Using ancient DNA to quantify losses of genetic and species diversity in seabirds: a case study of Pterodroma petrels from a Pacific island

The largest anthropogenic extinction events during the Holocene occurred on Pacific islands, where thousands of bird populations were lost. Although ancient DNA approaches have become widely used to monitor the genetic variability of species through time, few studies have been conducted to identify the potential cryptic loss of genetic and species diversity within Pacific seabird species. Here we used heterochronous sampling of mitochondrial DNA (Cytochrome b) in the genus Pterodroma from Norfolk Island to quantify potential loss of genetic and species diversity. We particularly focused on the providence petrel P. solandri whose main breeding colony (~ 1,000,000 breeding pairs) became extirpated from Norfolk Island following European settlement circa 1800. We sampled subfossil bones consistent with Pterodroma spp. from Norfolk Island, and performed genetic comparisons with other populations of P. solandri and congeneric species. The majority of subfossil Norfolk Island individuals exhibited the most common mitochondrial haplotype from Lord Howe Island P. solandri, suggesting no appreciable loss of genetic variation as a consequence of the Norfolk Island extirpation. Our findings provide an example where a large seabird population was rapidly extirpated by humans without loss of species-level genetic diversity, probably as a consequence of high connectivity with other populations. However, past connectivity was insufficient to prevent the extirpation itself, which has conservation implications for predicting the resilience of threatened seabirds. In contrast, ancient DNA analyses of smaller Pterodroma bones from Norfolk Island indicate the loss of a second species, potentially P. pycrofti, P. brevipes or another closely related, possibly undescribed taxon, from the Tasman Sea.