KEY WORDS : Australia; Conservation; Evidence; Genetic rescue; Ornithology; Population Monitoring; Translocation.
1. INTRODUCTION: When populations decline and become fragmented, changes to patterns of gene flow and reductions in effective population size can exacerbate population declines through depressed individual fitness associated with inbreeding and genetic drift (Frankham 1995, Harrisson et al. 2019). Therefore, the importance of understanding the dynamics of threatened species’ populations at the molecular level is increasingly being acknowledged in conservation (Allendorf et al. 2010). Without such knowledge, potentially significant drivers of population decline can be overlooked if the focus of monitoring efforts is solely at the individual or population level (Stojanovic et al. 2022). Aided by ever-improving sequencing and analytical techniques, there is little doubt that the value of population genomics to threatened species conservation will continue to increase (Willi et al. 2022).
For small, inbred populations, genetic rescue can have significant conservation benefits (Frankham et al. 2015). Meta-analysis revealed that outcrossing through the introduction of novel genes from external populations had beneficial effects on over 90% of inbred populations examined, with a median increase in composite fitness of over 145% in stressful environments (Frankham et al. 2015). Examples of successful genetic rescue include the mountain pygmy possum Burramys parvus , whose population more than doubled in three years following the introduction of six males from an external population (Weeks et al. 2017), and the bighorn sheep Ovis canadensis , whereby experimental restoration of immigration into a small inbred population led to increases in fitness-related traits of 23 – 257% (Hogg et al. 2006).
Here we address a key knowledge gap in our understanding of an Australian bird considered the third most likely to go extinct within fifteen years; the critically endangered King Island scrubtit Acanthornis magna greeniana (Geyle et al. 2018). King Island scrubtits are a subspecies of the nominate Tasmanian scrubtit Acanthornis magna magna restricted to King Island, a 1098 km2 island within the Bass Strait between Victoria and mainland Tasmania. Expert elicitation estimated the King Island scrubtit had an 83 % probability of extinction within 20 years (95 % CI = 66 – 93 %, Geyle et al. 2018). The primary driver of King Island scrubtit population decline is habitat loss (Webb et al. 2016). While the exact habitat preferences of King Island scrubtits are not certain, surveys suggest the birds prefer swamp forest containing dominant or sub-dominant swamp paperbarks Melaleuca ericifolia (Webb et al., 2016; Bell et al. 2023). It is likely the King Island scrubtit would once have been widespread in suitable native vegetation prior to European colonisation > 100 years ago, but extensive surveys of potentially suitable habitat fragments suggest it is now confined to three patches: Colliers Swamp in the south, Pegarah State Forest and surrounding forests in the east, and Lavinia State Reserve and a small area of private land between The Nook swamps and Granite Lagoon in the northeast (Webb et al., 2016; Bell et al. 2023, Figure 1). The total estimated area of occupancy of the King Island scrubtit is < 1 km2 and likely declining (Webb et al. 2016).
The three known putative subpopulations of the King Island scrubtit are separated by 18 to 20 km, between which lies a matrix dominated by agricultural land, sand dunes and potentially unsuitable scrub and heath vegetation types (Figure 1). The extent to which individual scrubtits can permeate the matrix and facilitate gene flow between the subpopulations is unknown, but predicted to be low based on the species’ habitat preferences. Field surveys estimate the Colliers Swamp subpopulation contains approximately 30-40 birds, Pegarah Forest 15-30 birds and Nook Swamps 10-20 birds (Webb et al. 2016, Bell et al. 2023), but the effective population size of each subpopulation is unknown. There is a risk that genomic impacts associated with small effective population size and limited dispersal capacity may be contributing to the decline of the King Island scrubtit population (Frankham 2015). Other threats facing King Island scrubtits include acid sulphate soils associated with drainage for agriculture, wildfires, predation by feral cats, and habitat deterioration through windthrow and sea-level rise (Fielding et al., 2022; Webb et al., 2016). The King Island scrubtit is therefore a priority species for conservation actions under the Australian Government’s Threatened Species Action Plan 2022-32 (Commonwealth of Australia 2022).
In contrast to the King Island scrubtit, Tasmanian scrubtits are not considered threatened and are widespread in suitable habitats on mainland Tasmania and close offshore islands, though there is currently no reliable population estimate (BirdLife International 2022). The species prefers wet forest habitats which dominate the western side of mainland Tasmania but are patchier on the eastern side (Figure 1). There is no information on dispersal in Tasmanian or King Island scrubtits, though the species is understood to be largely sedentary (Higgins et al. 2006).
To assess the need for and to inform potential genetic management of the King Island scrubtit, our study had four aims: firstly to sequence the scrubtit genome; second to estimate the current spatial genetic structure of the King Island scrubtit population; third to quantify current levels of genetic diversity, inbreeding and relatedness within the King Island scrubtit population; and finally to consider the population genetics of the King Island scrubtit within the context of the genetics of the Tasmanian scrubtit. Following population genetic theory and available demographic data (Frankham 1995; Webb et al. 2016), we predicted that, relative to the Tasmanian scrubtit, the King Island scrubtit population would have higher levels of population structure and inbreeding and lower genetic diversity.