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Public Roads - March/April 2000

Linking Habitats and Reducing Roadkill

This article is an adaptation from CRITTER CROSSINGS: Linking Habitats and Reducing Roadkill, a brochure recently published by the Federal Highway Administration (FHWA). The publication describes transportation's effects on wildlife, and it highlights exemplary projects and processes that are helping to reduce the adverse effects.

Wildlife and Highways: An Overview

About 6.3 million kilometers (almost 4 million miles) of public roads crisscross the United States. Wildlife

Wide-ranging large carnivores, such as grizzly bears and wolves, and slow-moving animals, such as salamanders and turtles, are particularly vulnerable to becoming roadkill.
Photo Credit: Chuck Bartlebaugh

experts say these roads affect animals in several ways: roadkill, habitat loss, and habitat fragmentation.


Millions of animals - birds, reptiles, mammals, and amphibians - are killed every year by vehicles traveling on America's roads. For example, partially as a result of roadkills, the population of an endangered cat - the ocelot - has been reduced to about 80 animals.

Slow-moving animals, such as turtles and salamanders, are at high risk of becoming roadkill, especially when they try to cross a road to reach mating or nesting sites on the other side.

Wide-ranging, large carnivores, such as wolves, grizzly bears, and mountain lions are also vulnerable, simply because they routinely have to cross many roads.

Animals on roads are a risk to humans as well. Each year, more than 200 motorists are killed and thousands more are injured in animal-vehicle collisions, according to The Wildlife Society. The insurance industry estimates that the annual cost to society for these fatalities and injuries is $200 million. Individual motorists usually pay at least $2,000 for vehicle repair every time they hit a large animal.

Habitat Loss

Over the centuries, the habitat lost to wildlife has resulted not only from highway construction but also from timber harvesting, agricultural conversion, urban and residential development, and other factors.

Loss of habitat is permanent and severe. For example, the threatened grizzly bear is now confined to less than 2 percent of its former range south of Canada, and only two robust populations remain of the rare lynx that once prowled forested areas from Maine to Oregon.

The red-legged frog, which is on the federal endangered species list, has disappeared from 99 percent of its habitat in California's Central Valley. Another endangered amphibian in California, the Arroyo toad, can be found only in remote areas of the state. Still another wildlife species, Atwater's greater prairie chicken, is down to three isolated populations (42 animals) in Texas. The list goes on.

Habitat Fragmentation

Few species use all the patches of a landscape. Their survival depends on being able to move from one patch to another. Highways "fragment" landscapes and divide wildlife populations into smaller, more isolated units. Smaller populations are less stable. Over time, they may face extinction from predators or natural causes, and they may also be more susceptible to inbreeding and genetic defects.

Habitat fragmentation threatens all wildlife species that have to cross roads to meet their biological needs. "Forest carnivores are particularly vulnerable," says biologist Bill Ruediger of the U.S. Forest Service. "They're at risk because of their small populations, low reproduction rates, and large - even huge - home ranges."

An Uphill Climb for Fish

The effects on fish populations are less obvious. We drive over culverts without even realizing there are

In 1987, two tunnels were built in Amherst, Mass., to help salamanders migrate to and from the warm, fishless vernal pools (small temporary ponds) where they migrate every spring to mate and lay eggs. Two spotted salamanders, glossy black with two rows of bright yellow spots down their backs and tails, crawl safely through one of the tunnels under the highway.
Photo Credit: Scott Jackson

streams underneath. Even if we can see a stream, we usually can't see the fish swimming in the water. Nonetheless, the impacts are real and serious.

Anadromous fish - species that migrate from freshwater to saltwater and back to freshwater - are most severely affected by fish barriers. Most of the culverts we drive over today were built decades ago before we knew about the needs of these species.

Ability to migrate upstream is the critical issue for both anadromous and resident fish species. "Juveniles have a tougher time than adults," says Rose Owens, a biologist at the Oregon Department of Transportation. "They can't jump as high or sustain the same bursts of energy, and they're more sensitive than adults to turbulence and to changes in water temperature."

In the Pacific Northwest, the fish-passage issue is complicated by biology. Pacific salmon, unlike Atlantic salmon, exist as several species. Different populations spawn at different times and in different streams. What's more, Pacific salmon spawn only once and then die.

Fish populations in the Pacific Northwest have been steadily declining - not only because of transportation-linked barriers to upstream movement but also because of dams, pollution, over-harvesting, and other human disturbances. Populations have declined to such an extent that virtually all stocks of wild salmon and trout are federally listed in some region of the Pacific Northwest. For example, Chinook salmon stocks are federally listed in Puget Sound, in the Columbia and Sacramento Rivers, and in other watersheds.

Opportunities to Make a Difference

Under the Transportation Equity Act for the 21st Century (TEA-21), FHWA funding is available for wildlife crossings on both new and existing roads. Thanks to TEA-21 and an expanded "Transportation Enhancements" category, states and communities can get help not only for crossing structures but also for habitat connectivity measures.

"President Clinton has made protecting the environment one of his top priorities," said Federal Highway Administrator Kenneth R. Wykle. "TEA-21 offers an unprecedented opportunity for us to reduce highway impacts on wildlife."

Several strategies are being used to counteract roadkill, habitat loss, and fragmentation. The strategies range from site-specific projects, such as salamander tunnels and bear underpasses, to regional models that combine landscape ecology, conservation biology, and human safety concerns with long-range transportation planning.

For more information about "Critter Crossings," contact Paul Garrett at (303) 969-5772 or The full text of CRITTER CROSSINGS: Linking Habitats and Reducing Roadkill, is available on the FHWA Web site at".

