New Vulnerability Assessment Identifies Top Threats to Iconic Spotted Owls in Marin County, CA

By Senior Avian Ecologist Renée Cormier, Point Blue Conservation Science

Marin County, CA supports one of the highest known densities of iconic northern spotted owls. And the local population is relatively stable, at least for now. As a result, Marin is an area of particular conservation interest.

© MARGARET BROWN

The northern spotted owl (Strix occidentalis caurina) is a threatened spotted owl subspecies on both federal and state endangered species lists. An iconic species of the forests of the Pacific Northwest, it’s been declining in vast parts of its range. Marin County, California, including Point Reyes National Seashore, Golden Gate National Recreation Area, and Muir Woods National Monument, is an exception. Marin supports one of the highest known densities of these owls. And the local population is relatively stable, at least for now. As a result, Marin is an area of particular conservation interest. But successful conservation and management requires understanding the threats species face, and how those threats might change over time.

Vulnerability assessments are a common approach to evaluating species’ exposure and sensitivity to threats. So Point Blue Conservation Science and National Park Service researchers recently put that approach into practice for the National Park Service and other Marin land managers trying to identify conservation priorities and make informed management decisions. Our vulnerability assessment identifies three top threats to northern spotted owls in Marin: climate change, competition with barred owls, and severe wildfire. We also propose research priorities and conservation strategies based on our findings.

How did we assess the vulnerability of northern spotted owls in Marin County?

We assessed the vulnerability of northern spotted owls in Marin to eight distinct threats: 1) competition with barred owls; 2) climate change; 3) wildfire; 4) sudden oak death; 5) other habitat loss/disturbance from urban development or fuels management; 6) noise disturbance; 7) rodenticide poisoning; and 8) disease. We conducted a review of the scientific literature, and surveyed local experts to help us rank the owl’s vulnerability to each threat.

What are the key takeaways?

We identified three threats to which Marin’s northern spotted owls are currently most vulnerable: climate change, competition with barred owls, and wildfire (specifically high-severity fires). The frequency and intensity of exposure to each of these threats is expected to increase in Marin over the next 30–50 years. We also identified rodenticides and sudden oak death as moderate threats. Habitat loss (from urban development and fuels management), noise disturbance, and disease were identified as relatively low threats. However, the cumulative impacts of multiple threats may cause significant harm, and uncertainty around the owl’s sensitivity to some threats highlighted research needs.

What conservation strategies do we recommend in the vulnerability assessment?

Based on our findings, we made several conservation recommendations for northern spotted owls. These include:

• Continuing to protect habitat for northern spotted owls and dusky-footed woodrats, their main prey species;

• Managing barred owls to maintain the current low numbers;

• Continuing to avoid noise disturbance to nesting northern spotted owls; and

• Engaging in community education and outreach to help reduce threats from rodenticides, noise, and other types of human disturbance.

We also highlight that long-term northern spotted owl monitoring is essential for understanding emerging threats and detecting early warning signs of decline.

We identified three threats to which Marin’s northern spotted owls are currently most vulnerable: climate change, competition with barred owls, and high-severity wildfire. The frequency and intensity of exposure to each of these threats is expected to increase in Marin over the next 30–50 years.

© MARGARET BROWN

What data gaps and research needs did we find?

We identified research needs and uncertainties around how northern spotted owls and their prey in Marin might respond to changing weather and climate, including extreme events. How they might respond to habitat change, such as from wildfire fuels management and sudden oak death, is also uncertain. We also identified research needs around northern spotted owl responses to noise and other human disturbances.

We encourage further collaboration to address the uncertainties and research needs identified in this assessment. It is our hope that together we can further develop these conservation recommendations into an actionable conservation strategy for northern spotted owls in Marin County.

For more information

Cormier RL, Brown MM, Humple DL, Merkle WW, Press DT, Dybala KE. 2024. Vulnerability assessment and conservation recommendations for the northern spotted owl in Marin County, California. Science Report. NPS/SR—2024/148. National Park Service. Fort Collins, Colorado. https://doi.org/10.36967/2303475