Many ecological and biological populations oscillate with the remarkable property that their period length is quite constant while their abundance levels have large amplitude fluctuations. The earliest evidence for fluctuations comes from records of the Hudson Bay Company regarding the populations of lynx and hares. This company bought and sold lynx pelts. Their records (Fig. 1) - studied by Charles Sutherland Elton and Mary Nicholson  - show that the number of animals fluctuates with a period of about nine years, so that about a dozen cycles were observed in the data during the century. The amplitude of these cycles varies significantly from year to year, as would be expected for a chaotic system as observed in many three species models (see , ,  among others).
The counts from the Hudson Bay Company are undersampled since only around ten points are available per cycle (one point per year for a cycle which has an average period equal to 9.2 year). These data have been interpolated to allow a global modelling . A 4D model producing transient chaos was obtained (Fig. 2).
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 R. K. Upadhyay, S. R. K. Jyengar & V. Rai, Chaos: an ecological reality?, International Journal of Bifurcation & Chaos, 8 (6), 1325-1333, 1998.
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 J. Maquet, C. Letellier & L. A. Aguirre, Global models from the Canadian Lynx cycles as a first evidence for chaos in real ecosystems, Journal of Mathematical Biology, 55 (1), 21-39, 2007.