PVA Highlights Different Management Actions Required to Prevent Extinction in Fenced and Unfenced Populations of Reintroduced Western Quolls

Abstract

Population Viability Analysis (PVA) is a broad tool which can be used to estimate the likelihood of population persistence in translocated populations. We used PVA to evaluate the viability of two reintroduced populations of western quolls, a threatened marsupial carnivore, using population-specific parameters. Quolls were reintroduced to two sites in South Australia: Arid Recovery (AR), an arid cat- and fox-exclusion site comprised of a 12 300-ha fenced safe haven, and Ikara Flinders Ranges National Park (IFRNP), a 93 800-ha semi-arid unfenced conservation reserve where introduced predators were controlled through aerial baiting. We predicted that factors influencing population viability would vary significantly between the two sites due to the differences in release size area and predation risk. Site-specific genetic, mortality, and population size data of quolls were gathered and incorporated into the PVA. At the unfenced site, scenarios with higher carrying capacity and the lower rates of mortality recorded after aerial cat and fox baiting significantly reduced the probability of extinction (PE). Increases in juvenile mortality rate increased PE to a greater extent than the adult mortality rate. Increasing the size of the area under intensive cat and fox control was the management action most likely to result in a viable population through increasing carrying capacity. In comparison, at the fenced AR site, extinction risk was related to the small population size, and, despite higher survival, supplementation of extra individuals was deemed necessary to avoid extinction and inbreeding. Low mortality rates allowed for less frequent supplementation. Population harvesting from IFRNP of 21 individuals for up to 3 years did not significantly increase the risk to this population of inbreeding depression if carrying capacity was increased. These results demonstrate the need for different management strategies for different populations of the same species to avoid extinction within 100 years.