Theoretical studies have shown that variation in density regulation strongly influences population dynamics1, yet our understanding of factors influencing the strength of density dependence in natural populations still is limited2. Consequently, few general hypotheses have been advanced to explain the large differences between species in the magnitude of population fluctuations3,4,5,6. One reason for this is that the detection of density regulation in population time series is complicated by time lags induced by the life history of species7,8 that make it difficult to separate the relative contributions of intrinsic and extrinsic factors to the population dynamics. Here we use population time series for 23 bird species to estimate parameters of a stochastic density-dependent age-structured model. We show that both the strength of total density dependence in the life history and the magnitude of environmental stochasticity, including transient fluctuations in age structure, increase with generation time. These results indicate that the relationships between demographic and life-history traits in birds9,10 translate into distinct population dynamical patterns that are apparent only on a scale of generations.
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We thank A. P. Møller for help in organizing this database and for comments on the manuscript. This study was supported by the Research Council of Norway (Strategic University Programme in Conservation Biology), the Natural Sciences and Engineering Research Council of Canada, the Institut Polaire francais-program 109 and the US National Science Foundation.
Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.
The estimates of the total strength of density dependence, the autoregression coefficients for different time lags, the stationary variance in the time series and the environmental variance (including transient fluctuations due to variation in the age-distribution) for the time series. The estimates of the life history characters clutch size, age at maturity and adult survival rates are also included. (PDF 21 kb)
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Sæther, B., Lande, R., Engen, S. et al. Generation time and temporal scaling of bird population dynamics. Nature 436, 99–102 (2005) doi:10.1038/nature03666
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