia. In 1997, the Victorian Government Roads Corporation built Slaty Creek wildlife underpass at a cost of $3 million (Abson and Lawrence, 2003). Scientists used 14 different techniques to monitor the underpass for 12 months in order to determine the abundance and diversity of species using the underpass (Abson and Lawrence, 2003). During the 12-month period, 79 species of fauna were detected in the underpass (compared with 116 species detected in the surrounding forest) including amphibians, bats, birds, koalas, wombats, gliders, reptiles, and kangaroos (Abson and Lawrence, 2003). The results indicate that the underpass could be useful to a wide array of species but the authors suggest that Slaty Creek could be improved by adding fences along Calder Freeway and by attempting to exclude introduced predators such as cats and foxes from the area.


Wildlife crossings appear to be highly effective at reducing wildlife-vehicle collisions and increasing road permeability for a wide variety of species (especially ungulates and some species of carnivores). The combination of wildlife crossings and roadside fencing has helped reduce the mortality of several keystone species such as elk and the Florida panther, but has proven ineffective for bears in some localities. These results merit additional research and development of these mitigation tools, both to enhance their effectiveness and expand their use to protect wildlife.

It remains unclear whether wildlife crossings can reduce the extensive habitat fragmentation caused by roads but preliminary results seem encouraging. The relationship between fragmentation and wildlife crossings will become more evident as conservation biologists begin to analyze DNA markers to determine the magnitude of gene flow across these structures. The research, implementation, and improvement of wildlife crossings will be critical in determining the usefulness of these structures. Money for protecting wildlife is limited, so conservationists need to act quickly to determine if wildlife crossings are the best way to maximize the impact of their time, effort, and resources.

Every year there are cases of stunned animals, such as moose and deer, which have been hit by cars and trucks on the highways. When these animals are hit, not all of them die, but some are just stunned. The drivers, not wanting to waste the food, bring the animals in to their cars and carry on with their journey. These animals wake up and can cause the drivers to crash. There were 11 cases last year of car crashes caused in this way.

Works cited

Abson, R. N. and R. E. Lawrence. 2003. Monitoring the use of the Slaty Creek Wildlife Underpass, Calder Freeway, Black Forest, Macedon, Victoria, Australia. Proceedings of the 2003 International Conference on Ecology and Transportation: 40-45.

Bank, F. G., C. L. Irwin, G. L. Evink, M. E. Gray, S. Hagood, J. R. Kinar, A. Levy, D. Paulson, B. Ruediger, R. M. Sauvajot, D. J. Scott, and P. White. 2002. Wildlife habitat connectivity across European highways. U. S. Department of Transportation: Federal Highway Administration: 1-45.

Beier, P. and R. F. Noss. 1998. Do habitat corridors provide connectivity? Conservation Biology 12:1241-1252.

Bennett, A. F. 1991. Roads, roadsides, and wildlife conservation: A review. Nature conservation: The role of corridors 2:99-118.

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

Clevenger, A. P. 2007. Highways through habitats: The Banff Wildlife Crossings Project. Transportation Research News 247:14-17.

Clevenger, A. P. and N. Waltho. 2000. Factors influencing the effectiveness of wildlife underpasses in Banff National Park, Alberta, Canada. Conservation Biology 14:47-56.

Clevenger, A. P., B. Chruszcz, and K. E. Gunson. 2001. Highway mitigation fencing reduces wildlife-vehicle collisions. Wildlife Society Bulletin 29:646-653.

Danby, D. 2004. A Green Latticework. website.

Foster, M. L. and S. R. Humphrey. 1995. Use of highway underpasses by Florida panthers and other wildlife. Wildlife Society Bulletin 23:95-100.

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

Hallstrom, W., A. P. Clevenger, A. Maher and J Whittington. 2008. Effectiveness of highway mitigation fencing for ungulates and carnivores. Journal of Applied Ecology - In Review.

Knapp, K. K., T. Oakasa, W. Thimm, E. Hudson, and C. Rathmann. 2004. Deer vehicle crash coutermeasure toolbox: A decision and choice resource. Wisconsin Department of Transportation, Madison.

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.

Primack, R. B. 2006. Habitat Destruction. Essentials of Conservation Biology (textbook) Ch. 9:189-193.

Reed, R. A., J. Johnson-Barnhard, and W. L. Baker. 1996. Contribution of roads to forest fragmentation in the Rocky Mountains. Conservation Biology 10:1098-1106.

Rich, A. S., D. S. Dobkin, and L. J. Niles. 1994. Defining forest fragmentation by corridor width: The influence of narrow forest-dividing corridors on forest-nesting birds in Southern New Jersey. Conservation Biology 8: 1109-1121.

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

Spellerberg, I. F. 1998. Ecological effects of roads and traffic: A literature review. Global Ecology and Biogeography 7:317-333.

The Humane Society of the United States website. 2007. Wildlife crossings: Wild animals and roads.

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