Application of the one-migrant-per-generation rule to conservation and management

Endangered species are commonly found in several (partially) isolated populations dispersed on different fragments of a habitat, natural reserve, or zoo. A certain level of connectivity among such populations is essential for maintaining genetic variation within and between populations to allow local and global adaptation and for preventing inbreeding depression. A rule of thumb widely accepted by the conservation community is that one migrant per generation (OMPG) into a population is the appropriate level of gene flow. This rule is based on Wright's study of his island model under a long list of simplifying assumptions. I examined the robustness of the OMPG rule to the violation of each of the many assumptions, quantifying the effect with population genetics theory. I showed that, when interpreted as one effective migrant per generation, OMPG is generally valid for real populations departing from the ideal model in the discrepancies of actual (N) and effective (N-e) population sizes and actual (m) and effective (m(e)) migration rates. I also addressed the issue of converting the effective number of migrants (M-e = N(e)m(e)) into the actual-number of migrants (M = Nm) of a certain age and sex. In particular, N-e < N, a case common for natural populations, did not necessarily require M > M-e to maintain a certain level of differentiation among populations. Rather, translating the elusive M-e into the manageable M depends on the specific causes (e.g., biased sex ratio, reproductive skew) that lead to N-e < N.