Population dynamics of the California spotted owl (Strix occidentalis occidentalis): A meta-analysis
Citation
Franklin, A. B., R. J. Gutiérrez, J. D. Nichols, M. E. Seamans, G. C. White, G. S. Zimmerman, J. E. Hines, T. E. Munton, W. S. LaHaye, J. A. Blakesley, G. N. Steger, B. R. Noon, D. W. H. Shaw, J. J. Keane, T. L. McDonald, and S. Britting (2004). “Population dynamics of the California spotted owl (Strix occidentalis occidentalis): A meta-analysis”. In: Ornithological Monographs. DOI: 10.2307/40166799. URL: http://www.jstor.org/stable/40166799.
Number of Citations: 112
Keywords
spotted_owl
Abstract
We conducted a meta-analysis to provide a current assessment of the population characteristics of California Spotted Owls (Strix occidentalis occidentalis) resident on four study areas in the Sierra Nevada and one study area in southern California. Our meta-analysis followed rigorous a priori analysis protocols, which we derived through extensive discussion during a week-long analysis workshop. Because there is great interest in the owl’s population status, we used state-of-the-art analytical methods to obtain results as precise as possible.
Our meta-analysis included data from five California study areas located on the Lassen National Forest (1990–2000), Eldorado National Forest (1986–2000), Sierra National Forest (1990–2000), Sequoia and Kings Canyon national parks (1990–2000), and San Bernardino National Forest (1987–1998). Four of the five study areas spanned the length of the Sierra Nevada, whereas the fifth study area encompassed the San Bernardino Mountains in southern California. Study areas ranged in size from 343 \(km^2\) (Sequoia and Kings Canyon) to 2,200 \(km^2\) (Lassen). All studies were designed to use capture-recapture methods and analysis. We used survival in a meta-analysis because field methods were very similar among studies. However, we did not use reproduction in a meta-analysis because it was not clear if variation among individual study-area protocols used to assess reproductive output of owls would confound results. Thus, we analyzed fecundity only by individual study area. We examined population trend using the reparameterized Jolly-Seber capture-recapture estimator (\(\lambda\)).
We did not estimate juvenile survival rates because of estimation problems and potential bias because of juvenile emigration from study areas. We used mark-recapture estimators under an information theoretic framework to assess apparent survival rates of adult owls. The pooled estimate for adult apparent survival for the five study areas was 0.833, which was lower than pooled adult survival rates (0.850) from 15 Northern Spotted Owl (S. o. caurina) studies. Estimates of survival from the best model on the Lassen (\(\phi\) = 0.829, 95% confidence intervals [CI] = 0.798 to 0.857), Eldorado (\(\phi\) = 0.815, 95% CI = 0.772 to 0.851), Sierra (\(\phi\) = 0.818, 95% CI = 0.781 to 0.850), and San Bernardino (\(\phi\) = 0.813, 95% CI = 0.782 to 0.841) were not different. However, the Sequoia and Kings Canyon population had a higher survival rate (\(\phi\) = 0.877, 95% CI = 0.842 to 0.905) than the other study areas. Management history and forest structure (e.g. presence of giant sequoia [Sequoiadendron giganteum]) on the Sequoia and Kings Canyon study area differed from all other study areas. There appears to be little or no evidence for temporal variation in adult apparent survival on any of the study areas.
Although we did not directly compare fecundity, estimates were highly variable among years within all study areas (CV of temporal process variation = 0.672–0.817). Estimates for fecundity among the study populations were Lassen (\(\bar{b}\) = 0.336, SE = 0.083), Eldorado (\(\bar{b}\) = 0.409, SE = 0.087), Sierra (\(\bar{b}\) = 0.284, SE = 0.073), Sequoia and Kings Canyon (\(\bar{b}\) = 0.289, SE = 0.074), and San Bernardino (\(\bar{b}\) = 0.362, SE = 0.038). During most years, the Sierra Nevada populations showed either moderate or poor fecundity. However, 1992 appeared to be an exceptional reproductive year for owls in the Sierra Nevada. In contrast, the San Bernardino population had less variable reproduction (CV of temporal process variation = 0.217), but experienced neither the exceptional reproduction of 1992 nor the extremely poor years that characterized all of the Sierra Nevada study areas. Because fecundity may be influenced by weather patterns, it was possible that the different weather patterns between southern California and the Sierra Nevada accounted for that difference.
Except for Eldorado, all estimates for \(\lambda\) were <1.0, but none was different from \(\lambda\) = 1.0 given the 95% confidence intervals (Lassen [\(\bar{\lambda}\) = 0.985, SE = 0.026]; Eldorado [\(\bar{\lambda}\) = 1.042, SE = 0.047]; Sierra [\(\bar{\lambda}\) = 0.961, SE = 0.024]; Sequoia and Kings Canyon [\(\bar{\lambda}\) = 0.984, SE = 0.047]; San Bernardino [\(\bar{\lambda}\) = 0.978, SE = 0.025]). However, additional evidence (in the form of realized population change based on \(\hat{\lambda}_t\)) strongly suggested that the Sierra population declined during the study period. Estimated trends in \(\lambda_t\) for the Eldorado and Sierra study areas were negative. Thus, we could not distinguish definitively between alternatives that the populations were stationary or that the estimates of \(\lambda_t\) were not sufficiently precise to detect declines on four of the study areas (Eldorado, Lassen, San Bernardino, and Sequoia and Kings Canyon).
Results of the trend analyses do not allow strong inference about the decline of the populations. Because \(\lambda_t\) reflects changes in owl numbers on the study areas (i.e. it integrates emigration, immigration, birth, and death rates), it does not allow inference about the larger populations in which those local populations are imbedded. That is, it is possible that local populations could be producing fewer young but are enhanced by immigration from surrounding areas. It also is possible that the conditions within the study areas may have been better, in terms of habitat loss, than surrounding areas because a Spotted Owl conservation strategy was imposed in the national forest study areas. That may be particularly true where high amounts of private land surrounded study areas (e.g. Lassen, Eldorado). The relatively low survival rates coupled with trend estimates that were either declining or <1.0 suggest a cautious approach to developing conservation strategies for the California Spotted Owl until further analyses can be conducted that couple climatic and habitat conditions with population parameters, such as adult survival and fecundity.