By Dr Laurence Berry, Senior Wildlife Ecologist
The recent release of 62 Greater Bilbies (Macrotis lagotis) from the 480-hectare breeding area into a massive 9,570-hectare feral predator-free fenced area at Mallee Cliffs National Park marks a key milestone in AWC’s partnership with the NSW Government. The successful establishment of the population demonstrates the effectiveness of AWC’s applied conservation approach with management actions informed by high quality monitoring data and best practice science.
A total of 50 Bilbies were released at Mallee Cliffs in October 2019 funded by the Government’s Saving our Species program. Founders were sourced from two reintroduced wild populations at Scotia Sanctuary in far western NSW and Thistle Island, near Port Lincoln, South Australia, and from captive breeding programs. The initial release of founders into a smaller ‘breeding area’ was to encourage the mixing of genetics from different sources throughout the new population prior to release of individuals into the larger fenced area.
The first few weeks following release are critical for the success of a reintroduction, as during this period newly translocated animals lack familiarity with the host environment and may have heightened exposure to native predators. The stresses associated with translocation may also exacerbate any pre-existing health conditions in translocated animals.
Following each reintroduction, AWC implements an extensive program of monitoring to measure survival rates, health and body condition, population size and recruitment. This monitoring allows AWC to evaluate the outcomes of each reintroduction and provides feedback allowing us to refine our reintroduction practice.
To monitor Bilby survival rates at Mallee Cliffs, we fitted a subset of 31 founders (62 per cent) with tail-mounted radio-transmitters. Survival was tracked for four months following release. The survival rate across all cohorts was 90.3 per cent, meaning that around 45 of the 50 founders released survived the initial period after translocation.
To monitor individual health and body condition of the reintroduced population, we undertook 400 live-trapping nights within the breeding area at 3-month intervals within the first year of release. Our results showed that, over an 18-month period, all founding groups increased or maintained mean body weight from that recorded at release. Individuals from captive breeding programs (reared on supplementary food) had significantly higher mean body weight than those sourced from wild populations, and this advantage was maintained over the course of the following year. The trapping results also track the increase in body weight in the new cohort of Bilbies born at Mallee Cliffs as new individuals survive and reach maturity. No signs of disease were observed in the reintroduced population, and mean body condition scores for individuals from different sources were all within an optimal range.
We used a capture-recapture statistical approach to estimate population size. Over an 18-month period, population size increased from 50 founders to an estimated 118 (± 28) individuals. This number exceeded the target of 110 individuals set as the trigger for releasing individuals into the fenced feral-free area at Mallee Cliffs.
Consequently, in June 2021, we captured 32 male and 30 female Greater Bilbies in the breeding area and released them into the larger fenced area. All individuals released were offspring of the original founders. We retained at least 50 individuals in the breeding area, including as many original founders as possible, to continue the process of genetic mixing.
We expect the Bilby population at Mallee Cliffs to grow to over 1,100 individuals. The reintroduction will contribute to rebuilding Mallee Cliffs’ historical assemblage, helping restore lost ecosystem processes (Bilbies are prolific ‘diggers’), and contributing to the maintenance of adaptive evolutionary potential in the Bilby by exposing populations to natural selection against environmental pressures that existed historically across its range.
Whilst extant Bilby populations persist in south-west Queensland, the Tanami, and Great Sandy Deserts, these are vulnerable to introduced predators. AWC’s work at Mallee Cliffs demonstrates that with appropriate planning, monitoring and management, it is possible to establish and maintain viable Bilby populations. AWC currently manages established Bilby populations within five feral-free safe havens at Scotia, Mt Gibson, and Yookamurra Wildlife Sanctuaries and at Pilliga and Mallee Cliffs National Parks. Further Bilby reintroductions at AWC’s Newhaven Sanctuary are also planned in 2022. It is estimated that AWC will protect a total of more than 5,000 Bilbies across these sites within the next decade, making a substantial contribution towards safeguarding the future of the species.
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