Population genomic structure
The patterns of population genomic structure in scrubtits add further evidence that Tasmanian biodiversity is broadly structured by biogeographic barriers related to climate, topology and anthropogenic impacts. King Island scrubtits likely diverged from Tasmanian scrubtits towards the end of the Pleistocene glacial period around 12,000 years ago, when sea level rise flooded low-lying marshlands in what is now the Bass Strait (Bowdler 2015). King Island scrubtits are therefore already occupying a climate refuge, and habitat loss following European settlement has fragmented this refuge into three isolated subpopulations. Our results suggest these subpopulations are now unlikely to be connected by natural gene flow, with genetic differentiation between the subpopulations resulting from vicariance and genetic drift. Pairwise FST values between King Island subpopulations are similar to or greater than those between Tasmanian scrubtit subpopulations, despite the substantially smaller geographic distances between those on King Island (circa 20 km) than those on the Tasmanian mainland (60 – 100 km). This suggests that the nature of the matrix surrounding the King Island subpopulations is more of a barrier to dispersal than the distances between them.
Among Tasmanian scrubtits, east coast birds are isolated somewhat from the rest of the population by a broad swathe of unsuitable habitats associated with a warmer and drier climate in the midlands (Corney et al. 2013), comprising predominantly dry sclerophyll forest that has also been heavily cleared since European arrival (Figure 1). The distribution of wet sclerophyll forest and rainforest within the east coast is patchy. Tasman Peninsula scrubtits are isolated by ocean on three sides and a narrow neck of land on the fourth, but dispersal is likely also limited by a lack of wet forest and land clearing on the nearby mainland (Figure 1). This pattern of genetic isolation of the Tasman Peninsula scrubtit is similar to the patterns observed in other taxa including the Tasmanian devil Sarcophilus harrisii (Jones et al. 2004; Farquharson et al. 2022) and mountain ash Eucalyptus regnans (von Takach et al. 2021). In contrast, wet forest is abundant on the mainland adjacent to south Bruny Island. This suggests occasional gene flow across the 4 km strait separating Bruny Island from the Tasmanian mainland can occur, as has been demonstrated in forty-spotted pardalotesPardalotus quadragintus (Alves et al. 2023).