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The effect of aggressiveness on the population dynamics of a territorial bird

Abstract

A central issue in ecology lies in identifying the importance of resources, natural enemies and behaviour in the regulation of animal populations. Much of the debate on this subject has focused on animals that show cyclic fluctuations in abundance1,2,3,4,5,6,7. However, there is still disagreement about the role of extrinsic (food, parasites or predators) and intrinsic (behaviour) factors in causing cycles2,8,9,10. Recent studies have examined the impact of natural enemies1,3,4,7, although spatial patterns resulting from restricted dispersal or recruitment are increasingly recognized as having the potential to influence unstable population dynamics5,6,11,12,13. We tested the hypothesis that population cycles in a territorial bird, red grouse Lagopus lagopus scoticus, are caused by delayed density-dependent changes in the aggressiveness and spacing behaviour of males. Here we show that increasing aggressiveness experimentally for a short period in autumn reduced recruitment and subsequent breeding density by 50%, and changed population trajectories from increasing to declining. Intrinsic processes can therefore have fundamental effects on population dynamics.

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Acknowledgements

We thank the British army, English Nature, and the Dalhousie and Dunecht estates for allowing us access to their moors. We particularly thank T. P. J. Helps, C. McCarthy, J. Adamson, D. Calder and A. Dykes for their help in organizing the fieldwork. We also thank R. Cox, N. Green, D. Luccini and J. Irvine for their help with the fieldwork. D. Elston helped with the statistical analyses. S. Albon, M. Harris, X. Lambin, M. Marquiss, R. Moss and R. Van Der Wal made helpful comments on previous drafts of the manuscript. This work was funded by the Natural Environment Research Council.

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The authors declare that they have no competing financial interests.

Correspondence to F. Mougeot.

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Figure 1: Changes in grouse numbers on the control (open circles) and testosterone (filled circles) experimental areas of each population.
Figure 2

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