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).