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Figure (clik to enlarge) – Applying P(0)=0.6; r=4; N=100000; for(n=0;n<=N;n++) { P(n+1)=r*P(n)*(1-P(n)); } *Robert May* Population Dynamics equation [1974-76] (do check on Logistic maps) for several iterations (generations). After 780 iterations, *P* is attracted to 1 (max. population), and then suddenly, for the next generations the very same population is almost extinguish.

“** Not only in research, but also in the everyday world of politics and economics, we would all be better off if more people realised that simple non-linear systems do not necessarily possess simple dynamical properties**.” ~

*Robert M. May*, “Simple Mathematical models with very complicated Dynamics”,

*Nature*, Vol. 261, p.459, June 10, 1976.

(…) The fact that the simple and deterministic equation (1) can possess dynamical trajectories which look like some sort of random noise has disturbing practical implications. It means, for example, that apparently erratic fluctuations in the census data for an animal population need not necessarily betoken either the vagaries of an unpredictable environment or sampling errors: they may simply derive from a rigidly deterministic population growth relationship such as equation (1). This point is discussed more fully and carefully elsewhere [1]. Alternatively, it may be observed that in the chaotic regime arbitrarily close initial conditions can lead to trajectories which, after a sufficiently long time, diverge widely. This means that, even if we have a simple model in which all the parameters are determined exactly, long term prediction is nevertheless impossible. In a meteorological context, *Lorenz* [15] has called this general phenomenon the “** butterfly effect**“: even if the atmosphere could be described by a deterministic model in which all parameters were known, the fluttering of a butterfly’s wings could alter the initial conditions, and thus (in the chaotic regime) alter the long term prediction. Fluid turbulence provides a classic example where, as a parameter (the

*Reynolds*number) is tuned in a set of deterministic equations (the

*Navier-Stokes*equations), the motion can undergo an abrupt transition from some stable configuration (for example, laminar flow) into an apparently stochastic, chaotic regime. Various models, based on the

*Navier-Stokes*differential equations, have been proposed as mathematical metaphors for this process [15,40,41] . In a recent review of the theory of turbulence,

*Martin*[42] has observed that the one-dimensional difference equation (1) may be useful in this context. Compared with the earlier models [15,40,41] it has the disadvantage of being even more abstractly metaphorical, and the advantage of having a spectrum of dynamical behaviour which is more richly complicated yet more amenable to analytical investigation. A more down-to-earth application is possible in the use of equation (1) to fit data [1,2,3,38,39,43] on biological populations with discrete, non-overlapping generations, as is the case for many temperate zone arthropods. (…) in pp. 13-14,

*Robert M. May*, “Simple Mathematical models with very complicated Dynamics“,

*Nature*, Vol. 261, p.459, June 10, 1976 [PDF link].

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