Conservation implications
The principal threat to persistence of the King Island scrubtit is habitat loss and its small population size (Threatened Species Section, 2012). Preservation of remaining habitat and restoration of lost habitat will therefore be critical if the taxon is to recover in the longer term (Webb et al. 2016). In the shorter term, our results suggest the Pegarah Forest, Colliers Swamp and Nook Swamps subpopulations of the King Island scrubtit should be managed as a single management unit with translocations between these small, isolated populations used to improve gene flow that has been lost due to habitat fragmentation and ensure that the current level of overall genetic diversity is maintained (Frankham et al 2017). Higher levels of inbreeding, lower estimated effective population sizes and higher probability of visible defective traits (i.e. baldness) relative to the Tasmanian scrubtit suggest that genetic factors may well be exacerbating population decline in King Island scrubtits. Future genetic management of the population is therefore warranted. There are currently no demographic data on dispersal and breeding success rates in King Island scrubtits. However, if breeding success is low and / or contemporary gene flow between subpopulations is as infrequent as our results suggest, genetic rescue through the translocation of scrubtits between King Island subpopulations could help (Harrisson et al. 2016).
Action paralysis is a conscious management decision when threats are known, and risks of adverse events may be worth taking if a species or population is already known to be on a course for imminent extinction (Webb et al. 2018, Canessa et al. 2020). For many years, the risk of outbreeding depression has been used as an excuse for inaction (Ralls et al. 2018), however evidence for outbreeding depression is limited and predictable (Frankham et al. 2011, Frankham 2015). A very real risk of any translocation is that introduced individuals may be vectors for the establishment of novel pathogens in threatened populations (Peters et al. 2014), however this risk can be mitigated with disease screening.
The current rate of King Island scrubtit population decline, and the extent to which this decline is exacerbated by genetic effects are currently unknown. Our results suggest two precautionary approaches to genetic management of King Island scrubtits are feasible. Assuming that delaying management will not compromise the chances of future King Island scrubtit population recovery, the Tasman Peninsula subpopulation of Tasmanian scrubtits could be used as a trial for genetic rescue, given the remarkable similarity of this subpopulation’s genetic parameters to the King Island scrubtit. If conservation actions are considered urgent, genetic rescue trials could occur in the Pegarah Forest King Island scrubtit subpopulation, which our data suggest is most at risk of inbreeding and extinction, with minimal risks of negative genetic effects spreading to the other two subpopulations via natural gene flow.