Quantifying the habitat requirements of an endangered marsupial predator, the northern quoll (Dasyurus hallucatus)

Australia’s mammalian fauna is in a state of precipitous decline, with at least 10% of the native endemic species declared extinct since 1788. While declines and extinctions were initially concentrated in southern arid and semi-arid regions of the continent, there is now growing recognition of a contemporary wave of declines occurring in Australia’s north. One species that has already suffered considerable declines is the northern quoll (Dasyurus hallucatus); an endangered marsupial predator that once occurred across much of Northern Australia, but is now restricted to a handful of isolated populations. The drivers of this decline are manifold, but undoubtedly include the cane toad, Rhinella marina, an introduced amphibian that is lethal when consumed by native predators, including the northern quoll. The cane toad now inhabits areas that overlap with the distributional range of northern quoll populations located in Queensland, the Northern Territory, and the Kimberley regions in Western Australia, but has not yet invaded the Pilbara bioregion; the most western northern quoll population, now considered a stronghold for the species. Here, identifying critical habitat for the northern quoll is a research priority. In this thesis, I set out to accomplish four research objectives: (i) Quantify changes in the northern quolls geographic range and ecological niche across the entire species distribution (ii) Develop appropriate and standardised methods for monitoring northern quolls using remote sensing cameras, (iii) Define characteristics of critical habitat for northern quoll within the Pilbara bioregion at the landscape scale, and (iv) Define characteristics of critical habitat for northern quoll within the Pilbara bioregion at the patch scale.
In chapter one, I provide a brief overview of the northern quoll in context to Australia’s other mammal declines. In my first data chapter (chapter two), I showed declines were largest in the eastern extent (Queensland) of the northern quoll range, and smallest in the West (Pilbara), likely reflecting the temporal spread of cane toads. Populations were most likely to persist in high-quality habitat, characterised by topographical ruggedness and high annual rainfall.
In chapter three, I compared the effectiveness of vertically and horizontally orientated camera traps as a survey tool for northern quolls. While studies have traditionally used horizontal camera traps to survey mammals, vertically orientated cameras may be advantageous when surveying quolls specifically, because they can capture high-quality images of spot patterning. Results indicated that vertically orientated cameras were able to reliably detect northern quolls when they were present and were equally as effective as horizontal cameras, but provided additional information at the individual level.
In chapter four, I examined factors influencing the occurrence and abundance of northern quolls in the Pilbara bioregion at the landscape scale, where both response and predictor variables were collected to represent entire landscapes. We used the vertically orientated camera traps piloted in chapter three, given they can collect data at the individual level. We found the spatial configuration of rocky habitats were more important than the amount of habitat for predicting quoll occurrence and abundance at the landscape scale; northern quolls were less abundant in more fragmented landscapes, probably because they are required to spend more time moving between patches, where they are exposed to increased levels of predation. Supporting my findings in chapter two, this study also showed northern quolls favoured areas with high topographical ruggedness, and areas that received more rainfall.
Finally, in chapter five, I expand on the findings of chapter four by measuring northern quoll responses to habitat at additional scales: at the patch scale and within patch scale. Here I showed quolls are more likely to use rocky patches with smaller amounts of edge habitat relative to patch area, and those that comprise habitat with high amounts of vegetation cover and denning crevices. In the spinifex grasslands surrounding rocky patches, quolls were also more likely to use sites with high vegetation cover. Similar to chapter four, I suggest these results are likely driven by predation risk; northern quolls may be more exposed to predators at patches with high amounts of edge, low amounts of vegetation cover, and low den availability.
Collectively, the results from this thesis advance our understanding of northern quoll habitat requirements across its entire range, but particularly in the Pilbara, the last stronghold for the species. We demonstrate the value of large aggregated patches of rocky outcrops and suggest the division of such habitat, through either the construction of mining infrastructure (e.g roads, rail lines) or other means, is likely to have substantial negative impacts on northern quoll populations. In addition, our findings highlight the importance of managing vegetation within and surrounding rocky habitat, particularly in patchy landscapes, given doing so may have the potential to mitigate increased rates of predation in these areas. While the objectives of this thesis were mostly targeted towards the conservation of northern quolls, the methods I developed to achieve them have broad non-species specific applications, and our findings provide important theoretical insight relevant to the conservation of threatened species globally.