Natural dispersal is better than translocation for reducing risks of inbreeding depression in eastern black rhinoceros ( Diceros bicornis michaeli )

Due to increasing anthropogenic impacts, many species survive only in small and isolated populations. Active conservation management to reduce extinction risk includes increasing habitat connectivity, translocations from captive populations, or intensive surveillance of highly protected closed populations. Advances in sequencing technology mean that it is now possible to consider the genomic impacts of such strategies, as a proxy for variation in individual fitness. Using whole genome sequences from critically endangered eastern black rhinoceros (Diceros bicornis michaeli), we compare the consequences of different types of conservation efforts, based on cohorts of offspring resulting from parents from different sources. Based on the fraction of the genome in runs of homozygosity (ROH) of different lengths, we found lower inbreeding in offspring of individuals that had either been translocated from ex-situ populations (FROH>1Mb = 0.047) or dispersed between proximate native populations (FROH>1Mb = 0.065) compared to the intensively managed closed population from which the migrant moved (FROH>1Mb = 0.112). However, the benefit of such movement was removed after only a few generations of closed breeding (FROH>1Mb = 0.149). Although sample size restricted power to detect significance of differences, the relative abundance of highly deleterious mutations was higher for offspring resulting from translocation compared to the other cohorts and this load was sheltered by higher heterozygosity, which could increase risks of inbreeding depression if inbreeding subsequently occurs. In contrast, native dispersers reduced the negative effects of inbreeding without compromising the benefits of past purging of deleterious mutations. Our study highlights the importance of natural dispersal and reiterates the importance of maintaining habitat corridors between populations.