Pleistocene Sea‐Level Fluctuation Shapes Archipelago‐Wide Population Structure in the Critically Endangered Lord Howe Island Cockroach <i>Panesthia lata</i>

ABSTRACT The Lord Howe Island Group (LHIG) is one of Australia's most renowned archipelagos. Although a number of studies have investigated the biogeography and genetic diversity of species on Lord Howe Island (LHI) itself, the evolutionary distinctiveness of populations across LHI's satellite islets remains poorly understood. In this study, we explored the genetic structure and health of the Endangered, endemic cockroach Panesthia lata across four islands of the LHIG, using a panel of nuclear single‐nucleotide polymorphisms and complete mitochondrial genomes. Our analyses reveal that the lineage on the permanently isolated islet Ball's Pyramid is highly divergent from the remaining populations, while those on the episodically connected LHI, Roach Island, and Blackburn Island may have diverged as recently as the end of the last interglacial period. These results offer the first evidence that Pleistocene sea level lowstands allowed for historical faunal connectivity across the LHIG. Further, while P. lata was believed to have been locally extirpated by rodents on LHI, we discovered a surviving, relict population, albeit with high pairwise kinship indicative of a strong population bottleneck. We also detect relatively high levels of kinship in the other populations, suggesting potential inbreeding that could necessitate ongoing management. Finally, the combination of shallow genetic structure and low diversity suggests that genetic rescue from another island may be a viable strategy to conserve the LHI population of P. lata , as well as other species that have been similarly impacted by rodents.