Human-caused changes in the landscape surrounding Olympic National Park, Washington, may be facilitating a park invasion of barred owls, a competitor of the federally threatened northern spotted owl.

scott_gremel@nps.gov
Wildlife Biological Technician,
Olympic National Park, Washington


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Native to eastern forests, the barred owl (Strix varia) has moved into the Pacific Northwest over the last several decades, likely as a result of human-caused changes in the landscape. The barred owl is closely related to the threatened northern spotted owl (Strix occidentalis caurina, the subspecies found in Olympic National Park, Washington), but is larger, more aggressive, and better adapted to a range of habitats. As recently as 10 years ago, the barred owl was rare in Olympic, found mostly adjacent to logged areas along the park boundary and in broad, naturally disturbed river floodplains at lower elevations. During monitoring activities in 2000, crews documented barred owls at 18 sites, many of which formerly supported northern spotted owls. More than 10 of the 53 currently monitored northern spotted owl sites are now unoccupied, or the northern spotted owls were displaced 750 meters or more following the first documented use of the site by the barred owl. This biological invasion may prove to be the primary threat to the northern spotted owl in otherwise protected landscapes such as national parks.

As with many of the more subtle ecological changes occurring in parks, the extent of this problem was revealed by a long-term monitoring program, in this case one focused primarily on another question. In 1993, President Clinton released the Northwest Forest Plan to address disagreements about the management of federal forestlands in the Pacific Northwest. The plan mandates “effectiveness monitoring” to measure whether the various federal entities are achieving the goal of protecting enough habitat to support viable populations of species that are dependent upon late-successional forest. Northern spotted owl monitoring sites within the park, together with those monitored by the USDA Forest Service on the Olympic Peninsula, constitute the Olympic Demographic Study Area. This is one of eight study areas where rates of reproduction and survival are being investigated throughout the range of the northern spotted owl through 2002. In 2002, planners hope to replace this intensive and costly monitoring with a model that would predict trends in northern spotted owl populations by tracking changes in habitat. The barred owl complicates these efforts by increasing the uncertainty surrounding estimates of northern spotted owl numbers in protected forests. Future monitoring at Olympic will address factors that predict which northern spotted owl sites are most vulnerable to displacement. This will allow barred owl competition to be incorporated into future habitat models.

Although designed to monitor demographic rates, this long-term study also offers insight into the natural history of the northern spotted owl. Olympic National Park contains the largest unfragmented area of suitable habitat within the range of this species. As such, it provides an exceptional control area against which to compare more highly managed forests and to test hypotheses about the effects of barred owl competition.

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Last Updated: 06/17/2001
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