For long-lived iteroparous vertebrates that annually produce few young, life history theory predicts that reproductive output (R) and juvenile survival should influence temporal variation in population growth rate (λ) more than adult survival does. We examined this general prediction using 15 years of mark-recapture data from a population of California spotted owls (Strix occidentalis occidentalis). We found that survival of individuals ≥1 year old (φ) exhibited much less temporal variability (CV = 0.04), where CV is coefficient of variation, than R (CV = 0.83), and that R was strongly influenced by environmental stochasticity. Although λ was most sensitive (ê log-transformed sensitivity) to φ (ê = 0.77), and much less sensitive to either R (ê = 0.12) or juvenile survival (survival rate of owls from fledging to 1 year old; ê = 0.12), we estimated that R contributed as much as φ to the observed annual variability in λ. The contribution of juvenile survival to variability in λ was proportional to its ê. These results are consistent with the hypothesis that natural selection may have favored the evolution of longevity in spotted owls as a strategy to increase the probability of experiencing favorable years for reproduction. Our finding that annual weather patterns that most affected R (temperature and precipitation during incubation) and φ (conditions during winter related to the Southern Oscillation Index) were equally good at explaining temporal variability in λ supports the conclusion that R and φ were equally responsible for variability in λ. Although currently accepted conservation measures for spotted owl populations attempt to enhance survival, our results indicated that conservation measures that target R may be as successful, as long as actions do not reduce φ.
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Acknowledgements We thank M. Crozier, M. Bond, Z. Peery and our field assistants for help in gathering demographic data, and B. Heald, F. Schurr, and C. Rambeau from University of California Berkeley Blodgett Forest Experimental Station for logistical help during fieldwork, and for their continuous support of the project. We thank L. Marschall, G. Zimmerman, A. Franklin, D. Grandmaison, and an anonymous reviewer for many helpful comments, and D. Andersen, L. Frelich, and J. Nichols for helpful suggestions on earlier drafts. We thank the U.S. Forest Service for funding this study (contract no. FS-53-9158-00-EC14 to R. J. G.).
- Fitness components
- Life history strategy
- Spotted owl
- Temporal variability