Date of this Version
Trends in Ecology and Evolution December 2011, Vol. 26, No. 12, doi:10.1016/j.tree.2011.09.014.
In a recent article in TREE , we reviewed evidence for a consistent standardized estimate of minimum viable populations (MVPs) across taxa [2–4] and found that the universal MVP of 5000 adults advocated by Traill et al.  was unsupported by reanalyses of their data. We identified shortcomings in the original analyses, and found substantial uncertainty in standardized MVP estimates, both within populations of the same species and among species. We concluded that neither data nor theory supported a generally applicable MVP.
No evidence refuting the technical problems that we identified in their original analyzes was presented by Brook et al. . Instead, they agreed with us that a universally applicable MVP is illusory and that no such ‘magic number’ exists. Brook and colleagues’ clear rejection of a universal MVP is important because both popular coverage  of their work and many statements in their own publications had suggested otherwise. For example, Frankham et al.  wrote that evidence against universality was simply ‘. . .an artifact of defining it for a fixed number of years, rather than generations’. Likewise, Traill et al.  stated that ‘The bottom line is that both evolutionary and demographic constraints on populations require sizes to be at least 5000 adult individuals’, judging 5000 to be a ‘. . .consensus. . . [and] useful benchmark’ . Even in their Letter , Brook et al. asserted that genetic arguments are sufficient to embrace a generalized MVP, overlooking statistical artifacts in the translation of effective size to census size and the substantive variation that characterizes these data . Their confidence in the merits of 5000 as an MVP conservation target is emphasized by its recent promotion as ‘. . .an empirically supported threshold MVP target’ for conservation triage . Given this backdrop of mixed messages, it is important to (re)emphasize the contingent nature of MVPs and the wide variability of standardized MVP estimates among populations and species .