Effective/census population size ratio estimation: a compendium and appraisal

With an ecological-evolutionary perspective increasingly applied toward the conservation and management of endangered or exploited species, the genetic estimation of effective population size (N(e)) has proliferated. Based on a comprehensive analysis of empirical literature from the past two decades, we asked: (i) how often do studies link N(e) to the adult census population size (N)? (ii) To what extent is N(e) correctly linked to N? (iii) How readily is uncertainty accounted for in both N(e) and N when quantifying N(e)/N ratios? and (iv) how frequently and to what degree might errors in the estimation of N(e) or N affect inferences of N(e)/N ratios? We found that only 20% of available N(e) estimates (508 of 2617; 233 studies) explicitly attempted to link N(e) and N; of these, only 31% (160 of 508) correctly linked N(e) and N. Moreover, only 7% (41 of 508) of N(e)/N ratios (correctly linked or not) reported confidence intervals for both N(e) and N; for those cases where confidence intervals were reported for N(e) only, 31% of N(e)/N ratios overlapped with 1, of which more than half also reached below N(e)/N = 0.01. Uncertainty in N(e)/N ratios thus sometimes spanned at least two orders of magnitude. We conclude that the estimation of N(e)/N ratios in natural populations could be significantly improved, discuss several options for doing so, and briefly outline some future research directions.