USA — Bighorn sheep in northern Washington are not thriving the way they should and Washington State University faculty members are working to figure out why.
Bighorn sheep were reintroduced into the Sinlahekin Wildlife Area in the northern reaches of the state in the 1950s; today there are about 100, according to Mark Swanson, an ecosystem analyst and associate professor of landscape ecology and silviculture in WSUs new School of the Environment.
“The question is: What really is preventing these animals from achieving more historic populations? said Swanson.
While poaching and exposure to domestic ovine diseases have played a role, another key factor is how suppression of natural fires on the wildlife area has changed its landscape. Swanson, working on a larger bighorn sheep project with wildlife ecology Professor Lisa Shipley, recently received another round of funding from the Washington Department of Fish and Wildlife to assess changes in bighorn sheep habitat and develop ways to assess the effectiveness of ecological restoration in the Sinlahekin.
What is happening
Prior to intervention by new settlers, a natural cycle of forest fires as well as Native American burning practices on the wildlife area kept smaller trees and shrubs at bay. That preserved prime sheep habitat wide-open spaces with lots of room to graze and to see approaching predators, said Swanson. When that historic fire cycle was interrupted, those smaller trees and shrubs filled in the open spaces between larger trees and have encroached on open grazing spaces.
“If you look at historic photos of the certain places in the Sinlahekin and then look at current shots of those places, there is a dramatic difference, Swanson said. “You can see the grazing areas getting smaller and smaller.
The effect of that encroachment is three-fold, he added. In addition to reducing grazing areas, a closed forest canopy “shades out many of the grasses, shrubs and herbs that are staples of the sheep diet. In addition, closed forests provide much better cover for predators such as mountain lions. “That, obviously, makes the sheep much more vulnerable, Swanson said.
Over the past two years, Swanson, Shipley and their research teams have worked with state fish and wildlife biologists to capture and tag a sample of sheep with GPS radio collars to track their movements and use of habitats on the wildlife area and adjacent lands. That information has helped focus research on the prime habitat that remains.
“Well be assessing the demographic consequences of fire suppression on the trees in the wildlife area, and along with that, how bighorn sheep respond to those changes, Swanson said.
He and graduate student Tiffany Baker have set up 48 different “transects in the wildlife area and every 50 feet assessed:
Trees and their height, age and diameter Shrub density Grasses and forbs Percentage of forest cover Visual obstruction (percent of vegetation obstructing vision) Scat pellet counts for both bighorn sheep and other ungulates, such as mule deer
They will analyze that data to see if the habitat “is really functioning the way we want it to, Swanson said.
Swanson and Baker will then make recommendations on how to restore lost habitat, including the use of strategic forest thinning and prescribed, low-severity fires.
“We need to consider putting low severity fires back into the forest management mix, said Swanson.
Both he and Baker acknowledge that recommendation could meet with concern from adjacent landowners and others.
“Another component of our work will be educating the public that prescribed burning and thinning is really good for the landscape, Baker said.