Why Was This Indicator Chosen?
Iconic and charismatic Northern Spotted Owls (Strix occidentalis caurina) are good indicators of Marin County’s forest health, as their success depends on the presence of diverse, robust evergreen forest ecosystems in this area. Northern Spotted Owls are important upper-level predators that feed on a variety of rodents, especially dusky-footed woodrats (Neotoma fuscipes).
One Tam land management agencies have a wealth of inventory and long-term monitoring data on this species covering most of Marin County. Data on long-term trends in Northern Spotted Owl territory occupancy, reproductive success, and nesting habitat preferences help managers track population trends, avoid nesting season disturbances, and evaluate the impacts of potential threats including encroaching Barred Owls (S. varia), Sudden Oak Death (SOD), and climate change.
What is Healthy?
A healthy population of Northern Spotted Owls on Mt. Tam would remain stable or increase over time. Additionally, existing high levels of pair occupancy and fecundity would be maintained within the observed normal range of variability, or above long-term average values based on monitoring data. Lastly, the threat from Barred Owls would remain low.
What Are the Biggest Threats?
- Barred Owls, which negatively impact Northern Spotted Owl reproduction and survival
- Habitat quantity and quality, as Northern Spotted Owls will nest in areas of relatively high recreational use and residential areas where they may be exposed to noise disturbances and rodenticides
- The effects of SOD on forest structure and dusky-footed woodrat abundance
What is the Current Condition?
Listed as threatened in 1990 under the Endangered Species Act, Northern Spotted Owl numbers appear to be dramatically decreasing across their range, which extends from southern British Columbia to Marin County, California (Dugger et al., 2016). In contrast, Marin County’s Northern Spotted Owl population appears stable (Ellis, 2016; Cormier 2015).
Based on the metrics used for this health assessment, Northern Spotted Owls in the One Tam area of focus are in Good condition. Pair occupancy of territories ranged from 74–97% of nests surveyed, depending on the year, with an average pair occupancy of 87%. Fecundity has been variable, but within an acceptable range, and there are currently no confirmed Barred Owls within monitored areas in Marin County (Ellis, 2016).
What is the Current Trend?
This species is showing No Change in pair occupancy of breeding territories and fecundity, and an improvement with the current lack of Barred Owls in Marin County.
How Sure Are We?
Confidence is High, as Northern Spotted Owls are monitored on an annual basis within and adjacent to Mt. Tam on both National Park Service and California State Park lands, as well as by Point Blue Conservation Science on and adjacent to Marin Municipal Water District and Marin County Parks lands.
What is This Assessment Based On?
- Inventories begun in the late 1980s (unpublished data), and continued in the early 1990s, with more complete inventories completed in 1997 and 1998, and again in 2006 for National Park Service and California State Park lands (Hatch et al., 1999, Jensen et al., 2007)
- Annual monitoring of Northern Spotted Owls in Marin County since 1999, with the National Park Service covering federal and state park lands and Point Blue monitoring on Marin Municipal Water District and Marin County Parks property
What Don’t We Know?
Key information gaps include:
- The effect of SOD on Northern Spotted Owl foraging or its primary food item, the dusky-footed woodrat
- The impacts of climate change on Northern Spotted Owl fecundity, survivorship, or habitat
- Factors affecting fecundity in the One Tam area of focus such as weather and climate, landscape and habitat factors, and any future presence of Barred Owls
- Dispersal patterns of juveniles
- Dusky-footed woodrat abundance across Northern Spotted Owl sites
- Survivorship data (not collected since 2003)
Cormier, R. L. (2015). Northern spotted owl monitoring on Marin County Open Space District and Marin Municipal Water District Lands (Unpublished report). Petaluma, CA: Point Blue Conservation Science.
Dugger, K., Forsman, E., Franklin, A., Davis, R., White, G., Schwarz, C., … Sovern, S. (2016). The effects of habitat, climate, and barred owls on long-term demography of northern spotted owls. The Condor, 118 (1), 57-116. Available from: http://www.bioone.org/doi/10.1650/CONDOR-15-24.1.
Ellis, T. (2016). Monitoring northern spotted owls on federal lands in Marin County, California (2014-2015 report currently in preparation). Natural Resources Technical Report NPS/SFAN/NRTR—2015/XXX. Fort Collins, CO: National Park Service.
Hatch, D., Allen, S., Geupel G., & Semenoff-Irving, M. (1999). Northern spotted owl demographic study Marin County, California (Unpublished annual report). San Francisco, CA: National Park Service.
Jensen, H. J., Adams, D. B., & Merkle, W. W. (2007). Northern spotted owl inventory on federal lands in Marin County (2006 annual report). Natural Resources Technical Report NPS/PWR/SFAN/NRR—2007/004. Sausalito, CA: San Francisco Bay Area Network, Golden Gate National Recreation Area, Fort Cronkhite. Available from: http://www.sfnps.org/download_product/1216/0.
U.S. Fish and Wildlife Service. (2011). Revised recovery plan for the northern spotted owl (Strix occidentalis caurina). Portland, OR: U.S. Fish and Wildlife Service. Available from: https://www.fws.gov/ecos/ajax/docs/recovery_plan/NSO%20Final%20Rec%20Plan%20051408.pdf.
Wiens, J.D., Anthony, R. G., & Forsman, E. D. (2014). Competitive interactions and resource partitioning between northern spotted owls and barred owls in western Oregon. Wildlife Monographs, 185(1), 1-50. Available from: http://onlinelibrary.wiley.com/doi/10.1002/wmon.1009/abstract.