Environmental variability: not quite as random as you might think

Environmental variability is often described using colour, with analogy with optics. This simple concept has completely revolutionised my view of ecology, and in particular with regards to Population Viability Analysis and Resilience.

It is often assumed that the environment within which we live varies completely randomly. However, there is evidence to suggest that the environment does not vary randomly, but in patterns.These patterns can be described in terms of auto-correlation. For instance if one year is similar to the next, it is positively auto-correlated. If one year is very different to the next, it is negatively auto-correlated.

gr1b1Fig. 1. Red noise is used to describe positive auto-correlation while blue noise is used to describe negative auto-correlation. White noise is random. Image from paper by Ruokolainen et al. (2009).

In fact, there is research demonstrating that temperature has become bluer over time and is redder in temperate regions. Population dynamics can transform when environmentally forced. With red noise (i.e. positive auto-correlation) “good years” follow “good years”, and with blue noise (i.e. negative auto-correlation), “bad years” follow “good years”. Red noise is therefore thought to increase extinction risk in biological populations, because successive runs of bad years have a cumulative negative effect on survival.

Including environmental noise of different colours into population models can therefore help make better predictions about extinction risk and population viability.  If you would like to learn more on the subject, this review paper is a good starting point.

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