Population genetic structure of Leadbeater's possum Gymnobelideus leadbeateri, and its implications for species conservation

Since European colonisation, Leadbeater's possum (Gymnobelideus leadbeateri) has declined across its range to the point where it is now only patchily distributed within the montane ash forests of the Central Highlands of Victoria. Population viability analyses (PVA) have modelled ongoing large-scale population declines under current timber management prescriptions. The loss of large hollow bearing trees and failure to maintain mature ash and recruit younger trees is predicted to result in the loss of up to 90% of Leadbeater's possums within the next 50 years.

There have been two recently discovered populations that occur in environs dissimilar to that of the majority of central highlands populations. The first at Yellingbo Nature Conservation Reserve occurs in lowland swamp, and the second at Lake Mountain, occurs in sub-alpine woodland. The population at Yellingbo is distinct, not only in terms of habitat differences, but also in being completely geographically isolated from other conspecific population. Breeding colonies at Yellingbo and Lake Mountain make use of artificial nesting hollows in addition to natural denning sites, the latter being uncommon at both sites.

This study uses a panel of 15-20 highly resolving microsatellite markers and mitochondrial D-loop sequence data, to infer historical gene flow and investigate current population structure. Populations in the northern part of the central highlands (including Lake Mountain) were highly admixed, and showed no signs of either current or historical barriers to gene flow. The isolated population of Yellingbo was highly genetically differentiated (on the basis of microsatellite data). Analyses of bottlenecking confirmed it to have undergone a recent reduction in population size. The extent to which the distinctiveness of the Yellingbo population might be expected solely through bottlenecking of central highlands populations, was tested by simulating population history scenarios seeded with genotypes from candidate sources. No bottleneck scenario reproduced the genetic characteristics of the Yellingbo population, suggesting that it does not share recent ancestry with other extant populations. Mitochondrial sequence data confirmed that, not only had Yellingbo been isolated from the rest of the species range since well before European colonisation, but that it may once have formed part of a larger genetic unit that is now extinct. It therefore harbours a unique subset of genetic diversity that is not represented by any other extant population.

In addition to inter-population genetic analyses, a fine-scale genetic analysis was also conducted at both Lake Mountain and Yellingbo. This included an investigation into the mating system and patterns of dispersal. The species’ was found to be largely monogamous at both sites, although there was evidence of extra-pair paternity in less than 5% of cases. At both sites, males and females were found to disperse much further than previously recorded (during an earlier study at Yellingbo). Maximum inferred dispersal distances were 2.5km in males and 2km in females. Importantly, population genetic structure was found to coincide closely with the spatial arrangement of breeding colonies, and genetic variation was highest within colonies than among them. Colonies or groups of colonies were found to be discrete genetic units that do not experience sufficient migration to homogenise them.

The mitochondrial genetic data collected in this study identified historic loss of maternal lineages in the northern central highlands, probably indicative of past shifts in local climate. This may translate to population declines across the broader range of the species. This suggests that in addition to declines detected in the field and in line with predictions from PVA, future declines are highly probable, potentially to the point of extinction of this species.