When the Northern Spotted Owl was protected under the Endangered Species Act in 1990, the primary threat to the species was the loss of the old-growth forest it depends on. However, new research published in The Condor: Ornithological Applications shows that the Northern Spotted Owl population in Washington's Mount Rainier National Park has declined sharply in the past two decades despite the long-term preservation of habitat within the park. The culprit? The spread of Barred Owls, a closely related, competing species that has moved into Spotted Owls' range from the east.
Biologists have seen Barred Owls in Spotted Owl territories within the national park more and more frequently since Spotted Owl surveys began in 1997. For their new study, Oregon Cooperative Fish and Wildlife Research Unit's Anna Mangan, the National Park Service's Tara Chestnut, and their colleagues analyzed two decades' worth of data from these surveys. "We found that Spotted Owls now occupy 50% fewer territories in the park than they did 20 years ago when the study began, despite the lack of habitat disturbance," says Chestnut. "Spotted Owls were less likely to be present in territories where Barred Owls were detected, and if Spotted Owls were there, sharing space with Barred Owls made them less likely to breed. Only 18 adult Spotted Owls were detected in the study area in 2016, down from a high of 30 owls in 1998."