It sounds like the start of a joke: “How would the Douglas squirrel cross the road?”
For the four scientists from Portland State University and Metro standing on the side of the bustling Tualatin-Sherwood Highway, the answer isn’t funny.
“They’re either not going to do that or they’re going to get killed,” said Lori Hennings, a senior natural resources scientist at Metro.
It’s a similar situation for a number of native wildlife and plants who can’t cross highways, bridges and urban development to connect from one piece of habitat to another to find food or breed.
But in an effort to improve habitat access by wildlife, the team of four scientists has embarked on a multiple-year effort to develop methods to map out and assess the quality of biodiversity corridors through computer mapping and on-the-ground verification. The Metro Regional Habitat Connectivity project is aided by a five-year, $125,000 contribution from Metro with money from the voter-approved 2013 parks and natural areas levy.
The project’s work is focused on the Tonquin area between the Tualatin and Willamette rivers, from the Tualatin River National Wildlife Refuge near Sherwood to Metro’s Graham Oaks Nature Park in Wilsonville. The hope is that the corridor mapping methods, when complete, will provide a tool for planners, scientists and others to design projects that are friendlier to wildlife and incorporate pathways for wildlife to move around.
“If you’re isolated in habitat, then the population can support only so many individuals,” said Catherine de Rivera, an associate professor of environmental science at Portland State. “To keep a population sustainable in an area over time, you have to maintain genetic diversity and you need individuals to be able to colonize.”
Biodiversity corridors work much like the roads and bridges that people rely on to find food, move to different areas and find shelter. Similarly, wildlife and plants rely on biodiversity corridors of habitat – trees, shrubs, streams, wetlands and others – to move between habitats and to find food and mating partners.
As roads and buildings get built, wildlife are locked into smaller patches of habitat, often times with no way of moving to another patch of habitat without risking their lives by getting run over by cars or other such challenges.
On the sidewalk along the Tualatin-Sherwood Highway, 16-wheeler trucks and cars whiz past at 45 mph on one side.
On the other side of the sidewalk, Rock Creek flows out from a culvert underneath the highway into a large natural area full of willow, ash and cottonwood trees. A common yellowthroat and dozens of other songbirds tweeted into the warm air of a sunny summer morning.
A little ways away, a recently deceased mourning dove lies at the base of some blackberry bushes near the sidewalk. The scientists think it might’ve been the victim of an encounter with a truck traveling on the highway.
The scientists spent 2014 using computer-mapped streams, trees and wetlands to model where they think wildlife species are accessing existing corridors between important habitat areas.
They chose several surrogate species that are representative of different types of animals and habitats. For instance, Northern red-legged frogs represent other pond-breeding amphibians that rely on wetlands to breed but that also need upland habitat. Beavers represent animals relying on riparian habitat. And the Douglas squirrel, which moves by jumping from branch to branch, represents animals that rely on canopies where trees touch one another.
But modeling goes only so far. The scientists are spending 2015 out in the field making sure their maps and models are accurate. The work in the field gives the scientists a chance to assess the current habitat conditions and whether a surrogate species could make it across a barrier.
Along the Tualatin-Sherwood Highway, the scientists note that it would be possible for red-legged frogs to cross where Rock Creek goes under the highway.
“But that’s a long way for a little Douglas squirrel to get to the trees,” says Martin Lafrenz, an associate professor of geography at Portland State. “This is a hard barrier.”
Following Rock Creek south, the scientists stumble on a vast field of dense, thick invasive reed canarygrass.
“It definitely has restoration potential,” says Leslie Bliss-Ketchum, a co-leader of the connectivity project and a Ph.D. candidate at Portland State who has studied biodiversity corridors. “I think for the frog, it’s more challenging because it’s hard to move. Accessing the upland areas and getting down here through the reed canarygrass would be hard.”
Designing projects with wildlife-friendly corridors and features can be done – and it works. A short drive away in Wilsonville is a 1.5-mile stretch of Boeckman Road that opened in 2008. The roadway features 13 undercrossing structures of different sizes, including a 400-foot section of habitat beneath a new bridge that’s nearly six feet tall.
Bliss-Ketchum maintains a 200-foot long strip of sand below the bridge that captures the footprints of many of the animals that use the undercrossing. In the 24 hours since she last raked over the sand, snakes, red-legged frogs, deer, river otters and other animals have all crossed through on their way from one habitat to the next.