Spotted Owls Face Genetic Bottleneck

June 25, 2009 -- The northern spotted owl has been a controversial conservation icon for years -- ever since large swaths of old-growth forest in the Pacific Northwest were set aside to protect the threatened bird 15 years ago. That decision angered logging companies and forced them to take a financial hit.

Still, despite the extra protection, spotted owl populations have continued to decline. A new study helps explain why: With a drop in numbers, the birds have lost genetic diversity.

In addition to habitat loss and competition from other owl species, this type of genetic bottleneck makes the species more vulnerable to inbreeding problems and less resilient in the face of disease, climate change, and other challenges.

"It provides additional evidence that spotted owls are not doing great right now," said Chris Funk, a population geneticist at Colorado State University in Fort Collins. "It also points out that we might have to think about another threat to spotted owls, which is the threat from loss of genetic variation."

Northern spotted owls live in old-growth forests throughout the Pacific Northwest, from southwest British Columbia to northwest California. The owls have brown feathers with white spots, deep dark eyes, and a nearly 4-foot wingspan. Their distinctive hooting helps define the untouched forests of the Pacific Northwest.

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"It's a species that a lot of people like and enjoy," said Robert Fleischer, an evolutionary and conservation geneticist at the Smithsonian National Zoo in Washington D.C. "It's hard to put a value on something like that, but it would be a far less rich experience to have Pacific Northwest woods that were lacking spotted owls."

Development has not been kind to northern spotted owls, which need lots of space and are sensitive to disturbances. The owls were listed as threatened under the Endangered Species Act in 1990. In 1994, the Northwest Forest Plan designated 77 percent of the bird’s range in federal forest land as off-limits to logging.

Yet, the owl's numbers have been dropping by 3 to 4 percent each year. One reason is competition with the barred owl, whose range has been expanding. Habitat loss remains a problem, too. Funk and colleagues suspected that genetic bottlenecking might also add to the owl's woes.

For their study, the researchers scanned DNA from more than 350 northern spotted owls across the animal’s range. Then, they ran a bottleneck test, which looks for the loss of certain rare gene-forms, or alleles. When a population shrinks, chances rise that uncommon alleles will disappear.