The three-year study – conducted in western Oregon through a research partnership including the U.S. Geological Survey and OSU –also confirms that barred owls not only use similar forest types and prey species as spotted owls, but also that a high density of barred owls can reduce the amount of those resources available to spotted owls.
"Interactions between invasive and native species can be multifaceted and complex, with the stakes being even higher when the native species is already threatened with extinction," explained USGS Director Marcia McNutt. "Careful scientific observation and analysis can tease out the critical areas of conflict or competition, the first step in finding solutions."
The northern spotted owl was designated as threatened under the Endangered Species Act in 1990. In recent years, the barred owl has expanded its range from eastern into western North America, where its geographic range now overlaps the entire range of the northern spotted owl.
Barred owls also have become more common than spotted owls in the forests of western Oregon, according to David Wiens, the USGS author of the study. Within the study area at least 82 pairs of barred owls were identified but only 15 pairs of spotted owls. The probability that spotted owls survived from one year to the next was 81 percent compared to 92 percent for barred owls, and barred owls produced more than six times as many young as spotted owls.
The value of old forest habitat for spotted owls was further demonstrated by the study. Both species frequently used patches of old conifer forest or stands of hardwood trees along streams while hunting for food and roosting, and both species survived better when there were greater amounts of old conifer forest within their territories.
The study occurred in the central Coast Range of western Oregon where barred owl populations have steadily increased over the past two decades. Of all of the owls identified in the study area, Wiens captured a sample group and outfitted 29 spotted owls and 28 barred owls with radio transmitters. He monitored the interactions among the radio-marked owls and how the two species used resources. The forested area where the study occurred included 52 sites that were formerly occupied by pairs of spotted owls.
"Despite two decades of dedicated management efforts, northern spotted owl populations have continued to decline throughout much of their range," said Eric Forsman, a Forest Service researcher who also participated in the study. "This study suggests that conservation of old forest habitat is still a critical need for spotted owls, so we will continue to work with our research and management partners to collect information and explore options for management."
This information is the result of a research partnership led by the U.S. Geological Survey. In addition to OSU, the partnership included other agencies of the U.S. Department of the Interior—the U.S. Fish and Wildlife Service, National Park Service and the Bureau of Land Management—as well as the U.S. Forest Service, Oregon Department of Forestry, and Boise State University.
In February 2012, the U.S. Fish and Wildlife Service released a draft Environmental Impact Statement that outlines options for experimental removal of barred owls from certain areas throughout the spotted owl's range to test the effect of such removal on spotted owl population trends. The Service is considering combinations of both lethal and non-lethal (capturing and relocating or placing in permanent captivity) methods for removing barred owls.
The full report, Competitive Interactions and Resource Partitioning between Northern Spotted Owls and Barred Owls in Western Oregon, is available as an Oregon State University doctoral dissertation.