Demographic evaluation of translocating the threatened northern quoll to two Australian islands

Context: Translocation is widely used to help avoid extinction of species from threatening processes. A fundamental objective of translocation is to establish self-sustaining populations; estimating demographic parameters is critical to assessing success of these programs and can also be used to support future management actions. 

Aims: We estimated demographic parameters to evaluate the success of translocating the northern quoll Dasyurus hallucatus to two islands (Astell and Pobassoo), in response to the threat posed by the introduced cane toad Rhinella marina on the Australian mainland. 

Methods: We used capture-mark-recapture methods to monitor both populations at regular intervals from initial release in 2003 until 2009 and a one-off survey in 2014. 

Key results: Relative abundance (trap success) increased exponentially in the first 4 years, declined, and then stabilised in subsequent years. The population of female northern quolls on Astell Island peaked in 2006 with an estimate of 3640 (95% CI 3022-4257), and on Pobassoo Island the peak was 2007 with 617 (95% CI 531-703) females. In 2014 the population had decreased to 2193 (95% CI 1920-2467) on Astell and 451 (95% CI 359-543) on Pobassoo. Apparent survival and body condition decreased significantly following the population peak, possibly related to density dependence. 

Conclusion: Both populations of northern quolls reached their regulation phase after going through establishment and growth phases, which included exceeding carrying capacity. The pattern was similar between the populations. 

Implications: Increased survival and recruitment at threat-free translocation sites contributes to successful establishment of self-sustaining populations.