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High-amplitude fluctuations and alternative dynamical states of midges in Lake Myvatn

Abstract

Complex dynamics are often shown by simple ecological models1,2 and have been clearly demonstrated in laboratory3,4 and natural systems5,6,7,8,9. Yet many classes of theoretically possible dynamics are still poorly documented in nature. Here we study long-term time-series data of a midge, Tanytarsus gracilentus (Diptera: Chironomidae), in Lake Myvatn, Iceland. The midge undergoes density fluctuations of almost six orders of magnitude. Rather than regular cycles, however, these fluctuations have irregular periods of 4–7 years, indicating complex dynamics. We fit three consumer–resource models capable of qualitatively distinct dynamics to the data. Of these, the best-fitting model shows alternative dynamical states in the absence of environmental variability; depending on the initial midge densities, the model shows either fluctuations around a fixed point or high-amplitude cycles. This explains the observed complex population dynamics: high-amplitude but irregular fluctuations occur because stochastic variability causes the dynamics to switch between domains of attraction to the alternative states. In the model, the amplitude of fluctuations depends strongly on minute resource subsidies into the midge habitat. These resource subsidies may be sensitive to human-caused changes in the hydrology of the lake, with human impacts such as dredging leading to higher-amplitude fluctuations. Tanytarsus gracilentus is a key component of the Myvatn ecosystem, representing two-thirds of the secondary productivity of the lake10 and providing vital food resources to fish and to breeding bird populations11,12. Therefore the high-amplitude, irregular fluctuations in midge densities generated by alternative dynamical states dominate much of the ecology of the lake.

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Figure 1: Population dynamics of T. gracilentus in Myvatn.
Figure 2: Simulated dynamics of the model given by Box 1 equations (1)–(3) for 50 generations.
Figure 3: Dynamics of the midge–algae–detritus model depending on resource input rate, c , and detritus retention, d.

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Acknowledgements

We thank K. C. Abbott, M. Duffy, K. J. Forbes, R. T. Gilman, J. P. Harmon and members of Zoo/Ent 540, Theoretical Ecology, University of Wisconsin – Madison, for comments on the manuscript. V.A.A.J. thanks R. A. Jansen-Spence for the time to do this research. This work was funded in part by National Science Foundation grants to A.R.I., and grants from the Icelandic Research Council and the University of Iceland Research Fund to A.E. and A.G.

Author Contributions A.E. and A.G. oversaw the data collection and are responsible for the long-term study on midge dynamics in Myvatn. A.E. and A.R.I. conceived the midge–algae–detritus model, and A.R.I. performed statistical analyses. V.A.A.J. and A.R.I. performed the mathematical analyses of the midge–algae–detritus model.

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Correspondence to Anthony R. Ives.

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The file contains Supplementary Notes, Supplementary Figures S1-S10 with Legends and Supplementary Methods and Supplementary Table S1. (PDF 480 kb)

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Ives, A., Einarsson, Á., Jansen, V. et al. High-amplitude fluctuations and alternative dynamical states of midges in Lake Myvatn. Nature 452, 84–87 (2008). https://doi.org/10.1038/nature06610

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