Minimum viable population size: not magic, but necessary

Letter: We agree with Flather and colleagues [1] that there is no ‘magic number’ or universal threshold around which one can plan for threatened species management to cover all contingencies; neither have we ever claimed so [2]. As Flather and colleagues reiterate [1], a minimum viable population size [MVP; the abundance above which the probability of extinction (over conservation-relevant timescales) is unacceptably low for any species] is illusory. There is substantial variation in MVP among species [3] and probably across subpopulations for widespread or spatially disjunct species, and there is no obvious ‘decision threshold’, as reviewed by Traill et al. [4] and elsewhere [5]. Yet even with this uncertainty, ignoring MVP because of concerns over its imperfections or risk of misuse, as Flather et al. [1] seem to prefer, would be imprudent.