CRITTER CROSSINGS: Linking Habitats and Reducing Roadkill was written by Ginny Finch, who is a program analyst in FHWA's Office of Natural Environment. Her other brochures include More Than Asphalt, Concrete, and Steel; Leaving A Place Better Than You Found It; Wetlands and Highways; and Environmental Research: Linking Transportation, the Environment, and the Future.

Bear Underpasses in Florida

When biologists and engineers from the Florida Department of Transportation (FDOT) and the Florida Fish and Wildlife Conservation Commission (FWC) designed the state's first underpass for black bears on a stretch of state Route 46 in Lake County, they had done their homework. They noted that bears had been using similar crossings built for Florida panthers on Alligator Alley (now I-75). They also observed bears routinely traveled across the targeted section of state Route 46 near County Road 433 to get to habitats within central Florida's Wekiva River Basin. And they learned bears were being hit by vehicles more often at this spot on SR 46 than at any other spot on the highway.

To ensure that bears could easily access the underpass from the south, FWC purchased a 16-hectare (40-acre) tract of land in the bears' travel corridor.

The SR 46 underpass built by FDOT is a bear-friendly, dirt-floor box culvert, 14.3 meters (47 feet) long by 7.3 meters (24 feet) wide by 2.4 meters (8 feet) high. Work crews elevated the two-lane road over the crossing to give skittish animals a clear view across to the other side. They also planted rows of pines in the open pasture on one side of the road to guide bears to the culvert entrance.

Bears did indeed use the SR 46 crossing. In fact, post-project research revealed that bears plus 12 other species, including bobcats, gray foxes, and whitetail deer, crossed through it.

Laurie Macdonald, field coordinator of Defenders of Wildlife and director of Defenders' "Habitat for Bears Campaign," sees the SR 46 crossing as an important step in connecting bear habitats. "The Florida black bear needs a half million acres of connected habitats to survive," she said. "The SR 46 underpass helps meet that challenge."

"Underpasses like this one, together with land acquisition and habitat protection, are tools we can use to minimize the impacts of highways on wide-ranging mammals," said Terry Gilbert, an FWC biologist and member of the SR 46 design team.

Gilbert and his colleagues have analyzed more than 20 years of data about black bears killed by motor vehicles. They identified and prioritized 15 "bearkill" problem areas within seven counties, in which at least 39 percent of Florida's bear roadkills occurred.

"Their research will help in the planning, project development, construction, and maintenance of existing and future highways in Florida," says FDOT ecologist Gary Evink.

For more information, contact Terry Gilbert at (850) 488-6661.

Studying Wildlife Linkage Areas in Washington State

The Northwest Forest Plan calls the Snoqualmie Pass/Interstate 90 corridor east of Seattle "a critical

Biologists and engineers from the Florida Department of Transportation and the Florida Fish and Wildlife Conservation Commission knew that black bears were using underpasses designed for endangered Florida panthers along Alligator Alley (I-75). So, when they discovered that black bears were being hit by vehicles more often at one spot on state Route 46 in Lake County than at any other place along the highway, they designed and built the state's first underpass for black bears at that spot.
Photo Credit: William J. Weber

connective link in the north-south movement of organisms in the Cascade Range." The corridor, which passes through old-growth forests, is a critical link because it separates the unique plant and animal species that live in the forests.

Early in 1998, the Washington State Department of Transportation (WSDOT) and the U.S. Forest Service (USFS) launched a cooperative research effort to study the effects of I-90 on wildlife movement. The researchers chose a 48-kilometer (30-mile) study area along I-90 from Snoqualmie Pass (and its popular ski resort) east to the town of Cle Elum on the edge of the Wenatchee National Forest. In many places, I-90 is eight lanes wide. Traffic averages 24,400 vehicles a day and is expected to grow to 41,400 vehicles a day by 2018.

Along the study section of I-90, dramatic changes in elevation and precipitation - snow depth can reach 10 meters at the pass but rarely exceeds 1 meter at Cle Elum - have created a variety of vegetation zones and associated wildlife communities. Wide-ranging, large carnivores, such as lynx and wolverines, have been reported, as well as little-known, old-forest species, such as the red-backed vole and the papillose tail-dropper.

"Many of our old-forest species hardly move at all," says USFS biologist Peter Singleton. "In fact, some spend their whole life on one log."

To learn about the wildlife in the study area, the researchers used a multiscale approach. They looked not only at where animals cross the highway, but how they get there in the first place. They analyzed habitats to determine where animals are likely to live and raise their young and which other habitats they may travel through. They looked at wildlife distribution both along and near the highway.

"It's like buying a house," says Singleton. "You look at everything - the city, the neighborhood, the street, even the color of the carpet."

The research team used Geographical Information System (GIS) technology to evaluate habitat connectivity

Fish passage structures help salmon and trout negotiate barriers on their journey to upstream spawning habitats.
Photo Credit: Washington Department of Fish and Wildlife

and to analyze the distribution of roadkill of deer and elk. The team used snow-tracking to document highway crossings and animal distribution along the highway. They used off-road, automatic cameras to identify wildlife found near the highway; and they monitored bridges and culverts to find out which species use these structures to cross the highway.

The team discovered that:

  • Small mammals - but not medium and large carnivores - use I-90 drainage culverts.
  • Three locations in the corridor - Easton Hill and the north and south ends of Keechelus Lake - are important habitat connectivity areas.
  • No substantial differences were detected between species found more than a mile from I-90 and those found within a mile of the highway.

Both the research results and methodology are valuable to transportation planners.

"This work has major implications for how we adapt existing crossing structures and how we build new ones," says Marion Carey, a biologist at WSDOT. "It will be a vital resource for us as we try to make highways safer for animals and humans."

For more information, contact Peter Singleton at (509) 662-4315 extension 226 or phsingle/