Defining spatial genetic structure and management units for vulnerable koala (Phascolarctos cinereus) populations in the Sydney region, Australia

Context. Mammal populations around the world are increasingly threatened with population fragmentation because of loss of habitat or barriers to gene flow. The investigation of koala populations in the Sydney region not only provides valuable information about this vulnerable species, but also serves as a model for other species that have suffered major rapid declines in population size, and are now recovering in fragmented habitat. The peri-urban study region allows investigation of the impact of landscape features such as major roads and housing developments on koala gene flow. Aims. Animals originating from four geographic sampling areas around Sydney, New South Wales, Australia, were examined to determine population structure and gene flow and to identify barriers to gene flow and management units. Methods. The present study examined 12 microsatellite loci and used Bayesian assignment methods and genic frequency analysis methods to identify demographically separate populations and barriers to gene flow between those populations. Key results. Three discrete populations were resolved, with all displaying moderate to high levels of genetic differentiation among them (theta = 0.141-0.224). The allelic richness and heterozygosity of the Blue Mountains population (A = 6.46, H-O = 0.66) is comparable to the highest diversity found in any koala population previously investigated. However, considerably lower genetic diversity was found in the Campbelltown population (A = 3.17, H-O = 0.49), which also displayed evidence of a recent population bottleneck (effective population size estimated at 16-21). Conclusions. Animals separated by a military reserve were identified as one population, suggesting that the reserve maintains gene flow within this population. By contrast, strong differentiation of two geographically close populations separated by several potential barriers to gene flow suggested these land-use features pose barriers to gene flow. Implications. Implications of these findings for management of koala populations in the Greater Sydney region are discussed. In particular, the need to carefully consider the future of a military reserve is highlighted, along with possible solutions to allow gene flow across the proposed barrier regions. Because these are demographically separate populations, specific management plans tailored to the needs of each population will need to be formulated.