Evaluating the efficacy of protected habitat areas for the California spotted owl using long-term monitoring data

The USDA Forest Service has adopted a management strategy for the California spotted owl (Strix occidentalis occidentalis) in the Sierra Nevada that relies on protecting habitat (Protected Activity Centers [PAC]) around suspected owl territory centers. We discuss the history of the PAC concept and evaluate its efficacy by comparing owl core areas of use, derived from usage distributions based on long-term location data of territorial owls, with their associated PACs. The average size of core areas used by spotted owls (334.7 ac; SE = 40.2; N = 29; 95% usage distribution for roost and nest locations) was similar to the average PAC size (287.5 ac; SE = 4.3; N = 29; t = 1.16; P < 0.25; 28 df). The 50 and 90% usage distributions for owl use area were smaller than their corresponding PACs (t = 38.88, P < 0.0001, and 28 df; t = 2.31, P < 0.03, and 28 df, respectively). The spatial overlap between owl core areas of use and PACs was also high. The average proportions of each core area that coincided with a PAC area was 0.84, 0.70, and 0.61 for the 50, 90, and 95% usage distributions, respectively. Moreover, there were more owl locations found inside (x = 36.0; range, 8 -76; SE = 2.96) than outside (x = 6.9; range, 0 -26; SE = 1.03) of PACs (t = 9.289; P < 0.0001; 68 df). We concluded that PACs, even though derived through an ad hoc but reasoned method, appear to be a key element for conservation of California spotted owls because owls have used these areas over long periods of time (up to 24 years). We also suggest that location data collected during long-term monitoring programs may be useful for identifying core areas for habitat protection not only for spotted owls but also for other species.