Effects of non-random juvenile mortality on small, inbred populations

An individual's ability to produce surviving descendants defines its evolutionary fitness, and loss of family lineages (i.e. having no surviving descendants or relatives) diminishes allelic diversity within closed populations. This high variance in individual reproductive success is difficult to detect and measure, so potential demographic impacts are poorly understood. We identify the frequency and demographic impacts of lineage loss from non-random juvenile mortality of brood mates in critically endangered wild orange-bellied parrots Neophema chrysogaster over 22 years. We posit that non-random juvenile mortality is the mechanism that underpins lineage loss and reproductive skew in animals with few opportunities to breed in their lives. Only ~8% of parrot mothers bred more than once and non-random juvenile mortality was more prevalent than expected by chance. Nine of ten wild maternal lineages died out in the wild population over only three years. Using population viability analysis and genetic data, we show non-random juvenile mortality results in lower observed heterozygosity than random mortality scenarios. Failure to account for diminished population genetic diversity when family lineages die out may result in worse conservation outcomes. High individual variance in reproductive success can result in rapid loss of most families from a population if there is no conservation intervention. We identify warning signs and management approaches to address this threat. Looking for and correcting lineage loss early may be an important practical step for conserving population genetic diversity of such species.