Explainer: Stochastic extinction

Stochastic extinction is extinction caused by chance fluctuations (in births and deaths, in gene frequencies, and in the environment, including its rare extremes) rather than by a deterministic decline to zero. The phrase is an umbrella over two classes of randomness: environmental fluctuation, which is external to the population and whose rare extremes ("catastrophes") are the tail of that same distribution rather than a separate class; and demographic accidents, which are internal and unavoidable in finite numbers, comprising the vagaries of birth and death, and the loss of genetic variability. It also covers a separate palaeontological usage in which "stochastic" instead means extinction non-selective with respect to traits. Most productively it is a population-level idea: even a population with, on average, a positive or stable growth rate can be lost to chance once it becomes small, so that stochastic extinction is best read as a probability: the chance, over a stated horizon, that a self-replacing population completing its full life cycle in the wild is no longer attainable. Whether a given decline is labelled "stochastic" or "deterministic" frequently depends on the scale and time horizon of observation rather than on the biology itself. The term therefore requires four things to be stated: the source of randomness, the biological scale (individual, population, metapopulation or clade), the time horizon and whether the claim concerns an underlying process or an observed pattern.