Longevity can mask low Nb if only Ne of mixed-age samples is estimated in threatened and mobile species

Abstract

Knowing about a threatened species’ effective population size (Ne) 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 (Nb) to Ne using two life history traits. We evaluated current methods to estimate Nb and Ne 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 Nb ca. 700 based on sibship assignment and linkage disequilibrium (LD) methods. Mean estimates of LD Ne based on the conversion of Nb and two life history traits was 668, while LD Ne based on samples of chicks and adults was 3216. The low ratio of Nb/Ne when using the Ne estimate based on mixed-age samples is probably driven by a combination of Nb 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 Nband Ne estimates are more appropriate to assess the population status rather than solely relying on Neof mixed-age samples. For black-fronted terns, we recommend increased conservation management and genetic monitoring based on Nb and Ne.