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.