Abstract
A major debate in ecology concerns the relative importance of intrinsic factors and extrinsic environmental variations in determining population size fluctuations1,2,3,4,5,6. Spatial correlation of fluctuations in different populations caused by synchronous environmental shocks2,7,8 is a powerful tool for quantifying the impact of environmental variations on population dynamics8,9. However, interpretation of synchrony is often complicated by migration between populations8,10. Here we address this issue by using time series from sheep populations on two islands in the St Kilda archipelago11,12,13. Fluctuations in the sizes of the two populations are remarkably synchronized over a 40-year period. A nonlinear time-series model shows that a high and frequent degree of environmental correlation is required to achieve this level of synchrony. The model indicates that if there were less environmental correlation, population dynamics would be much less synchronous than is observed. This is because of a threshold effect that is dependent on population size; the threshold magnifies random differences between populations. A refined model showsthat part of the required environmental synchronicity can be accounted for by large-scale weather variations. These results underline the importance of understanding the interaction between intrinsic and extrinsic influences on population dynamics14.
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Acknowledgements
We thank Scottish Natural Heritage and the National Trust for Scotland for permission to work on St Kilda, and their staff for assistance, support and encouragement; J. M. Pilkington, A. McColl, A. Robertson and I.R. Stevenson for help with the project; P. Rohani for comments on the manuscript; H. Tong for statistical advice and encouragement; and the Royal Artillery for logistical support on St Kilda. The work was funded by grants from the Natural Environment Research Council (to T.H.C.-B., M.J.C., B.T.G., K.W. and S.D.A.), the Royal Society (to M.J.C., K.W. and S.D.A.) and the Biotechnology and Biological Sciences Research Council (to J.M.P.).
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Grenfell, B., Wilson, K., Finkenstädt, B. et al. Noise and determinism in synchronized sheep dynamics. Nature 394, 674–677 (1998). https://doi.org/10.1038/29291
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DOI: https://doi.org/10.1038/29291
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