Use of Population Viability Analysis to evaluate a koala ( Phascolarctos cinereus ) source population and its response to a gradual translocation harvest

This study applied Population Viability Analysis (PVA) retrospectively to explore the response of a source population of koalas (Phascolarctos cinereus) in southeast Queensland to a translocation harvest between 2009 and 2013, with concurrent habitat loss. The study involved a rare case where the source population was monitored over 10 years, allowing us to measure changes in demographic rates and forecast impacts from translocation harvests. Field surveys to estimate population size were undertaken in 2006 and repeated in 2017. Data collected during monitoring of the source population (2009-2014) enabled refinements to key demographic parameters such as age-class distribution, female reproductive rates, and mortality rates. Scenarios that included the translocation harvest and loss of ~34% of habitat between 2007 and 2017 were firstly modelled separately. The population was predicted to increase after the last translocation harvest, but a further 30-year period was required for the population level to match that of the scenario without translocation and without habitat loss. Population projections over 50-year timeframes were not substantially influenced by either omitting the translocation component or confining the translocation harvest to one year rather than the actual 5-year period. In comparison to the translocation harvest, habitat loss (reduced carrying capacity) was predicted to have a far more substantial and lengthy negative impact. Sensitivity analysis confirmed that levels of mortality and female reproduction were likely to be highly influential on model forecasts. The refined demographic settings and reduced variability in carrying capacity due to environmental variation predicted a population size approaching that estimated from the 2017 field surveys. These findings provide guidance for evaluating potential impacts concerning the scale and timing of koala translocation harvests and affirm the value of establishing detailed demographic profiles for source populations.