Slow motion extinction: inbreeding, introgression, and loss in the critically endangered mangrove finch (Camarhynchus heliobates)

The critically endangered mangrove finch is now limited to one small population on the west coast of Isabela Island in the Galapagos, but 100 years ago multiple populations were found on the islands of Isabela and Fernandina. By accessing genetic datasets through museum sampling, we are able to put current levels of genetic diversity and hybridization with congenerics into a historical context for enhanced conservation. In this study, we compared neutral genetic diversity of the now extinct Fernandina population to historical and current diversity of the Isabela population using 14 microsatellite markers. We found that current genetic diversity of the last remnant population (similar to 80-100 individuals) is far below levels 100 years ago, with only about half of the allelic diversity retained. Current genetic diversity is close to levels in the Fernandina population that went extinct by the 1970s. Bottleneck analysis did not show a strong signature of recent decline, and instead implies that this species may have consistently had low population sizes with wide fluctuations. Hybridization with congeneric woodpecker finches was found in the modern Isabela population, implying that some individuals within the few remaining breeding pairs are finding mates with woodpecker finches. Within the context of historical low population sizes and wide fluctuations, current conservation efforts may help the mangrove finch face current extinction threats and avoid the fate of the Fernandina population. However, this historically small lineage will likely continue to face challenges associated with small specialist species surrounded by a widely-distributed sister lineage producing viable hybrids.