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At least 3.9 millionmiles of public roads crisscross the United States.
Wildlife experts say these roads impact animals in several ways:
Roadkill. Millions of vertebrates - birds, reptiles, mammals, and amphibians - are killed every year by vehicles traveling on America's roads. For example, roadkill has helped reduce the population of a federally endangered cat - the ocelot - to about 80 animals. Slow-moving animals like turtles and salamanders are at high risk of roadkill, especially when they try to cross a road to reach mating or nesting sites on the other side.
Wide-ranging large carnivores like wolves, grizzly bears, and mountain lions are also vulnerable, simply because they routinely have to cross a lot of roads.
Roadkill threatens humans as well as animals. 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 in vehicle repair every time they hit a deer.
[Photo of grizzley and car - Wide-ranging large carnivores like wolves and grizzly bears and slow-moving animals such as turtles and salamanders are particularly vulnerable to roadkill.]
Habitat loss. Over the centuries, 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 federally 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 federally endangered red-legged frog has disappeared from 99% of its habitat in California's Central Valley. Another federally endangered amphibian in California - the Arroyo toad - can only be found in remote areas of the state. Still another wildlife species - Attwater'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.
When highways "fragment" landscapes, they divide wildlife populations into smaller, more isolated units. Smaller populations are less stable and, over time, face extinction from predators or natural causes. They may also be more susceptible to inbreeding and to 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."
Impacts on fish populations are less obvious. We drive over culverts without even realizing there are streams underneath. Even if we can see the streams, we usually can't see the fish swimming in their waters.
Nonetheless, the impacts are real - and serious.
Anadromous fish - species that migrate from freshwater to saltwater and back to freshwater - are the most severely impacted by fish passage barriers. Most of the culverts we drive over today were built decades ago before we knew about the needs of these species.
[Graphic: Imperiled Wild Salmon Trout Stocks in the Pacific Northwest]
Ability to migrate upstream is a 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 because of dams, pollution, over-harvesting and other human-caused 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.
[Photo of a crossing structure]
Under the Transportation Equity Act for the 21st Century, or TEA-21, Federal Highway Administration funding support 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 for habitat connectivity measures.
In the following pages, you'll read about strategies being used to counteract roadkill and habitat loss and fragmentation. The strategies range from site-specific projects like goat bridges and bear underpasses to regional models that combine landscape ecology, conservation biology, and human safety concerns with long-range transportation planning.