Abstract
Knowing about a threatened species' effective population size (N-e) is vital to assess population trends and guide conservation efforts. Highly mobile and migratory species are some of the most threatened yet hardest to monitor groups. Recently, a conversion has been proposed to convert estimates of the effective number of breeders (N-b) to N-e using two life history traits. We evaluated current methods to estimate N-b and N-e to inform conservation management using the black-fronted tern/tarapirohe (Chlidonias albostriatus) as a case study. Black-fronted terns are endangered, highly mobile and traditional monitoring methods have proven difficult. Based on 17 microsatellites and extensive sampling throughout the breeding range, we estimated mean N-b ca. 700 based on sibship assignment and linkage disequilibrium (LD) methods. Mean estimates of LD N-e based on the conversion of N-b and two life history traits was 668, while LD N-e based on samples of chicks and adults was 3216. The low ratio of N-b/N-e when using the N-e estimate based on mixed-age samples is probably driven by a combination of N-b being more sensitive to the large recruitment failure of individuals, large variation compared to mean reproductive success between same-age and same-sex individuals, and the ongoing overall population decline. We highlight that in relatively long-lived and threatened species, a combination of N(b)and N-e estimates are more appropriate to assess the population status rather than solely relying on N(e)of mixed-age samples. For black-fronted terns, we recommend increased conservation management and genetic monitoring based on N-b and N-e.