Skip to content U.S. Department of Transportation/Federal Highway AdministrationU.S. Department of Transportation/Federal Highway Administration
Office of Planning, Environment, & Realty (HEP)
[Text only]
Paw Print Wildlife and Highways: An Overview Tortoise Underpasses Badger Tunnels Four Tools to Assess Wildlife Linkage Areas Programs to Remove Fish Passage Barriers Bear Underpasses Salamander Tunnels Passages for Large Mammals Goat Underpasses Computer Model Highway-Wildlife Relationships Amphibian-Reptile Wall and Culverts An Overpass for Animals and Humans
PreviousPrevious Table of Contents NextNext

Four Tools to Assess Wildlife Linkage Areas

Photo of a wolverine The study area included wide-ranging animals like the wolverine (left) and less-mobile animals like the papillose tail-dropper (right). The powerfully-built, dark brown wolverine looks like a small bear (except for its bushy tail), but it acts and moves like a weasel. Human disturbance has reduced its vast historic range in the U.S. to the northern Rockies in Montana, Wyoming, and Idaho.
Photo of a papillose tail-dropper The papillose tail-dropper is a 1/2-inch-long (12-millimeter-long) slug, brown with bumps ("papilla") and black spots. It can drop and regenerate its tail as a defensive tactic against predators. Old forests offer the tail-dropper the dampness and food (fungi) it needs.

The President's Northwest Forest Plan calls the Snoqualmie Pass-Interstate 90 corridor east of Seattle "a critical connective link in the north-south movement of organisms in the Cascade Range." The corridor is considered a "critical" link because it passes through old-growth forests, separating the unique plant and animal species that live in them.

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 30-mile (48.3-km) 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 Forest. In many places, I-90 is 8 lanes wide; traffic averages 24,400 vehicles a day and is expected to grow to 41,400 a day by 2018.

Dramatic changes in elevation and precipitation (snow depth can reach 30 feet - 9 meters - at the pass but rarely exceeds 3 feet at Cle Elum) have created a variety of vegetation zones and associated wildlife communities. Wide-ranging large carnivores like lynx and wolverine 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 multi-scale 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 or simply 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 GIS technology to evaluate habitat connectivity and analyze deer and elk roadkill distribution; snow-tracking to document highway crossings and animal distribution along the highway; off-road automatic camera surveys to identify wildlife found near the highway; and monitoring of bridges and culverts to find out what species use these structures to cross the highway.

Here's what the team of researchers discovered:

  • 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; and
  • There are no substantial differences between species detected more than a mile from I-90 and those found within a mile of it.
Photo of a deer at a culvert Photo of a woodrat at a culvert
A mule deer (left) and a bushy-tailed woodrat (right) approach a culvert under I-90.

Transportation planners will be able to use not only this information but the research methodology itself - a methodology that can be used in I-90 and in other transportation corridors in the Pacific Northwest.

"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

Previous | Table of Contents | Next
Updated: 7/18/2012
HEP Home Planning Environment Real Estate
Federal Highway Administration | 1200 New Jersey Avenue, SE | Washington, DC 20590 | 202-366-4000