Minimum viable population size: not magic, but necessary

We agree with Flather and colleagues 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. As Flather and colleagues reiterate, 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 and probably across subpopulations for widespread or spatially disjunct species, and there is no obvious ‘decision threshold’, as reviewed by Traill et al. and elsewhere. Yet even with this uncertainty, ignoring MVP because of concerns over its imperfections or risk of misuse, as Flather et al. seem to prefer, would be imprudent.